Most of us are aware that windows provide us with natural light, ventilation, and a view. But windows still remain the least understood building design component, even though many homeowners place much importance on the functioning and energy efficiency of their windows.
Windows transmit not only sunlight but also both indoor heat and solar heat through the building envelope. That’s why they may account for major heat losses in winter as well as major solar heat gains in summer. Since the energy crises of the 1970s, however, manufacturers have improved window design to allow less unwanted heat transmission during both hot and cold seasons. Many energy-conscious consumers, except those in the warmest climates, prefer insulated or double-paned glass windows.
A window’s heat transmittance is measured by U-factor. A smaller U-factor pro-vides more insulating value than a larger one. The smaller the number, the better. With today’s technology, a window is considered energy efficient if its U-factor is less than 0.40. To achieve this energy-efficiency standard, the glass is coated with a very thin layer of material that is engineered to transmit or reject certain frequencies of radiation. This coated glass is called low-emissivity (low-e) glass.
Glass’s transmittance is measured by solar heat gain coefficient (SHGC), which is a decimal number less than one. A number of 0.60 means that 60 percent of the solar radiation passes into the house and 40 percent is rejected back into the environment.
Passive solar heating requires a high SHGC—in other words, a window that lets solar radiation pass into the space.
Quite often passive solar homes are built using glass that rejects solar energy (low SHGC). This can be a costly mistake. When selecting the glass, here are some general rules of thumb you can follow:
Typically, the low U-value windows also reject most solar gains (low SHGC). Therefore, it may be difficult to buy a low U-value window with a high SHGC. The right choice is dependent upon the climate.
As you can see, selecting windows can be complicated. That’s why it’s best to have an experienced solar design professional use a simulation tool to determine the best windows to use in your home’s climate.