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What to look for in new windows

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).

Window Features

Several features contribute to the ENERGY STAR® and NFRC ratings, including:

  • Multiple glazings. Generally, the more panes of glass (or glazings) the better the insulating quality of the window. Double-paned windows are the standard in new windows; triple-panes in certain applications (large or fixed windows, for example) can improve thermal performance.
  • Gas filling. The space between the glazings can be filled with inert gases (such as argon or krypton) that are better insulators than air.
  • Low-e coatings. Adding a metallic coating to the glass layers can lower the window’s ability to transfer heat.
  • Insulating spacer. The material that separates the panes of glass can also conduct heat. Wood or other insulating material is better than metal.

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.

  • Solid steel and aluminum . Although durable, these materials are poor insulators and have higher rates of expansion and contraction, making weather-stripping and caulking joints more prone to failure.
  • Wood . A traditional material, wood is a good insulator and has much less expansion and contraction than other materials. However, wood is susceptible to moisture damage and requires more maintenance.
  • Vinyl . Nearly maintenance free and similar to wood in insulating value, vinyl has higher expansion and contraction than wood. It can also be subject to sun damage, peeling, warping, and discoloration.
  • Fiberglass . Offers high insulating, lower expansion, and contraction.
  • Wood clad . A wood frame covered with metal or vinyl offers good insulation and low maintenance, with lower expansion and contraction.

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.