Many home-improvement magazines and contractors tout the benefits of replacing old windows with new energy-efficient models. What they don’t tell you is that window replacement is rarely the most cost-effective, energy-saving strategy. Think about it: You’re upgrading a relatively small area of your home’s exterior by only about two to three points on the R-value scale. Your payback will take many years, even if you figure in tax credits and increases in the cost of energy.
In many cases, you’d be wiser to invest in insulation, weatherstripping and sealants such as caulking and duct mastic — or even in insulating window treatments. In all likelihood, more heat escapes from your home (or, in the summer, enters it) through attic floors, attic hatches, recessed light fixtures, fireplaces and other penetrations in the envelope of your house than through the windows.
By simply sealing the heating and cooling ducts in your home, at a cost of a few hundred dollars, you can probably save more energy than you would by replacing old double-pane windows — or even sound single-pane windows, assuming you have serviceable storm windows.
When replacement is the answer
That said, sometimes old windows need to be replaced for reasons other than saving energy. For example, wood windows that have deteriorated because of water infiltration and rot are prime candidates for replacement. So are windows that no longer operate properly and would be expensive to repair. You may also want to upgrade your windows to make maintenance easier: Today’s new designs open inward so you can clean the exterior glazing from inside your home. You can even order some windows with a self-cleaning coating on the glazing that helps to prevent dirt buildup.
Aesthetics can be a factor in window replacement, too. Many homes of historical note have been marred by the installation of inappropriate window styles. Replacing them with the right style of windows will improve the look and value of the home (photo 1). This upgrade doesn’t have to be wasteful: If existing units are in good condition, take them to a building-materials recycling center.
Whenever you’re replacing old windows, it pays to invest in energy-efficient models. You can choose from two installation approaches: inserts and full-frame replacements. Inserts fit inside the old window frames. Only the window stops and sashes need to be removed during installation. Existing trim, inside and out, is usually not affected. You can install replacement inserts with less labor, less cost and less mess than installing full-frame replacements. Inserts are normally custom-built to the exact sizes of your openings and to match the angle of your existing sill. (You’ll use caulk and molding to hide the small gap between the old window frame and the new insert.) However, installing inserts is an option only if the old window frame is in good shape, has no rot and is square.
If the old window frame has deteriorated or is out of square, or if you want a different style or size of window, you’ll need to install a full-frame replacement. This process typically requires removing the entire existing window, including the casings, frame and exterior trim. Although full-frame replacements involve more labor, cost and disruption, they will allow you to ensure that the area around the window frame — a common location of energy leakage — is well-insulated. When the trim has been removed, you can spray closed-cell insulation between the window frame and the studs — and on any existing fiberglass insulation. You can usually use standard window sizes, but you can also order custom-built models. Another bonus: Unlike installing inserts, installing full-frame replacements does not reduce the glazing area.
Glazing and reflective films
When selecting an efficient window product for your home and climate, first focus on glazing. Efficient windows typically have two layers of glass and are called “double-glazed” (photo 2). The small gap between the glass layers creates a barrier to heat loss by conduction. That barrier may be enhanced with an additional layer of glazing (typically a suspended sheet of plastic film); in this case, the window is called “triple-glazed.” Some windows have two layers of suspended plastic film and three gaps or spaces. The more spaces, the better the barrier will be at stopping heat loss. The gaps between layers of glazing are usually filled with a gas that further reduces heat loss by conduction. Argon or krypton, or a combination, are commonly used gas fills. Krypton has the edge in performance but is more expensive.
In addition to glazing, you will have to decide about reflective films, glazing tints and low-emittance (low-E) coatings when buying an efficient window. Reflective films block much of the radiant energy striking a window, but they block much of the visible light, too. In addition to looking like poor-quality mirrors, they may cause occupants to use more electric lighting to compensate for the loss of natural light.
Tinted glazing is preferable to reflective treatments because it reflects radiant energy and reduces cooling loads without blocking as much of the visible light entering the home. A visual transmittance (VT) of 60 percent (versus 90 percent for clear glass) is common for bronze- or gray-tinted glazing. Spectrally selective tints (usually light blue or light green) are improvements over bronze and gray tints, allowing less solar gain but even more visible light to enter the house.
Low-E coatings and fiberglass
Low-E coatings are more versatile than either tints or reflective films and are virtually invisible. These microscopic layers of metal or metallic oxides suppress radiant heat flow out of the window and can be formulated to allow in varying degrees of solar radiation. In climates where heating is the dominant concern, low-E coatings can be used to prevent radiant heat transfer out of the house but to allow high solar heat gain. In climates where both heating and cooling are required, low-E coatings can reduce radiant heat loss while allowing moderate heat gain. In climates where the dominant concern is cooling, low-E coatings are primarily used to reduce solar heat gain. Other factors, including your window orientation and house design, may affect your choice of coatings.
In addition to glazing and low-E coatings, the material with which the window frame is built will significantly affect efficiency. Insulated vinyl and fiberglass perform better than wood, wood-clad materials and uninsulated vinyl. Aluminum, even with a thermal break, performs worse than all of the above.
To simplify the process of window selection, the Efficient Windows Collaborative (EWC) provides a window-selection tool on its Web site. Choose the nearest city and state to access a list of recommended window types, specifications and projected cost savings as well as the Web site addresses for manufacturers that offer such windows. Although the EWC consists mostly of window manufacturers, the site provides unbiased information on the benefits of energy-efficient windows, descriptions of how they work and recommendations for their selection and use.
To find the definitive rating of a window product you’d like to buy, visit the National Fenestration Rating Council (NFRC). This nonprofit organization administers the only independent rating-and-labeling system for the energy performance of windows, doors and skylights. NFRC labels include ratings for U-factor, solar heat-gain coefficient, visible transmittance, air leakage and condensation resistance.
Terms to Know
Air leakage (AL): A measure of the amount of air that passes through a closed window. The lower the AL rating, the better.
Condensation resistance (CR): a product’s ability to resist forming condensation on its interior surface, measured on a scale of 0 to 100. The higher the CR, the better.
Emissivity: a material’s ability to radiate absorbed energy, measured on a scale of 0 to 1. Aluminum foil has a very low emissivity (0.04), whereas asphalt has a high emissivity (0.93). Clear glass also has a high emissivity (0.92). Low-emittance (low-E) coatings are applied to reduce glass’s ability to emit radiant energy.
R-value: A measure of a product’s resistance to heat flow. A high R-value product has greater resistance to heat flow than one with a low R-value. (Windows’ R-values range from about 0.8 to 5.) R-value is the reciprocal of U-factor (see below).
Solar heat-gain coefficient (SHGC): how much solar radiation a product transmits, measured on a scale of 0 to 1. The higher the SHGC, the more solar radiation (and heat) enters your home.
U-factor: A measure of a product’s ability to prevent heat loss. The lower the U-factor, the better its insulating value and the greater its resistance to heat flow. Windows’ U-factors range from about 0.2 to 1.2.
Visible transmittance (VT): how much visible light comes through a product, measured on a scale of 0 to 1. The higher the VT, the more light enters your home.