What is the difference between windows that are used in green homes versus those used in homes that are not green?
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Windows have many aspects that contribute to their “greenness,” from their orientation relative to the sun and prevailing winds, their effectiveness for daylighting, as well as the selection of the window type based on the thermal properties of their frames and glazing.
The location of the windows relative to the movement of the sun throughout the day is the first consideration in designing a green home. As the sun rises in the east and sets in the west, east and south facing windows provide morning and afternoon sun, with the west-facing windows late afternoon sun. As the sun is high in the sky midday facing south, the amount of light and heat can be controlled with overhangs that act as sunshades.
East and west facing windows are more difficult to control as the sun is low and only vertical sunshades can effectively limit the amount of sunlight. Houses that were built before the time of heating and air conditioning typically oriented the rooms of the house to take advantage of the sun, with the breakfast room located to the east, parlors to the south and kitchens at the north.
Their location relative to the prevailing winds will enhance the ventilation of the home with high top lights serving for an escape of heat due to the stack effect.
Daylighting your home through optimal window placement also contributes to a home’s “greenness.” Windows, in conjunction with light shelves, will reflect sunlight onto the ceiling which in turn will reflect light deeper into the space. As incandescent lighting produces as much heat as light, daylighting will not only save the energy cost of lighting but also save on cooling costs in the summer.
A window’s thermal properties are an essential consideration, as it can be the weakest element in the building envelope contributing to heat gain or heat loss. Technical advances of glazing have given us a range of products to select from, and the evaluation of a window’s performance has become a complex topic. A window can be evaluated for its insulation value (U-factor), solar gain (Solar Heat Gain Coefficient, SHGC), and air leakage (AL).
Dual or triple-pane windows, made up of 2 or 3 layers of glass separated by an air space, are very effective insulators. Low-E (emissivity) coatings can drastically cut the amount of heat gain in a room by reflecting the sun’s radiant infrared energy before it enters the house, while letting in visible light. Also, when considering heat transmission, the material of the window frame should be considered, as a metal frame is a better conductor of heat and cold than a vinyl frame. Heat loss or gain can also occur through cracks in the window assembly.
The National Fenestration Rating Council (NFRC) is a very good reference for what window properties are appropriate for your climate. In some climates free heat in the winter might outweigh concerns about overheating in the summer. The website offers a state-by-state guide for window selection. The Window Selection Tool compares average simulated energy costs for your location based on various window types.