A wood-frame window with insulated window glazing. | Photo courtesy of ©iStockphoto/chandlerphoto
Windows come in a number of different frame and glazing types. By combining an energy-efficient frame choice with a glazing type tailored to your climate and application, you can customize each of your home’s windows.
Types of Window Frames
Improving the thermal resistance of the frame can contribute to a window's overall energy efficiency, particularly its U-factor. There are advantages and disadvantages to all types of frame materials, but vinyl, wood, fiberglass, and some composite frame materials provide greater thermal resistance than metal.
Aluminum or Metal Frames
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 and the U-factor, 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. 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 usually 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 also expand and contract in response to weather conditions. Wood frames also require regular maintenance, although aluminum or vinyl cladding reduces maintenance requirements.
Types of Window Glazing or Glass
In addition to choosing a frame type, you will need to consider what type of glazing or glass you should use to improve your home's energy efficiency. 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.
To improve the thermal performance of windows with insulated glazing, some manufacturers fill the space between the panes with inert gas -- commonly argon or krypton -- that has a higher resistance to heat flow than air.
Heat-absorbing window glazing contains special tints that change the color of the glass. Tinted glass absorbs a large fraction of the incoming solar radiation through a window, reducing the solar heat gain coefficient (SHGC), visible transmittance (VT), and glare.
Some heat, however, continues to pass through tinted windows by conduction and re-radiation, so the tint doesn't lower a window's U-factor. Inner layers of clear glass or spectrally selective coatings can be applied on insulated glazing to help reduce these types of heat transfer.
The most common gray- and bronze-tinted windows are not spectrally selective, and reduce the penetration of both light and heat. Blue- and green-tinted windows offer greater penetration of visible light and slightly reduced heat transfer compared with other colors of tinted glass. In hot climates, black-tinted glass should be avoided because it absorbs more light than heat. Tinted, heat-absorbing glass reflects only a small percentage of light, so it does not have the mirror-like appearance of reflective glass. Note that when windows transmit less than 70% of visible light, indoor plants can die or grow more slowly.
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 glazing or glass control heat transfer through windows with 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 different types of low-e coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain. A low-e coating can also reduce a window's VT unless you use one that's spectrally selective.
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.
Reflective coatings on window glazing or glass reduce the transmission of solar radiation, blocking more light than heat. Therefore, they greatly reduce a window's VT and glare, but they also reduce a window's SHGC. Reflective coatings usually consist of thin, metallic layers, and come in a variety of colors, including silver, gold, and bronze. Reflective window glazing is commonly used in hot climates to control solar heat gain. The reduced cooling energy demands can be offset by the need for additional electrical lighting, so reflective glass is used mostly for special applications.
Spectrally Selective Coatings
A special type of low-e coating is spectrally selective, filtering out 40% to 70% of the heat normally transmitted through insulated window glass or glazing while allowing the full amount of light transmission. Spectrally selective coatings are optically designed to reflect particular wavelengths, but remain transparent to others. Such coatings are commonly used to reflect the infrared (heat) portion of the solar spectrum while admitting more visible light. They help create a window with a low U-factor and SHGC but a high VT.
Spectrally selective coatings can be applied on various types of tinted glass to produce "customized" glazing systems capable of either increasing or decreasing solar gains according to the aesthetic and climatic effects desired. Computer simulations have shown that advanced window glazing with spectrally selective coatings can reduce the electric space cooling requirements of new homes in hot climates by more than 40%.