Many 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.
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, 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 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 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 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.
Many more glazing types and combinations are covered elsewhere; visit the Efficient Windows Collaborative to learn about the specific properties and efficiency of different glazing options.
Below are some of the 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 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.
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.
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%.
Gas Fills and Spacers
Gas fills between glazing layers minimize heat transfer between the interior and exterior of the window. Argon or krypton are the gases typically used; both are inert, non-toxic, clear, and odorless.
Krypton can be used when the space between glazing layers must be thin—about ¼ inch. It has better thermal performance than argon but is also more costly.
Argon can be used when the spacing can be a bit larger—1/2 inch. Sometimes it is mixed with krypton to keep cost low while increasing thermal performance.
Spacers 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; see the Efficient Windows Collaborative for more information on the different spacer types.