Many design features and technologies make windows more energy efficient and improve the durability, aesthetics, and functionality. When selecting new windows, consider the frame materials, the glazing or glass features, gas fills and spacers, and the type of operation. All of these factors will influence the overall energy-related properties of the window. To assess and compare the overall window properties, look for an NFRC label.
Frames and Sash
Although very strong, light, and almost maintenance free, metal or aluminum window frames conduct heat very rapidly, which makes metal a very poor insulating material.
To reduce heat flow, metal frames should have a thermal break -- an insulating plastic strip placed between the inside and outside of the frame and sash.
Composite window frames consist of composite wood products, such as particleboard and laminated strand lumber, and some are mixed with polymer plastics. These composites are very stable, they have the same or better structural and thermal properties as conventional wood, and they have better moisture and decay resistance.
Fiberglass window frames are dimensionally stable and have air cavities that can be filled with insulation, giving them superior thermal performance compared to wood or uninsulated vinyl.
Vinyl window frames are made of polyvinyl chloride (PVC) with ultraviolet light (UV) stabilizers to keep sunlight from breaking down the material. Vinyl window frames do not require painting and have good moisture resistance. The hollow cavities of vinyl frames can be filled with insulation, which makes them thermally superior to standard vinyl and wood frames.
Wood window frames insulate relatively well, but they require regular maintenance, although aluminum or vinyl cladding reduces maintenance requirements. Metal clad wood frames may have slightly lower thermal performance.
Glazing or Glass
In most windows the most important decision regarding energy efficiency is the selection of the glazing. Based on various window design factors such as window orientation, climate, building design, etc., you may even want to choose different types of glazing for different windows throughout your home.
For more detailed information about glazing options visit the Efficient Windows Collaborative to learn about the specific properties and efficiency of different glazing options.
While single glazing is common in older buildings, virtually all new efficient buildings use double or triple glazing units. The "insulating glazing units", or IGUS, are available in a wide range of properties based on the type of glass selected, the coatings on the glass, the gas used to fill the space between the panes, and the spacers that keep the glazing separated. Below are some of the most common coatings and technologies you may find when shopping for windows:
Insulated window glazing refers to windows with two or more panes of glass. To insulate the window, the glass panes are spaced apart and hermetically sealed, leaving an insulating air space. Insulated window glazing primarily lowers the U-factor, but it also lowers the SHGC.
Low-emissivity (low-e) coatings on glass control heat transfer within the insulated glazing. Windows manufactured with low-e coatings typically cost about 10% to 15% more than regular windows, but they reduce energy loss by as much as 30% to 50%.
A low-e coating is a microscopically thin, virtually invisible, metal or metallic oxide layer deposited directly on the surface of one or more of the panes of glass. The low-e coating lowers the U-factor of the window, and can manage the daylight transmittance as well as the solar heat gain through the glazing system. Different types of low-e coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain, and and they can also be tuned to control the amount of visible daylight transmitted.
Although low-e coatings are usually applied during manufacturing, some are available for do-it-yourselfers. These films are inexpensive compared to total window replacements, last 10 to 15 years without peeling, save energy, reduce fabric fading, and increase comfort.
In those climate where cooling loads dominate, you will want a glazing that provides daylight and view but transmits as little of the sun's non-visible infrared radiation as possible, Some low-E coatings are designed to be spectrally selective, filtering out 40% to 70% of the heat normally transmitted through insulated window glass or glazing while allowing the full amount of daylight.
Gas Fills and Spacers
In order to minimize heat transfer between the interior and exterior of the window, the space between glazing layers, usually about 1/2", is filled with argon or krypton gas; both are inert, non-toxic, clear, and odorless.
Argon is most commonly used because it is inexpensive and performs well in the typical 1/2" space. Krypton can be used when the space in thinner than usual - usually about ¼ inch. It has better thermal performance than argon but is also more costly.
Spacers with associated sealants are used to keep the layers of glazing the correct distance apart. In addition, they provide accommodation for thermal expansion and pressure differences, while also preventing moisture and gas leaks.
A variety of spacers are available with different impacts on the window U-factor. Look for "warm edge" spacers which are designed to lower the window U-factor and reduce condensation at the edge of the window. See the Efficient Windows Collaborative for more information on the different spacer types.
Operating Types
Another important consideration is how the windows operate. Since all operable windows may have undesired air leakage which will decrease your home's energy efficiency, the choice of operator type can impact overall energy use. Air leakage rates will vary with the type and quality of the weatherstripping and seals, and will generally increase over time due to operational use. Traditional operating types include:
- Awning. Hinged at the top and open outward. Because the sash closes by pressing against the frame, they generally have lower air leakage rates than sliding windows.
- Hopper. Hinged at the bottom and open inward. Like both awning and casement, they generally have lower air leakage rates because the sash closes by pressing against the frame.
- Single- and double-sliding. Both sashes slide horizontally in a double-sliding window. Only one sash slides in a single-sliding window. Like single- and double-hung windows, they generally have higher air leakage rates than projecting or hinged windows.
- Fixed. Fixed panes that don't open. When installed properly they're airtight but are not suitable in places where window ventilation and egress is desired.
- Single- and double-hung. Both sashes slide vertically in a double-hung window. Only the bottom sash slides upward in a single-hung window. These sliding windows generally have higher air leakage rates than projecting or hinged windows.
- Casement. Hinged at the sides. Like awning windows, they generally have lower air leakage rates than sliding windows because the sash closes by pressing against the frame.
Links and Sources
- Window Selection Tool
- Climate Zone Recommendations - Efficient Windows Collaborative
- Product Ratings - National Fenestration Rating Council
- Energy Performance Label - National Fenestration Rating Council
- Windows and Daylighting - Lawrence Berkeley National Laboratory
- Window Technologies: Frame Types - Efficient Windows Collaborative
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