Buying windows can be confusing. There are multiple options available, including the materials used in the frames, the finishes, the types and quantities of insulating and sealing materials, coatings, and more. But for evaluating energy efficiency, there are some basic things to look for:
The NRFC label
The first thing you should look for is a label from the National Fenestration Rating Council (NFRC). The NFRC is a nonprofit organization that provides consistent energy and performance ratings of windows, doors, and skylights. It evaluates products according to several categories, including:
U-factor: The ability of a window to conduct heat (the inverse of an R-value, used to evaluate products like insulation). U-factor ratings generally fall between 0.20 and 1.20; the lower the number, the better the energy efficiency of the unit. The recommended U-factor for windows is 0.30 or less.
Solar Heat Gain Coefficient: Measures a window’s ability to reduce heat gain in the summer, thus reducing cooling loads. Based on a zero to 1 scale, a lower number will block more sunlight, reducing solar gain. In Minnesota, a good balance of about 0.50 is recommended.
Window options - If new windows are needed, there are several materials and finishes available (above). Check the NFRC label for information about efficiency and performance.
The ENERGY STAR® label
The U.S. Department of Energy and Environmental Protection Agency have developed an ENERGY STAR® designation for products meeting certain energy performance criteria. Since the energy performance of windows, doors, and skylights can vary by climate, product recommendations are given for four climate zones: a mostly heating zone (Northern), two heating and cooling zones (North/Central and South/Central), and a mostly cooling zone (Southern).
Several features contribute to the ENERGY STAR® and NFRC ratings, including:
Sash and frame construction
Important to the air leakage and U-factor of a window, the materials that compose the sash and frame also have an effect on maintenance and durability.
Proper window & door installation is critical to good performance
As with any product, proper installation of doors and windows will provide the best performance. That means suitable insulation and air-sealing between framing and units, and correct flashing on the exterior to prevent water intrusion.
To ensure warranty coverage, manufacturer’s instructions must be carefully followed, as well as applicable building and energy code requirements.
Remember, a gain in efficiency can be quickly negated by substandard installation or lack of attention to detail. In other words, buying windows and doors with high thermal performance and durability won’t do you much good if they are improperly installed.