Below is a text version of the webinar titled "Design Strategies for Ducts in Conditioned Spaces," originally presented in March 2014 and part of DOE Challenge Home's Tech Training Webinar Series. In addition to this text version of the audio, you can access the recording of the webinar (video).
Hi, everyone. Thank-you so much for your patience. My name is Lindsay Parker. Welcome to the U.S. Department of Energy Challenge Home technical webinar series. We're really excited that you could all join us today. I'm going to give you a short intro to the webinar. Today's session is on ducts in conditioned spaces, and it's one of a continuing series of technical training webinars Challenge Home is putting on to support our partners in designing and building DOE Challenge Homes. So just a couple of housekeeping issues: We'll have all attendees in listen-only mode. But feel free to ask questions using the question function on GoToWebinar. And we'll cover as many of your questions as possible near the end of the webinar. We're also going to be recording this, so we'll post it online, onto the DOE Challenge Home website, in which you'll be able to receive -- I'll send you an email with information on how to access the recording online. So now I'll hand over the reins to Jamie Lyons with Newport Partners, who is the technical director of the Challenge Home program.
Great. Thank-you, everybody, for joining us and sorry for the small delay. It might look like it, but we weren't delayed because we were finishing our NCAA brackets, even though the games start right about now. We just had to work out some audio details and I think we're good to go. So before I turn this over to our presenter, I just want to sort of frame up the context for today's webinar.
Over the last year or so, we've been going around the country and working with our partners on training sessions. So this entails working with builders and raters and architects, designers, utilities, efficiency program staff, that whole sector of the industry. And in these training sessions, which will run three or four hours, we cover all the key facets of zero energy ready homes through the Challenge Home program. So that covers things like the business case, the value propositions and the messaging, key marketing resources, and then the technical specs. But in that framework, it's tough to spend too much time on the details of the tech specs, which brings us to today and why DOE's running this technical training webinar series throughout the course of the year. So today we're covering one of the key mandatory items for Challenge Home, which is design strategies for locating the ductwork within the conditioned envelope of the building. And this is sort of a must-have for zero energy ready homes, because if these homes are going to be zero energy ready, the energy penalties that are at stake along with comfort, indoor air quality, and potential moisture issues that we can get with poor duct design, we have to do better than that in zero energy ready homes, so for that reason, one of the mandatory requirements is to locate the ducts within conditioned space. As our speaker will show you, though, there's a number of tools in the toolkit for accomplishing this. So I'm going to turn it over to Bill Zoeller here in a second. Just to introduce Bill, he's a registered architect and a senior VP with Steven Winter Associates, with over 30 years of experience in building design, construction, post-construction evaluation, tech transfer, training, and building performance research. He's consulted on product development and marketing analysis work for major building material suppliers, and Bill also occupies the efficiency seat on the State of Connecticut's Codes and Standards Committee. Bill is a frequent speaker and trainer on high-performance buildings at national conferences and educational workshops. And with a little stroke of luck, Bill's going to join us right now, assuming his audio is working. And we'll get started with the training. Bill, are you with us?
I think so. Confirm that I am. Alright, great. My screenshot just went away. Let me see if I can get that back. ... Did it go away for everybody, or just me?
Let me take a look here. Yea, we're not seeing your slides. So you need to click back to the ... share your screen settings. GoToWebinar ... Go to your slideshow in PowerPoint.
OK. If you run your slideshow, we'll see that.
I think part of the slowness here is our servers acting up a little bit for some reason. So ... I'm sure it will behave itself.
(Presenters talking) ... We can run it from here. Lindsay will queue up the slides.
I got it. OK. Alright. Thanks, everybody, for joining us. As Jamie discussed, what we're going to be talking about today is the design options for locating ducts within conditioned space. And briefly mentioned as well is the need to do that, why we would want to do that, and the performance benefits of accomplishing just that.
So the first question is, why ducts in conditioned space? And there's really many answers for that. But most significantly is, energy performance and building performance in terms of indoor air quality, comfort, durability, etcetera. When we have ducted HVAC systems where the ducts are located outside of the conditioned space, obviously they're located in spaces that have higher thermal loads, losses in terms of leakages to the outside, which changes the pressure boundaries within the house, pressure in attics, causing a whole lot of other problems. We've seen, from our experience in testing systems where ducts and air handlers are located outside the conditioned space, of losses ranging from 10 to 45 percent. Ten percent would really be a best case scenario. So if we have a really good duct system but we're still hanging our ducts in the attic, you're really not going to get better than 90 percent from a distribution standpoint. If we had some leakage into that, especially on the return side, we could see effectiveness down to 50 percent of what it's supposed to be. So when we are doing our best to produce energy, thermal energy efficiently with our equipment, and then throwing half of it away, obviously that's not an optimal situation, so we're looking to prevent that and build on in terms of performance and benefit and so on.
Also, when we put our ducts in attics and not in conditioned space, every time the duct penetrates the ceiling plane, for instance, with register boot, that's just another hole connecting the unconditioned space to the conditioned space. So it's either something we have to pay attention and deal with, or it's something we ignore and pay the price in terms of energy, comfort, etcetera, etcetera. On the indoor air quality aspect, probably the biggest negative of not having ducts in conditioned space is you're going to have your ducts located in places where it may be a vented attic space, you may have some return side leakage. Return side ducts typically are under peak pressurization, so when they operate, they're like little vacuums. They will pull in debris from the attic space or wherever they're located, a crawl space, etcetera, into the airstream, into the house. So we're actually pumping in pollutants into the house when we do that, so it's clearly something you want to avoid.
OK, when we're looking at how Challenge Home is approaching the whole metric of building better houses, one of the goals is to get to that zero energy ready place that Jamie was talking about before. From a prescriptive position, there are multiple things that are specifically required in the Challenge Home metric, in order to be able to certify yourself as a Challenge Home, and to perform at the levels that are anticipated in the Challenge Homes be constructed. One of those is in fact that ducts need to be within the home's thermal and air barrier boundaries. Now, it's not a one-size-fits-all scenario. There's multiple ways of doing it. And what we're going to be doing today is talking about what the options are, when they best fit a particular circumstance, and what the costs and benefits of each of those approaches might happen to be.
The first thing we see is that there are some mandatory requirements for Challenge Home. Simple things like the Energy Star baseline for a lot of the performance criteria, envelope issues, and then we're going to get to the duct system here, and you can see that the ducts within the conditioned space is clearly something that needs to be accomplished. Now even though it's a mandatory requirement to place your ducts in conditioned space in Challenge Home, it doesn't necessarily spell out how to do it. Only that, what the end result needs to be. So what we've done is sort of looked at all the different viable options and your different circumstances that make sense for putting ducts in conditioned space. And we've tried to provide some graphic instructions on those, some pluses and minuses, and help guide builders and designers to what approach might be the most appropriate for a particular house in a particular location.
There are a couple of really small exceptions that allow the ducts to end up in the attic space. Now we don't necessarily advocate these, or all of them, in any case. But for instance, if there is a short duct run up to 10 feet in total length, that does penetrate the home's thermal and air boundary, because of an outlying master bathroom or something that is impractical to get through in another means, there's sort of a little out. Now this is, it's sort of a designer give-back. If there's a circumstance where it's just really impractical to do it, that's kind of an out. We actually -- I will be demonstrating one location where that might make sense in the examples I give later, but for the most part, that's a practice that we try to stay away from because we would be penetrating the thermal boundary somewhere. We've got some extra leakage in ceiling to deal with. So if we can avoid it, even better. So that's what we try to do. In terms of jump ducts, return air pads from bedrooms, etcetera, that's also an exception that we can put in the outside conditioned space, in an attic, for instance, and still not violate the mandatory requirements of all ducts in conditioned space. The logic behind that is that AV air flow isn't really significant. If I'm pushing say 100 CFM into a master bedroom, for instance. On the return side, there's only 100 CFM coming back. And it's a low-pressure 100 CFM. It's not in a pressurized system, you know, basically the same pressure as the house air. Also, the temperature of that air is the same as the space in the house itself, maybe 75 degrees, 70 degrees, versus chilled air at 55 degrees or heated air at 120 degrees. So the delta P's not as much. It is required, however, that those jump ducts be sealed with the same sort of good practice as the pressurized duct system, and then lastly is the ductless HVAC system that we're seeing more and more of, the mini splits and the variations on the mini splits.
OK, so when we're looking at the ducts in conditioned space, typically, we're looking at ducts that can be placed within the conditioned floor area of the building. Basically, when we're looking at a typical home, we're looking at the walls. If it's a slab, the slab itself, the attic ceiling, and a place that is -- the area that is specifically defined by the thermal boundary of the house itself. We look for places that we can put ducts in that already existing defined boundary. We could also look at things like unvented crawl spaces and basements, if the crawl space is made to be sealed conditioned space similar to a basement, that's a place that we can use to place our HVAC system, air handler, ducts. etcetera. Same thing with an unvented attic. If we take our thermal boundaries and place them up at the roof deck instead of at the ceiling, that gives us some additional volume within the structure of the building to place our ducts and thereby keeping all that equipment within conditioned space. Lastly, we can also look at options with using a vented attic. Even though the vented attic itself is not a conditioned space, and would seem to violate the mandatory requirement, Challenge Home has provided a methodology that is as effective and efficient as the other methods that allow ducts to be placed in the unconditioned space of the attic. And that's the last of the six options we'll be talking about today. We'll be getting into that in some detail.
OK, of the options that are available to us, my diagram is actually showing that there's five different applications. We actually have a sixth that we'll be talking about, as well. And as I mentioned before, selecting the best option depends on a lot of factors: where the house is located, how the house is designed, what the house is constructed out of, the climate zone the house is in, etcetera, etcetera. So if we look at the top left graphic, that's kind of the standard practice that you see most production builders still doing, and that's ducts in an unvented attic, or I'm sorry, that's a variation on the standard practice, where we're taking the insulation and moving it from the ceiling plane up to the roof deck, basically encapsulating an entire space within the attic in which to place our air handlers or ducts, etcetera. The image number two is ducts in the dropped ceiling, where we're taking the space that is below the primary ceiling plane and creating a special niche in there for which to place our ducts, so basically integrating it into the architectural floor plan of the space. Image number three is where we're actually doing something similar to the dropped soffits that we showed you in image number two but rather than create that volume below the ceiling plane, we're creating that volume above the ceiling plane. And we're doing that with a specially configured truss setup that defines that space, and then we're moving our thermal and air barrier up above the ceiling plane to encapsulate that newly defined space. Image number four is, I've got some truss floor construction. Typically you'll find this in a two-story home. And if I have a truss floor construction, that gives me plenty of space both vertically and horizontally, looking at it from a plan standpoint, where I can run trunks, ducts, etcetera, to pretty much get to all the areas within my home. Then number five as I mentioned is ducts in the vented attic, and there are options available to thermally and air-sealed protect those ducts, such that they in fact mimic the performance of true inside conditioned space ducts. And we'll be talking about that. The last one, the graphic of which is not included in this particular slide, is using the sealed crawl space, for a basement, for the location of air handlers and ducts. So in essence we've got six different options. And then with the six different options available, the best option to select again depends on a multitude of factors that we'll be talking about a little bit further as we get into the presentation.
OK, so the first one I'm going to delve into in a little bit of detail, is the unvented attic approach. What happens with the unvented attic approach, is typically -- in a typical house we've got the insulation, the thermal barrier, the air seal barrier, at the ceiling plane. What we're doing here is we're removing the thermal barrier from the ceiling plane and instead moving it up to the roof deck. So we're encapsulating that entire attic volume within the conditioned boundary. Obviously, that gives us a lot of interior volume to place equipment. It gives us a lot of flexibility in terms of where that equipment can be located and what spaces that equipment can serve. In concept, it's really simple. It's kind of an intuitive thing that people grasp immediately, and they say, OK, I've got to move my insulation from the ceiling plane and put it up on the roof deck. It's one of those 3-D things that kind of pop into your head, and it's like, yea, that makes a lot of sense. There are, however, a lot of code performance characteristics that need to be reconciled for optimal application. It's not just a matter of putting some insulation in the roof deck and calling it a day. There's a whole lot of other components, interactions that we really need to be aware of, to make sure that we can use this method consistently, economically, and efficiently.
OK. Most of what controls how we can apply the insulation to the roof deck, in terms of how it's currently generally applied, falls under the International Residential Code, Section R-316.5.3, and that specifically deals with unvented attic spaces. What you see in the image on the left is basically a utility space located up in an attic. If I use an open- or closed-cell foam in this application, and the space is created solely for the purpose of placing mechanical services, and servicing those mechanical systems, then I don't need to install what the code defines as a thermal barrier. And a thermal barrier is basically a 15-minute fire barrier that's defined specifically under an AST impasse that requires that the temperature on one side of the 15-minute barrier remains low enough so that combustion will not occur. The slide on the right is actually the same attic space, a portion of which is sort of a little lost area that's used for storage, and in that area we've got a typical half-inch drywall applied that does constitute the required thermal barrier. So in this attic I've actually got both scenarios, where I've got no thermal barrier in one portion where I'm not using it for storage, and the other portion I am using it for storage and I do have a thermal barrier. On the image on the left, what I do, however, need is an ignition barrier. An ignition barrier can be as simple as an intumescent paint coating, something that essentially chars under low temperature and then prevents fuel, fire from reaching the fuel of a foam. A lot of other criteria fall into that. There are actually a couple of foams that don't need the ignition barrier under that circumstance, as well. That specifically intended if they did a specific ASPM test and did their evaluation services report, and it's listed that they don't need the ignition barrier, then that could be eliminated, too. Typical ignition barriers, as I mentioned, are the intumescent paint, it could be one and a half inch of mineral fiber insulation, a quarter inch of structural panels, where three-eighths chip board. For the 15-minute thermal barrier, generally it's going to be a half-inch drywall application.
The insulation to the roof deck can also be used to form a living space, as we see in the image on the left. It's a house that was fully enclosed in an open-cell foam product. It's going to be all covered with half-inch drywall, so it's got the ignition barrier, it's got the thermal barrier, and the cavities are full of insulation. The image on the right, we've got a small service area located up in the attic space where the ducts are located, so that will not get the thermal barrier. But depending on the foam that's used, it probably will have an ignition barrier applied to it.
OK, what we found in multiple applications is that the unvented attic approach, where we are placing the insulation at the roof deck, can work well in retrofit scenarios. Typically a retrofit scenario, especially down in the South, we're going to have roof truss construction, we're going to have a lot of ducts in the attic space, and maybe the air handler in the attic space. In this case, you can see we've got some copper plumbing. Happened to be a water heater up here. So given all the existing mechanical systems, and the fact that we wanted to improve the performance of this house, we looked at all the options that were available and decided that the unvented attic approach in this case was the best approach. It allowed us to keep our mechanical systems essentially where they are in the house. We didn't have to do a full-gut rehab in the rest of the building to provide space for the mechanical systems. So it allows us to use a volume that exists but use it in a different way simply by moving the thermal boundary out to the edge of the roof deck rather than at the ceiling plane.
OK, when we're looking at unvented attics, one of the code requirements that is in place, aside from the fire protection of the foam under different circumstances, is the issue of condensation control. And the reason we're concerned about condensation control, because if the inside layer of the foam, the space inside the building that is defined by the inside edge of the foam, is at room temperature, it's more of its conditioned space. There are circumstances where the roof deck on the outside edge can be very cold. At nighttime, in the winter, even in climate zone one, in the wintertime, on a clear night, there's a lot of heat loss coming off that roof deck, and the roof deck can get really cold. What happens then is that if I do get any moisture from inside the building contacting the inside of that roof deck, I can get potential condensation. And to prevent that, the code has a prescriptive table developed that tells us exactly how much of the insulation of all the total insulation that's required for that climate zone, how much of that insulation needs to be air- impermeable. It needs to be insulation that's specifically rated that air will not penetrate under normal pressures. In this case, this image is showing a rafter assembly that's filled with an open-cell foam product, maybe 3.5-per inch around that area, but specifically rated as air-impermeable. Now the product happens to be vapor-permeable, which I'll talk about in a little bit, but the code doesn't get into that. What the code gets into only is the air-impermeable nature of the material.
So what we see in this chart is sort of a synopsis of some of the condensation control insulation requirements for the unvented attic or insulation at the roof deck in the unvented attic. To use a specific example to explain what's going on, we've got our red box around climate zone 4A and 4B. So, you know, kind of a moderate zone, showing both the cold, excuse me, the moist, and the dry climate. So this could be somewhere in California, or it could be Baltimore, Maryland, or Washington, D.C., for instance. And in that climate, climate zone 4, Of all of the insulation that is required within that roof assembly, with the 2012 IECC, that would be an R-49, R-15 of that is required to be an air-impermeable insulation, applied directly against the roof deck. The intent of that is that we're trying to keep the dew point from not occurring within that insulation assembly in that R-15 or anywhere underneath the roof deck by virtue of having the impermeable insulation at R-15.
The code also allows an alternate assembly where there is a combination -- well, here it is, actually -- the air- impermeable insulation up against the roof deck. You can see the thinner gray band on top and then the air-permeable insulation underneath it. So of the total insulation that we've shown here, I would be showing a total aggregate amount of an R-49, but the R-15, the top slice of that, would be the air- impermeable material. And again, that is to prevent moisture from inside the house from migrating up through that material, contacting a cold roof deck, and condensating and causing a moisture problem.
Another alternate to this is to take the air-impermeable insulation and put it on top of the roof deck. We see this more in commercial installation, maybe a flat roof assembly or a shallow pitch assembly, where there may be multiple layers of poly-iso or XPS insulation on top of the roof deck, and then the cavity is left with just loose-fill insulation or a dense- pack type product inside that assembly. The result is the same. I've still got the same thermal boundary between the outside and my roof deck, so theoretically my roof deck is going to -- the first condensing plane, basically, would be at the same temperature regardless of my insulation is underneath or on top, so the code allows this sort of an assembly, as well. Either one will work. Obviously if we do this on top of a typical sloped roof, you've got different edge details and architectural trim details to work out, which is why we see this more often on flat roof assemblies. We've seen this on green roofs, planted roofs, etcetera. And it's just another option that we can use if we're using the approach where we want to go to an unvented attic situation.
OK, these next two slides I think do a pretty good job of explaining a lot of the more complicated issues that have to do with the unvented attic space. And when I started talking about this method, I talked about the fire barriers that were required, either an ignition barrier or a thermal barrier. In this case, I'm showing a section through a house. And in that attic space, I'm showing the insulation at my roof deck. The attic space will be used for the location of mechanical equipment only, and for the service of mechanical equipment. And there'll be no storage allowed. In that case, if I'm using a spray foam, spray polyurethane foam insulation atthe roof deck, all I need to do is use an approved ignition barrier, as opposed to the thermal barrier. Again, that could be as simple as an intumescent paint coating. There are hard boards you can use, as well. It tends to be a little bit more expensive, a little bit more labor involved in that. And again, there are certain foams that have been specifically tested to not require an ignition barrier under these circumstances. One of the things the code did in a recent change was they used to say, for the service of utilities, we didn't need to have the thermal barrier where we did use the ignition barrier. No one quite knew what "service of utilities" was, so they went ahead and changed the language so for repair and maintenance of equipment, basically, is what it is now. So it's a little bit more clear in language as to exactly what they're getting at. And I think that helped clarify a lot of the question and confusion on this issue. When we looked at interior space, the whole building is essentially envelope on the inside with that thermal barrier protecting us from the foam both in the walls -- we've got the thermal barrier at the attic ceiling -- and then the three-quarter-inch subfloor in this case acts as the thermal barrier for a crawl space. Which also is the same as the attic, insofar as I can have my equipment and I can service and maintain it, but I can't use it for storage if I have an approved ignition barrier.
Going on to the next slide is sort of a variation on that where in this case I am now upgrading my ignition barrier to the 15- minute thermal barrier throughout the building. And what that does is it allows me to use the space in the attic for storage. It allows me to use the space in the crawl space for storage. And the thermal barrier at the ceiling is removed, if there's like an open concept sort of space. I still do have the same subfloor at the floor framing, but it's not really being used as the rated thermal barrier anymore. So we've got a couple of options that we can use in looking at the unvented attic space, in terms of how I can place my equipment. Can I use the space for storage, etcetera. So, again, depending on exactly what the configuration of the house is, and how it's going to be used, and the cost parameters involved, the economics, there are some multiple options that are available to the designer and the builder.
OK, now, like all of the options for placing ducts in conditioned space, there's going to be some advantages and disadvantages. And in this case, the advantages start with providing the space to put the air handler as well as the ducts. That's sort of a big advantage. Most of these other options that we're talking about are really for duct placement and not necessarily for air handler placement. In those circumstances, we're basically relying on the designer to provide a space within the floor plan to place the air handler, and then the ducts can go into the other spaces that we're providing. But the unvented attic does provide the space for that air handler as part of its total system. It is not as plan-dependent as other options. What that means is that, I can basically run ducts and I can place registers wherever I want in that ceiling plane. If I've got sort of an outlier master bath somewhere, there's no issues with running a duct out to that location. No issues with placing a register where I need to in the ceiling of that space, no issues with return air. It's pretty flexible in terms of designs of systems. And as I mentioned prior, when we showed the other slide, of the attic space that we used for retrofit, it's pretty viable for retrofits. For deep energy retrofit where we don't want to completely gut the floor plan but we're going to improve the building envelope, it's something that can work pretty well. Looking at the limitations, it's typically, of the options we're looking at, and primarily looking at new construction, it's typically going to be one of the higher cost options. The materials that are used are expensive. You're talking about a lot of surface area that's impacted, that's probably the biggest issue. If we're talking about entire roof surface versus something much smaller in scope, there's some high costs to that. The type of insulation we're trending to use in the unvented attic space tends to be high R per inch, just by virtue of the fact that I have to get to pretty high R levels, like R-49 in a lot of climate zones now. And there may not be a lot of space to put that insulation in terms of the thickness of the assembly, so I've got to use the high R value per inch material. They tend to be the spray foams; tend to be more expensive. As we talked about in a little bit of detail, there's code limitations and requirements in the roof deck insulation both for fire protection and for condensation control. And they have to be taken into account and considered.
And then lastly when we do place the insulation at the roof deck versus ceiling plane, we have to understand that roof decks typically are sloped and not flat in most houses. A sloped covering over a certain square footage is going to be a larger area than a flat covering over that same area, so if I have got -- even if it's just generically, if I have a say a 6 and 12 roof pitch over a flat ceiling, that 6 and 12 roof pitch is about 11 percent greater an area than is the flat ceiling. If I had gables in that same attic space I've got to insulate and thermally protect those, as well. So you really are increasing the surface area of the thermal boundary sometimes considerably when you're using the unvented attic space. So that needs to be taken into account when we're looking at our energy models or total energy use of the building, because basically you've got to counteract that deficit of energy use to make up for the -- you're going to get an improvement for the duct system, you're going to get an increased load due to the envelope. So you've got to consider that and then put that into the equation to come up with the optimized solution. And then real quickly, the IRC Sections 806.4, unvented attic assemblies, spell out a lot of the specific issues that need to be considered and then the R316 Foam Plastic controls the remainder of it. So between those two, that may spell out the code requirements mostly for the unvented attics.
OK, moving on to option number two, and that would be ducts in dropped soffits, this works really well in simple plans, we've found. In this instance, a volume, a space volume is constructed below the primary ceiling plane. Those spaces are developed into soffits and dropped ceilings, and then the ducts are placed in those volumes to serve the spaces below. What's critical in using dropped soffits for the duct placement, is the architectural integration and the aesthetics of the solution. If you're going to use a dropped soffit, sometimes they can be hidden architecturally. For instance, if I have a nine-foot ceiling throughout a home but in a bedroom hallway, it drops down to eight feet and there's a cased opening between the main space and that bedroom hallway, that actually can almost not be noticeable and sometimes can actually be aesthetically interesting, because you've sort of got a series of volume transitions in spaces, something that architects will sometimes do anyway. So that could work out pretty well.
This option can work well as I mentioned in simple plans. It can also work well in more high-end design, depending again on what the intent of the designer is. These two images are showing a project we worked on a number of years ago in Orlando. It was one of Sarah Susanka's designs. It was actually the not so big showhouse, which was a showhouse at the International Builders Show a number of years ago. And it's a fairly, not a large house, maybe 2,600 square feet. But it used a lot of casework, a lot of dropped soffits, sort of a lot of Midwestern prairie-style Frank Lloyd Wright kind of details that tend to be pretty sympathetic with this sort of a treatment. So if the house style will accept this sort of an architectural treatment, it can work pretty well.
What we found really is, we kinda classify these dropped soffit designs really into two different categories. The one we've got here is sort of a linear dropped soffit configuration. And the reason we call it a linear dropped soffit configuration, if we look at the bedroom hallway at the bottom of the slide, you can see we've got a trunk that's kinda comes a little over to serve it and then the bedroom -- ceiling -- hallway ceiling is dropped about 12 inches or so. And within that space, we're just doing parallel duct runs to serve each of the adjoining spaces. Doing the same thing on the master bedroom side, and on the top of the drawing, you can see there we did a dropped soffit around the room similar to what we saw on the other slide to give us a place for the ducts on one side, and kind of continue the treatment around architecturally, so it kind of made some sense. In a smaller house, we probably wouldn't have the large trunk that's transversing both of the, both sides of the house. There might just be a simple linear effect where we're providing all high wall registers to get into the rooms that are adjacent to the ducts.
The other approach, and this is sort of more common to the more ornate plan that we showed before -- the Susanka house -- was what we term as a perimeter approach. In this case there's sort of a core space within the building. And that core space is basically wrapped in a architectural dropped soffit. Running the ducts in that perimeter space allows us to access the truly perimeter rooms around that core space and provide air to each of those rooms in the required amount. Now, there's two things that we really need to pay attention to, in terms of design and air flows in this type of a system. One is the throw of the air. What we find is that we're basically limited to somewhere around a 12-foot throw using typical air velocities and CFM amounts out of high wall registers or these types of soffit registers to get across a room. So that means that the duct register and therefore the dropped ceiling needs to be within 12 feet or so of the space. Now we can stretch those limits to, we have much higher performing building enclosures which we're attempting to recommend, as well. But that's sort of a number you'd want to start with, a 12-foot throw or so. The other thing we have to look at -- I'm going to go back a slide here for a second -- is this plan, this configuration, is more or less kind of an extended plenum, where that big trunk that's in the middle of the plan when the air comes out of the supplied plenum from the air handler, the supply riser, the pressures in that trunk are sort of equalized all the way throughout it, at least it is if it's designed correctly. And the amount of air that therefore is going to each of the registers is predicated on how big the duct hole is that penetrates that trunk. By equal pressures throughout, the size of the hole that penetrates it, is going to determine how much air gets out of that particular hole. So that's how we can sort of design the CFM flows to each of those registers that we're seeing here. With this plan, it's a little bit more difficult. We don't have that extended plenum. Basically it's kind of a diminishing trunk, where the trunk size diminishes when I get to each of my out runs so that the ability for it to contain air becomes less and less until I get all the way to the end, where it's a lot less. So to get to the exact CFMs we want for each of these spaces, it becomes a little bit more difficult in this design. An experienced HVAC designer who knows his stuff can absolutely get to the right solution, but it's just not as easy as it would be with the extended trunk that we saw in the linear approach.
In terms of building these soffits, there's really a couple of approaches that really are based on three different material options. The one you see on the left is kind of the default drywall approach, where prior to the soffit being installed some drywall will be installed, it will be taped and mudded, and then the soffit kind of gets assembled around it, which can work well. The problem with it is, it's a little bit out of sequence, insofar as I've got my drywaller coming in, doing some work, then, you know, going off and doing something else. The drywallers like to come in and just bang out all the drywall and then go home. It's just a more production-friendly approach. So we've got a little break in sequence when we do it that way. Another approach is rather than use the drywall, is to use a laminated fiber material like a thermal-ply type product, which -- if you get it really thin, like a sixteenth of an inch or so -- which is essentially dimension-less, so that I'm going to the drawing section on the right, where the red line is, if I've got my rough framing up, and insulation on top of the wall in this case and the framer installs the thermal-ply product in there, it's really not out of sequence. And then the soffit can get installed around it. The duct kit can get installed. And when the drywaller does come back to do the ceiling and the walls and so on, they just do a sealant where the drywall laps over that very thin product and we get our air seal that way. So it's a little simpler of an approach. The other thing we can do is where we bend this material is we can score the back of it and bend it, and so we don't have to worry about that tape joint. We can just -- I mean, the facing of it remains continuous, so if someone starts to get skilled at doing that, they can understand that there's ways of manipulating the product to limit the work that's involved. The last thing I'll say about that is, if this drop were not at a quarter soffit but at a dropped ceiling in a hallway, for instance, the thermal-ply product or the laminated sheeting could be installed directly on the ceiling rafters -- ah, not the rafters, excuse me, the bottom cords of the trusses or the ceiling joist -- prior to the top plates being installed, such that the material kind of laps underneath where the top plates are going to go. So when the ceiling drywall on either side of the hallway is installed, it again just laps over it, and is sealed into place. So there's no break in sequence and it becomes fairly simple to accomplish. You could also use plywood or OSB for this. It's something that I've seen used. I'm not sure that it's the best material to use, but it's also obviously an air barrier and that's really what we care about. The material is an air barrier and that the joints between it and whatever is going to be up against it can add an air seal.
This is just kind of an attic-side image of what we don't want to see. Where you see this horrible opening where the duct is exposed, there's insulation pulled out of the way, and you know it's going to be leaky, because you know it's (inaudible), obviously an older installation. But obviously it's not going to perform well. So that's exactly what we're trying to avoid.
Advantages and limitations for ducts in dropped soffits. In low plan -- in simple plans, it can be low-cost. Again, it's kind of like one of those easy-to-understand and implement applications. There's kind of a short learning curve. And there's really minimal load -- excuse me, minimal code restrictions. Code doesn't have much to say about it. Because it doesn't impact negatively any of the fire code issues or condensation control issues that we had with the unvented attic and some others. On the limitation side, it is heavily plan- dependent. Advanced planning and design is essential not only to get it in but to make sure that the air flows are what we want them to be. As I mentioned before, it could be limited by the throw distance of the air coming out of the register. There are additional steps for air sealing and air barriers. And again, there's no provision for the air handler. It has to be located somewhere in the floor plan.
Moving on to option number three, this is a modified truss assembly that we at one time called a plenum truss, and you probably see some language included in places calling it that, so when you see that language it's basically what we're referring to. What we're doing here is we're creating a space above the attic plane by using specialized trusses that give us that volume. When we do this type of truss, the structural portion of the truss is in essence where the insulation barrier is. The ceiling joist is actually a nonstructural member that would be installed after we install the air barrier that wraps in that space. That allows us to use larger pieces of material without smaller fittings, smaller patches, and that makes it a lot more faster and production-friendly to install.
This was just some really sort of proof-of-concept thermal testing we did a bunch of years ago, when we started experimenting with the thermal, the plenum space. Really, it's two streamlines. In the middle we're looking at the purple line and the blue X's. The blue X's is the thermostat location in the dining room, and the purple line is the temperature conditions within the plenum space itself. And basically what we're seeing is that the temperature within the plenum space, even though not directly conditioned, is staying basically within an inch -- excuse me, one degree Fahrenheit of the thermostat set pretty much under all conditions. So, I mean, it's the result we expected, but it's always good to check because you never know what might pop up.
Here you're seeing a couple images of the plenum space installed, with the ductwork installed in place. The ductwork has been mastic-sealed. Even if the ductwork is in conditioned space, we want to get it as effective as possible at moving the air, so we don't want leakage even to the inside. So we're still advocating the very good mastic sealing application. In this case, the space is lined with the laminated fiber thermal ply product, just like the dropped soffit is, and again, the joints are sealed mastic or otherwise. Made such that they would be virtually air-tight. In this type of system, the ceiling registers are all in the ceiling, so therefore the ceiling registers will have horizontal directional throw registers on them.
This plan layout shows the overlay of what that particular truss space looked like, overlaid the plan. You can see we tried to make it as narrow as we can, to the point where we were overlapping the rooms we want to serve. So we can get ceiling registers in each of those rooms. And that's as big as we want to make the thing. No bigger. The reason for that is if it's bigger, it costs more, there's more additional air barrier, more liners, more labor, more time to do it. So from a cost performance we want to make this thing as small as we practically can.
When we have a plan that's a little bit wider than the plan we just looked at, we went ahead and worked with one truss manufacturer and developed another approach. And this is basically a modified scissor truss. And the modified scissor truss is the exact same thing as the other truss we saw. But does it in a way that we're actually using less lumber per truss. And trusses are priced on board foot of material, so the less board foot of material you can use, the less the truss is going to cost. So structurally, this truss happened to be a little bit more efficient. But it also gives us some more space in which to place our ductwork horizontally across the building. This image actually shows a gable truss, because the struts are vertical. It really is a structural scissor truss where the webs are actually structural in handling the vectors of the structure and not just vertical as they're showing here.
When we overlay that type of scissor truss over the plan, you can see that now we've got kind of a larger layout. That allows us to get further into the spaces below. So we don't have to worry so much about throw locations. We've got some -- we can get closer to the perimeter. If it's a more square plan, not a linear plan, it allows us to get better to the perimeter rooms. You can see on the top right we actually did penetrate the plenum space with one little duct to get into the master bath supply. That's the caveat that the 10 linear foot of ductwork that Challenge Home is kind of giving us an out on, if we wanted to, In this case, we actually just extended a little box around it, which is a little fussy, but it worked as well.
OK,what we're seeing here is a couple images of a scissor truss installed. The image on the left is a view of the scissor truss from the attic side looking down at the membrane that was installed as the air barrier. And the image on the right is what the ducts look like below that air barrier space. This happens to be kind of a northern Midwest project. It was out in rural Nebraska. The material that we had actually spec-ed for the liner wasn't available. That was the laminated fiber material. So we tried the foil wrap product, knowing that it was probably going to have issues, and we were right, because it wouldn't air-seal properly.
But what we found was a skim coat of closed-cell foam on it actually worked really well. And what we like to do anyway, in an attic space is use closed-cell spray foam and hold the top plates and junction boxes or electrical boxes such as the smokestack boxes that pop up in the attic little boxes around bathroom fans, etcetera. So even though it was a little bit more material, it wasn't a sequence change to do this. And it worked pretty well. The expense is higher than the rigid work material, but the effect was actually a little bit better. It worked quite well.
Getting into the advantages and disadvantages of the modified truss approach, again, low cost in simple plans. And that's going to be a pattern that we're going to see. Everything that we do is low-cost in simple plans. The first thing we should do is have simple plans. It's not as plan-dependent as the dropped soffit. It's got some of the advantages of the sealed attic, the unvented attic, where we have a little bit more flexibility, not as quite, there are minimal code restrictions. It does work best in a linear plan. You know, if you have a skinny townhouse type plan, it could work really well. There is additional air sealing steps required, and rather unique air sealing steps required, so it's a little bit of a learning curve in getting it right. It does require custom nonstandard trusses, which can be a challenge the first time through, if the builder or the truss manufacturer doesn't quite understand what you're trying to accomplish. And there's no provision for the air handler. One last comment on that, We had termed this thing plenum truss early on. We actually did use it as a plenum because you can use it for return air flow if you wanted to. However, if you do that, you can't run things like electric Romex wiring through it because that's not allowed in plenums. So it can be used for return air path, but you've got to be very careful about the other things that are in it when you do so.
OK, option number four is the space within the floor trusses. This works really well with two-story buildings. If I've got a two-story house, one of the first things I'll recommend is some sort of floor truss structure. It just makes integration of the mechanical a whole lot easier when we do this. We can place the registers either as floor registers going up or ceiling registers coming down. We could run our trunks and registers pretty much in any direction. It could go laterally or transverse through the building.
We do have to make sure that we're carefully planned in terms of where we put our ducts. It's a good idea to go with at least a 12-inch deep floor truss with a large trunk opening in the center of the stand. Also works well. It's absolutely essential to coordinate the location of your ducts with where the plumber thinks he needs his waste lines. Invariably, they're the same location. So that just needs to be thought through, and a little negotiation happens, and it can generally be worked out satisfactorily for everybody.
OK, in this case I'm showing ceiling registers blowing down. I've got floor registers going up. In hot-humid climates, floor registers are not always ideal: furniture placement issues, air-throw issues, etcetera. In that instance, on the second floor of a two-story building in a hot-humid climate, you can also use a high wall register, which is a matter of extending the duct up through wall framing and blowing out toward the exterior perimeter of the building. That actually works really well. Our studies have shown that in terms of air flow and distribution, that's probably one of the best approaches to go, anyway. An extra step with these plans, but it is even more effective than the typical ceiling. register.
OK, advantages and disadvantages of the floor truss integrated. You know, again like before, it's low-cost in simple plans. It's easy to execute. We're not changing the thermal boundaries of the house at all, which is a plus. It does use the existing conditioned volume. We're not creating any space above the attic plane, in the attic plane, below the attic plane, or in the crawl space, so that's a benefit. It's pretty flexible in terms of register locations and again, minimal code restrictions. Works really well in two-story homes. Here's the really important one: it's the structural, HVAC, and architectural coordination. You definitely want to get into more advanced planning with this approach to make sure that there's no issues that weren't thought through. It seems like it's very flexible and is, but you still need to think all about the issues through. A minimum of 12-inch deep truss is a good idea.
Next option I'll mention is the sealed crawl space. Sealed crawl spaces are now prescriptively allowed and defined by code. We find them in a lot of high-performance buildings. We think they work great. And basically what we're doing is we're taking the crawl space and making it the conditioned space rather than making the floor the thermal boundary. A lot of benefits for doing that for the performance of the building enclosure, but it also gives us a place for HVAC equipment. In terms of requirements, the code basically defines this under Section R408.3 and there's a couple of small caveats. Along with including the insulation requirements for the walls or the crawl space, there is a ventilation requirement in the sealed crawl space of 1 CFM continuous per 50 square feet, exhaust ventilation, and we also want this space to communicate with the living space above or if there's a ducted mechanical system, which is what we're talking about primarily here, the system needs to receive one cubic foot per minute of conditioned supply air for every 50 square feet. And that does not need to be continuous, just when the system is operating.
Quickly, advantages and disadvantages. It does improve thermal enclosure performance. By buttoning up that crawl space, we're improving indoor air quality and doing some other positive things to the building. It does give us a place for the air handler, which is great. We don't have to worry about putting a place in the floor plate for it. Flexible register locations -- they can go pretty much anywhere we want in the floor. If you want the high wall registers, that works, too. That does take a little bit more coordination, but it's not typically that difficult. And all the equipment is accessible for service, swap out, or whatever you need to do with it. In terms of disadvantages, you just have to follow the IDCC energy code for thermal enclosure requirements and then there's the mechanical ventilation requirements that I just briefly mentioned, as well.
OK, the last approach that we're going to talk about today is something we've been working on for a while, as well. And that's the buried duct approach. My opening slide here says "Buried Encapsulated Ducts." We're going to talk about that, but also just buried ducts alone. Under certain circumstances, insulation buried ducts without the encapsulation also will meet the Challenge Home requirement for ducts in conditioned space. So in this case really all we're doing is we're placing the ducts on the attic floor, air-sealing them to typical levels of the Challenge Home requirements, which we'll mention in a second, and then filling the void with insulation. And we're talking about R-49 in a lot of climate zones, and insulation is pretty deep, so varying the ducts is really pretty simple to do.
There are really three categories of buried and/or encapsulated ducts. The first two we're looking at here, either the buried ducts or the buried and encapsulated ducts, will be prescriptively allowed as alternate measures for Challenge Home ducts in conditioned space. The one we're showing below, the encapsulated ducts, is just something that we've used on retrofit scenarios where it was just nearly impossible to move a duct somewhere where we couldn't fully bury it. We found that that's actually a pretty effective approach, too, but not quite as effective as burying the ducts where the insulation would be.
So in a dry climate, what we're seeing here is we're just really taking the ducts, placing them low to the attic floor -- I'm sorry, moist climate, taking the ducts, placing them low to the attic floor, encapsulating them with an inch and a half of closed-cell foam and then burying the insulation, burying the ducts, the whole assembly in our loose-fill insulation. And what we've done is we've come up with some definitions. .... Hold on a second, I think ... There we go; I just have an extra slide.
OK. I'll move to this one. Where we buried the ducts in the loose-fill insulation, and the definitions that we've come up with are, "buried" is ducts buried not quite to the top of the insulation. "Fully buried" is the duct is buried to the top of the insulation. "Deeply buried" is the insulation is covered by -- the duct, rather, is covered by three and a half inches of insulation on top of it. In order to qualify for the Challenge Home requirement, the duct is a deeply buried duct. So that's what we're looking at here. Now, in the moist climate, you can see the map at the top of the page. Everything in the moist zone A would be required to be in the encapsulated duct that we're looking at here. In addition to the insulation requirement on a duct, there's also a leakage requirement, and that leakage is 3 CFM per 100 square feet of -- 3 CFM 25 per 100 square feet of conditioned floor air. So these are really tight. And even though we've got good thermal protection, we want to make sure that these ducts are really sealed tight. And that the advantage is that the closed-cell foam actually gives us some additional air sealing, as well. We do, however, recommend that a full mastic approach still be applied. Maybe a full belt and suspenders, but to make sure we get it right, that's something that we absolutely recommend. You can see the image on the upper right does show the duct laying on the floor with the foam added prior to the installation of the loose-fill insulation.
And now in a dry climate, where we're not so worried about condensation control -- just to back up a quick second -- the condensation control really is a cooling condition where if I'm running 55-degree air through the ducts, and I've got hot, sticky air in the attic, I want to make sure that I don't get condensation on the surface of the duct, which could prove problematic over the long term. So that's what control means doing. In the dry climate, we don't need to do that. We just have to do the full burial of the duct and the same metric for air sealing and then we get pretty comparable performance with the inside of the conditioned space.
And the reason for that is really described by this little image here. Even though if I add up my nominal insulation numbers, how deeply the duct is buried, or what the insulation value of the duct is, it's not explaining the phenomenon of what's happening here. And what's really happening is that by using this approach, the only thermal losses coming out of the duct are really on the top of the duct. The horizontal or the side of the duct, you're seeing essentially infinity levels of insulation. Not quite, but R a whole lot. R to prevent really any thermal conductivity from occurring. And then any thermal lost outward is to the conditioned space anyway, so it's not really a loss. So when we look at the duct that way, we end up with not a nominal R value, but an effective R value for the duct. And we've come up with some charts and some metrics defining what those nominal R values are.
And this is sort of a snapshot of what those ducts would be, the R values would be. As an example down at the bottom, the encapsulated, and fully buried duct, and this would only be like an R-38 insulation, would be over R-30 equivalent for the ducts, an R-31. If this were in an R-49 attic, that number would be even higher. So you can see by using these strategies, you're actually getting a lot more insulation value, effective insulation value on the duct, just be added to the numbers. The last page of my slides, I've got an image of a measured guideline that we've produced for the Building America program. In that document, which is available online, it's got some expanded tables and lists different insulation burial depths, different duct sizes, etcetera, where you can pull the effective R values out, and that's the number that you would place into your energy model to get overall building energy use, as well as for sizing the systems. Here we go ...
Real quick, this chart just shows on the foam insulated duct, between 10 o'clock and 1 p.m. in the afternoon, you can see I've got some line convergence of my added dew point, and the solid lines are the temperature sensors on the side of the duct. And what we're trying to do is avoid those crossings so we don't have any condensation occurring. And according to all our measurements, all our studies, and all our analysis, the metric that we've developed, the inch and a half of closed-cell foam and the R-8 duct, given these burial circumstances, will avoid condensation, pretty much under all design conditions.
OK, I've got a couple of images here of ducts laying on the attic floor, the one on the left before the ceiling drywall is installed, and the one on the right after the ceiling drywall is installed. So we've got some flexibility there.
Again, we do recommend that full mastic seal protocols be used regardless of whether spray foam will be applied or not. It's really critical, since these ducts are still in the attic, that we limit to the best levels practical any leakage. And again, so it's 3 CFM per 100 square feet of conditioned space, at 25 pascals.
Here you see one duct on the left is foamed in place before the ceiling drywall. The one on the right is foamed in place after the ceiling drywall, which sort of cocoons the duct to the ceiling of the residence and really buttons it down tightly.
Image on the left, duct in place and then the two images on the right show what it looks like when it's covered with loose-fill fiberglass insulation.
One important point here is that the ASTM does classify loose- fill -- or does classify fiberglass insulation as a mineral fiber insulation, and that is the code requirement for the ignition barrier, so when I do this sort of application I don't need any other ignition barrier over my duct. The loose-fill fiberglass insulation, if it's at least an inch and a half thick, qualifies as that ignition barrier. Cellulose does not, however, qualify as that material. There's other foam just like in the unvented attic, that are exempt from that requirement by virtue of their formulation and their testing.
All the requirements for the foam insulation and the ductwork are in multiple sections of the code. The mechanical section, Section 1601.3 and also foam insulation in R316.5.3 and there's flame spread indexes, smoke-development indexes; pretty much all Class 1 foams meet those first two requirements. And again, if I've got the exposed foam, no attic storage or occupancy, similar to the sealed attic space, and then lastly, the foam is protected by that ignition barrier or the foam itself is tested to not require it.
Advantages and disadvantages of this approach. It's low-cost in simple plans. It's easy to execute with no changes to the building enclosure except for where we're placing the ducts. Minimal plan coordination, meaning that it's pretty much as flexible as the unvented attic space in locating my registers, and just using the attic floor plane for the distribution allows me access to pretty much anywhere in the floor plan to put a supply register if one is needed. It does require HVAC design coordination insofar as the system needs to be low- profile and designed and installed on the floor of the attic, which is not typical standard practice. And there's no provision for the air handler within that space, as well.
OK, I've just got two more slides that I'm just going to go through really quick. And this one is just kind of relative energy performance of the different approaches, and some of the different approaches we talked about. I'll point out that most of this is really based on insulation levels that we'd see in the '09 IECC, the exception being in the improved benchmark model, which is the second line down. We're using the air sealing for the 2012 IEC levels, which is the 3 ACH50, which is a lot tighter. And the way you can see, if you look at the performance numbers here, we're seeing a lot of equivalency in terms of how far we can get in terms of performance. So any duct system that is truly inside conditioned space is going to get to a performance level right around, you know, the 13-, 14 -, 15-percent level. What we're seeing is that with the buried encapsulated ducts, we can get to the same place. With the unvented attic space, we can get close to that when we have a shallow roof pitch, because we're not increasing the envelope area by much. When we get to the larger roof pitch, we're increasing the envelope area more so our energy use goes up. One thing important we should point out: go all the way to the top of this chart by going to the improved benchmark. What we're doing is we're improving our air sealing to 3 ACH50 over 6, or 5 or 6 ACH50, and getting a really big bounce. What that tells us all is that thermal envelope and air sealing is really the most critical place in all this discussion, so that's the first place we should go. No matter how fast your boat is, if it's going to leak, it's going to sink. So button it up first. Get those loads low, and then we'll get to the efficient mechanical systems after that.
There are some -- I've went through most of this already, in terms of applicable code sections. I believe that this information, this slide deck will be available via PDF later on, so I won't repeat this stuff. And -- I thought I had one more slide ... but I don't. So that's my last slide; thank-you very much. I'm sorry if that seemed a rush but there's a lot of stuff here, I realized, that I had to go through.
Well, that generated a lot of good questions. Let me read out a few I've already answered and throw out a couple that I have not yet answered. We have a few minutes left. So thanks everyone for sending in a lot of great questions. Let me read a couple ... Let's see, so ... We heard from several of you who asked when the slides will available. DOE is planning to make them available as Bill just said, and also, it may be more equally important in the fact that this webinar will be recorded and then posted online, so you can get sort of the dialogue piece of it, as well. What else ... Bill, have -- has any of the testing take -- looked at the pressure levels in the plenum truss area, as measured during a blower door test? So the attendee is wondering how successfully air sealed that space might be, and how pressure levels might look during a blower door test of that plenum truss area.
Yes, we have. When it's done successfully, there's zero pascal difference between the interior space and the plenum space. If not done successfully, you will get a pressure difference, so it's a matter of execution at that point. But we have done it on multiple occasions where the difference was zero.
OK. We had a couple questions about the DOE Challenge Home spec and what we leads to spec, what does not, so there are a couple questions about return air ducts being located in unconditioned spaces that would not lead to spec. So from a design standpoint, we see a lot of HVAC design using like a central hard-ducted return located in conditioned space, possibly with the use of transfer grills or jumper ducts to still get return air pathways back to the unit. Obviously a ductless system is exempt from ducts being in conditioned space, so those are a couple systems that we see fairly often. Bill, are you familiar with the quanda effect on air flow, as air leaves a supply air register near a celing? If so, I'll queue up another question, or else I'll do my best to answer it.
Well, that's, we're kind of getting into the technical weeds now. Yea, that's a laminer flow issue. And we looked at that a long time ago using computational fluid dynamics. I'm guessing that there's a more specific component to that question, though.
I'm trying to find the question -- it was regarding whether that was considered in the air flow recommendations ... still looking for it ... Have you considered the impact of the quanda effect on throw along the ceiling line, so, this probably was when you were looking at the dropped soffit application, or design approach.
Umm, yea, we -- I mean, we looked at -- we could have a dropped soffit with essentially a high wall register, or it could be a ceiling register with a directional register on it. We've looked at air flows before. I mean, I know IBICUS has done a bunch of work with smoke chambers and that sort of thing, looking at the effects of register locations relative to different planes and different interactions. But we haven't found anything that would cause any of the recommendations that we went through in terms of these locations to alter.
Is it safe to say, with these low-load homes, that these houses will eventually end up being, we're not moving around 2,000 CFMs of air. We're moving around half that amount, if that much, so good duct design, low duct leakage levels, and diffuser selection, to make sure the air circulates out into the room, are all sort of integral to the process.
Yea, absolutely true. I mean, we're getting down to 8 CFM for fairly significantly sized houses. So when you're down to those kinds of low loads -- and then, you know, with multiple speed equipment on top of it, so that it's only 800 CFM, but it's on high, and you know, that doesn't happen that often, you know, if they're designed correctly. So, if we're at 500 or 600 CFM in a 2,000-plus square-foot house, you know, these are pretty low air flows. So you've really got to be careful about the placement of your registers, to make sure that you're going to get good comfort. The other side of that is that the better envelope, tightly sealed insulation levels, good windows in particular, diminish the negative effect of not getting the air flows exactly right.
Thanks. Just a couple minutes left here, so I'm going to throw one more question your way, Bill. We talked about -- while I do that, we're going to take the presenter reins back from you and just show a couple last slides to point our attendees toward. So Bill, the question is sort of just a general one on the issue of cost. Can you comment on the cost across the different options for a typical home? So I know you have some estimate data on that question that you can probably refer to.
Sure. I'll try to be as quick as I can on that. First answer is it's highly variable. And as I mentioned a few times, looking at different options, can be cost-effective in simple plans. And, you know, that's true across the board. If I'm going to take the typical, let's say a 2,400-square-foot house and compare those across the board, the unvented attic one is still going to be the most expensive. We've costed it out multiple ways using means, we've costed it out looking at basically surveying builders. We've costed it out just talking to vendors. And for that kind of a house, it's probably a -- it's basically about a 40 cents per board foot for open-cell foam installed, and about a dollar a board foot for closed-cell foam. So if you do the math on the size of the house, it works up to about, you know, 4 to 6,000 dollars, probably, in that range. The buried duct approach if you're looking at the encapsulated ducts, it's closed-cell foam again, a dollar a board foot for the material applied. Typical duct layout on that same house shouldn't be more than 600 or 800 square feet of coverage. So you know, maybe 1,000 dollars. For the dropped soffit approach, again, it depends on how complicated or how simple the house is. The plenum truss is -- so I would say, that the dropped soffit and/or the buried duct approach is going to be equivalent in terms of least expensive. Next up the ladder is going to be the truss approach. Probably the least expensive -- because you don't have to do really anything more -- is the ducts in the floor truss. And if you're doing an unvented crawl space or basement anyway, those are really zero cost options.
OK, nice job summarizing that quickly, Bill, thanks. I'm going to go just as quickly. The slide you should be seeing now lists just a few resources we want the group to be aware of. It's a website, obviously, and then there's an events tab there which lists all sorts of upcoming events, whether it's an in-person training, which happen with our training partners all over the country, these technical training webinars, which we have one of next week on Wednesday, and they'll be recurring about two per month for the remainder of the year on all kinds of technical topics like this one. And then conference presentations, where we might be at an industry conference, are also listed. The website has a partner locator for our builder and verifier partners, there's the program specs, and coming soon, you'll have webinar recordings from events like today.
So with that, we're going to wrap it up. On behalf of DOE, and especially Bill Zoeller, of Steven Winters, we want to thank everybody for attending, and spending part of your day with us. And please get in touch with us via email or via the website with any further questions. Thank-you.