Windows That Work
Paul FisettePaying attention to details can save you energy, hassles and money. When it comes to buying windows, we can be awfully shallow. Whether for a new home or a remodeling project, we usually select our windows - casement, awning, double-hung or fixed glass - for their looks, not because they provide a tighter seal or the best natural ventilation. Convenience is important, too, as demonstrated by the fact that an entire industry has been built around the tilt-out window that allows us to clean the glass without climbing a ladder. Then there's the cost: We're mesmerized by the price at the pump when it comes to windows. Sometimes shocked by the expense of top-of-the-line units, we fail to take into account performance and cost effectiveness. In other words, we buy cheap. That may turn out to be the most expensive option. The cost of a window really depends on its durability and the amount of energy that is pumped through it year after year. Energy-efficient windows save money each and every month, and they can lower the overall cost of your new house by allowing you to install a smaller heating and cooling system. Buying windows doesn't have to be an either/or proposition. With a little thought, you can have everything you want - looks, high performance and a price that won't bankrupt you. But it will take some effort, some time and, most of all, some patience to sort through everything that's on the market these days. A Matter of Energy If we could see energy loss the way we see color, energy performance would top our wish list of window options. Windows are thermal holes. Most are 10 times less energy efficient than the wall area next to them, and an average home may lose 30 percent of its heat or air-conditioning energy through them. Fortunately, window technology is improving rapidly. There are even some cases where new windows can be net energy gainers. Selecting energy-efficient windows is cost effective in any climate, with payback periods for the best of them ranging from two to 10 years. Before we take a look at specific choices, let's talk a bit about the thermodynamics of windows and why the details of their construction are so important. Heat is lost and gained through windows by the forces of conduction, convection, radiation and air leakage. The transfer of heat by conduction, convection and radiation is expressed in U-values. An R-value is the mathematical inverse of U; think of U as flow and R as resistance to flow. Air leakage is expressed separately as cubic feet of air that leaks through a square foot of window area per minute. Conduction is the movement of heat through a solid material. Heat flows through the glass, spacers (metal strips that separate the panes of glass) and frame of a window. Interrupt that pathway with a less conductive material and you impede the flow of heat. Less conductive materials, like insulation, do this by trapping dead air between the solid fibers of insulation. Multiple-pane windows do this by trapping low-conductance gas like argon and krypton in the space between the glass. Thermally resistant spacers and window frames reduce conduction too. Convection is another way that heat moves through windows. In a cold climate, heated indoor air rubs against the interior surface of the window glass. The air is cooled, becomes denser and drops to the floor. As this stream of air moves to the floor, more warm air rushes to take its place at the glass surface. The cycle, called a convective loop, is self-perpetuating. The cold glass surface methodically strips heat from the indoor air. Sitting on a sofa, you may recognize this convective movement as a cold draft and raise the thermostat. Unfortunately, each single-degree increase in a thermostat setting increases energy use by 2 percent. But there's another solution: Raise the temperature of the glass. By selecting multiple glazings with low-conductance gas fillings, warm-edge spacers and thermally resistant frames, you can raise the inboard glass temperatures, slow convection and improve comfort. Radiant transfer is the movement of heat from a warmer body to a cooler body through energy waves. A good absorber is a good emitter. Most wood stoves are black for a good reason: Black absorbs and emits radiation best. You can feel a stove's radiant heat on your face across a room. In turn, your face feels cool when it radiates (emits) its heat to a cold sheet of window glass. This uncomfortable sensation, like convection, persuades you to boost the thermostat. Clear glass absorbs heat and passes it outdoors. But absorption and emission potentials of glass can be greatly reduced by placing coatings on the glass that reflect specific wavelengths of energy. These low-E coatings allow less long-wave heat energy to pass through. In cold climates, more heat stays in the house. In hot climates, the heat stays outdoors. Low-E coatings improve the insulating value of a window by about the same amount as adding an additional pane of glass. So a double-glazed low-E window works as well as a triple-glazed clear window. Air leakage can siphon half of an average home's heating and cooling energy to the outdoors. Air leakage through and around windows is responsible for much of this loss. Well-designed windows have durable weather stripping and high-quality closing devices that effectively block air leakage. Hinged windows, like casements and awnings, clamp much more tightly against weather stripping than do sliding and double-hung windows. But well-made double-hungs are acceptable. Air leakage is also affected by how well the individual pieces of the window unit are joined together. Glass-to-frame, frame-to-frame and sash-to-frame connections must be tight. Values for air leakage are listed on a window's technical specifications as cubic feet per minute per square foot of window. Look for windows with certified air-leakage rates of less than 0.30 cfm/foot. The lowest values are best. Letting Energy In Well-designed windows block the flow of energy from our conditioned indoor environment. But we don't want to block our entire supply of free solar energy. In a cold climate we welcome the sun's heat and light most of the time, and once we capture the heat we don't want to give it up. In a warm climate we don't want the heat, but we do want the light. Advanced window technology lets us have it both ways. Less than half of the sun's energy is visible. Longer wavelengths, beyond the red part of the visible spectrum, are heat, or infrared. Shorter wavelengths of energy, beyond purple, are ultraviolet, or UV. When the sun's energy strikes a window, visible light, heat and ultraviolet radiation are either reflected, absorbed or transmitted into the building. Today's high-tech windows are designed to select the correct mix of heat and light for a given climate and exposure. We choose the blend of glass coatings, gas fillings, glass spacers and window frames that work best in our application. Here are the choices you have when selecting windows: Glazing: Forty years ago the choice was simple: Use single-pane clear glass with a storm window. As recently as 1980, 50 percent of the windows sold were single-glazed. Today, more than 90 percent are at least double-glazed and half have low-E coatings. The added cost for low-E coatings and low-conductance gas fillings is only a few dollars per square foot, about 5 percent of the window's overall cost. It's a no-brainer. These improvements boost energy efficiency by nearly 100 percent over clear glass, reduce condensation in cold climates and cut down on fabric fading. Glass coatings are designed to select specific wavelengths of energy. They can block UV, heat, light or any combination of the three. Windows with high visible transmission, or VT, are easy to see through and admit natural daylight. Besides having a nice view, you save energy because you use less artificial light. Some tints and coatings that block heat also reduce visible transmission, so be careful here. The VT of a window is listed in manufacturers' literature as a comparison to the amount of visible light that would otherwise pass through an open hole in the wall. (VT is sometimes expressed as a whole-window value, including the effect of the frame - pretty silly since you can't see through the frame. What is important is our ability to see through the glass. (Be sure you get the VT of the glass, not the entire unit.) The VT range in residential windows extends from a shady 15 percent for some tinted glass, up to 90 percent for clear glass. Glass with VT values above 60 percent look clear. Any value below 50 percent begins to look dark and/or reflective. People have different perceptions of what is clear and what has a tint of color, especially when you look through glass at an angle. The best advice is to look at a sample of glass and judge for yourself before you order the window. Look through the glass outdoors, not through the showroom window. Hold the glass at different angles. If your supplier says they can't show you a sample of glass, you are shopping at the wrong dealer. Manufacturers have long used the term shading coefficient, or SC, to describe how much solar heat is transmitted by their glazing systems. A totally opaque unit scores zero and a single pane of clear glass scores 1.0 on this comparative scale. A clear double-pane window scores 0.84 because it allows 84 percent as much heat to pass as a single pane of glass. However, solar heat gain coefficient, or SHGC, is the new and more accurate tool being adopted to describe solar heat gain. SHGC is the fraction of available solar heat that successfully passes through a window unit. It too uses a scale of zero (for none) to 1.0 (for 100 percent of available solar heat). The key difference is that SHGC looks at a percentage of available solar heat rather than looking at a percentage of what comes through a single pane of glass. It considers various sun angles and the shading effect of the window frame. As a result it is about 15 percent lower than SC values. Glass coatings are formulated to select specific wavelengths of energy. It is possible to have a glass coating that blocks long-wave heat energy (low SHGC) while allowing generous amounts of shorter wave light energy (high VT) to enter a home. This formulation is ideal in warm climates. A low SHGC will reduce air conditioning bills more than if you increased the insulative value of your window with an additional pane of glass. An SHGC under 0.40 is recommended for hot climates. In cold climates, you want both high visibility and high solar heat gain. An SHGC of 0.55 and above is recommended in the chilly north. In swing climates like Washington, D.C., choosing an SHGC between 0.40 and 0.55 is reasonable because there is a trade-off between cooling and heating loads. Windows that block ultraviolet radiation reduce fabric fading. Choose windows that provide high UV protection. Expect to find windows off-the-shelf that block more than 75 percent of UV energy. Window manufacturers sometimes boast of R-8 (U-0.125) values. Be very careful. This may only be the value at the center of the glass. Don't settle for high glass values. Look for whole-window values of U-0.33 or better. Windows with low U-values are widely available in all styles. Some manufacturers stretch low-E, coated plastic film within the gas-filled airspace of double-glazed units to provide an effective third or fourth pane of glass. The weight of these windows is comparable to double-glazing, and the true overall window performance is boosted to levels above R-6 for some. These units are pricey, but these high-tech versions can be more energy efficient than walls in very cold climates. The R-value is lower than a typical wall, but if the triple-glazed units are designed with a high SHGC, they can be net energy gainers in some designs. Spacers: If you've lived in a cold climate, you've seen condensation and even frost on windows. When warm indoor air is cooled below its dew point, liquid water is squeezed from the air and condensation collects on the cold surface. Condensation typically develops around the edges of window glass. The edge is where most double-paned glazing is held apart by aluminum spacers. Aluminum spacers are highly conductive, so the coldest part of a glazed unit is around its edges. Moist conditions support the growth of mold, decay and failure of finishes, and condensation affects durability and comfort. It is the top reason for window-related callbacks of contractors. Warming the edges reduces the chance for condensation to form. The material and shape of the material used to make the spacer can substantially affect the rate that heat travels through a window's edge. Many window makers now offer warm-edge spacers as standard fare. Conventional aluminum spacers are not acceptable. The best windows use less conductive materials like thin stainless steel, plastic, foam and rubber. Warm-edge spacers can improve the U-value of an entire window unit by 10 percent. More importantly, condensation is reduced. These spacers boost the edge temperature by about 5¡ F. There are many trademarked versions like Swiggle Seal, Super Spacer and PPG Intercept. What's important is that the window you order has a warm-edge spacer system. And if you are concerned that the argon gas will leak out of the window, all indications are that a properly constructed seal will easily last 20 years. Check the warranty. Frames: Far and away, the most popular and widely available window frames are wood and hollow vinyl. Aluminum runs a distant third. Alternative materials such as wood-resin composites, fiberglass, PVC foam and insulated vinyl are slowly gaining speed, but they remain a tiny part of the market. Some 53 million residential windows were sold in 2001, of which 42 percent were wood (including vinyl- and aluminum-clad), 44 percent were vinyl, 14 percent were aluminum and 1 percent were made from some other material. Wood dominates new construction, holding a 49 to 35 percent market edge over vinyl. Vinyl has gained new construction share in recent years, however, and it holds a 52 to 34 percent lead in the remodeling and replacement market. Durability and performance are the most important issues for builders and homeowners, and vinyl has made strides because of that. About 25 percent of a window's area is represented by its frame. So the frame material should be thermally non-conductive. For the most part, wood and vinyl perform equally well. Aluminum frames are typically poor energy performers. Connections where the frame is held together must be tightly sealed, and top-quality hardware and weather stripping should be thoughtfully fastened around the sash opening to limit air leakage. Look closely at this detail. Durability is important. Weather stripping needs to seal tightly after many hundreds of window closings, rain wettings, sun-dryings and winter freezes. Inexpensive, flimsy plastic, metal or brush-like materials don't cut it. High-quality compressible gaskets like those used to seal car doors are best. Closures must clinch windows tight. Look carefully at these components and ask your architect or builder about a particular brand's track record. Verified energy performance: Until recently, buying a window was a little bit like buying a mattress. Every manufacturer used its own set of standards to promise performance. Nowadays, though, there are standard measurements: U- and R-values. But what does the advertised R-value mean? And does it mean the same for all windows? No. Some manufacturers determine R-value by measuring conductance at a single point in the center of the glass and do not count heat transferred through the frame or metal spacers at the edges of the window. They do not account for the air that leaks around the sash. Nor do they measure radiant loss emitted from the entire window unit. Others honestly report whole-window values. So in 1989 the National Fenestration Rating Council was formed to level the playing field in the window industry (fenestration is a fancy word for windows and doors). The council's mission is to develop a national energy-performance rating system for windows and doors. All NFRC-rated windows are tested using a reliable, standard procedure that measures energy transfer through the entire window unit. Things like U-values, SHGC, visible light transmittance values and air leakage rates are listed on certified windows. When consumers see an NFRC label on a window they are considering, they can be sure that they have a reliable tool they can use to compare. The NFRC rating system works this way: Window manufacturers who want to be certified hire a lab certified by the NFRC. The lab simulates the thermal performance of the windows with computers. Entire unit performance - including the frame, spacer and glass - is measured. Windows with the highest and lowest simulated U-values in each product line (like casements or double-hungs) are physically tested to verify the computer simulation. An independent agency licensed by the NFRC reviews the computer simulations and randomly pulls window units from the factory floor as test samples. Test values must fall within 10 percent of the computer predictions for a product line to be validated. If the results fall outside the acceptable range, the product line fails certification and does not get an NFRC label. Certified products have temporary and permanent markings. The big, temporary label is placed in a visible spot on the window. A small, permanent NFRC serial code is etched on an inconspicuous part like a spacer or metal strip. Permanent labels are useful. Potential buyers of older homes often ask: "What kind of windows are these?" A call to the NFRC will provide the brand and rating for the unit. Labels provide builders, designers, code officials and consumers with information needed to verify code compliance and a reliable level of performance. Paul Fisette, a member of Smart HomeOwner's editorial advisory board, is director of the Building Materials Technology and Management Program at the University of Massachusetts in Amherst, Mass.