Improving the energy efficiency of home windows can vary from very inexpensive to quite pricey. Similarly, the cost-effectiveness of window improvements can vary widely. Use of storm windows and window treatments can provide moderate energy savings, while replacing old windows with new ones can provide larger energy savings but at a much greater cost.
Storm Windows and Treatments
In a study of Chicago homes conducted by the U.S. Department of Energy, low emissivity (low-E) storm windows were shown to reduce heating loads by 13-20% and had simple payback periods of 5-10 years. Savings were greatest when storm windows were added to single paned windows. Considering about 30% of Colorado homes have single pane windows, there is plentiful opportunity to improve the energy efficiency of windows by adding storms. Even plastic films added to windows over the winter can make a dent in your energy use.
Window treatments can also go a long way toward reducing energy use. Some treatments such as awnings and blinds are primarily intended to reduce solar heat gain through windows. Considering Colorado is a heating-dominated climate (meaning most of our home energy is used for space heating), these treatments will have limited year-round impact. Treatments such as insulated drapes, shutters, and cellular shades can both prevent unwanted heat gain and can keep heat in the home if properly installed. Cellular shades have shown particular promise in this respect.
Unless you are replacing single pane windows, replacing old windows with new ones typically shouldn’t be done for energy purposes alone. This is because the cost of new windows is very high compared to the potential financial savings associated with reducing energy use. That said, if you’re in the market for new windows, there are three main energy-related characteristics to pay attention to:
- U-factor – this is the inverse of R-value, meaning that the lower the U-factor the better insulation is provided by the window. The most energy efficient, triple pane windows on the market generally have U-factors of 0.1. Low U-factors are important in Colorado because of our cold winters and hot summers.
- Solar Heat Gain Coefficient – this is the amount of solar radiation let in through a window. If you already have an awning or shade trees to block out summer sun, you may want to purchase windows with a high SHGC in order to warm rooms in winter. If, on the other hand, you want to avoid overheating a room, look for windows with a low SHGC.
- Visible Transmittance – this refers to the amount of visible light let in by the window. If you are considering windows for a room with many reflective surfaces, you may want to look for a low VT number.
Other energy considerations for new windows include framing material and glazing features. Fiberglass and vinyl frames have the best thermal resistance of all frame types, with aluminum and metal windows performing worst. In terms of glazing features, both gas fills and low-E coatings can enhance thermal resistance. Certain low-E coatings can also reduce the solar heat gain coefficient unless you find coatings that are spectrally selective. If you are adding a low-E coating to a window in Colorado, the coating should be added to the inside of the window pane in order to reduce heat loss in winter.