Windows: Understanding Energy Efficient Performance

An update on how today’s high-tech windows work and what to look for when you make your next purchase.

by Paul Fisette – © 2003

When we choose windows for a new home or remodeling project, appearance is usually our first consideration. Most often we choose casement, awning, double-hung or fixed glass based on personal taste – not because they provide a tighter seal or the best natural ventilation. Convenience is important too. An entire industry is built around the tilt-out window. Liking a window’s appearance is a fuzzy proposition. But performance and cost are absolute. They are important values deserving thoughtful consideration.

Consumers are mesmerized by price at the pump: the cost of windows off-the-shelf. But cost really depends on durability and the energy dollars pumped through the windows each year. Energy efficient windows save money each and every month. They can lower your mortgaged investment by allowing you to install a smaller and less expensive heating & cooling system. Efficient windows make you feel more comfortable and last longer. Energy conservation and durability are critical components in life-cycle cost. With a little thought, you can have it all. Today, high-performance comes in every style of window.

Energy Matters

I am convinced that if we could see energy-loss like we see color and shape, energy performance would top our wish list of window options. Imagine having a dial that displays energy dollars spent as they pour through our windows!

Windows are thermal holes. Most are 10 times less energy efficient than the wall area they replace. An average home may loose 30% of its heat or air-conditioning energy through its windows. The good news is that window technology is improving by leaps and bounds. There are even some cases where new windows can be net energy gainers. Selecting energy efficient windows is cost effective in any climate. The payback period for selecting energy efficient units ranges from 2 to 10 years. High-tech windows are attractive and getting very easy to choose.

Energy Flow

Individual components of a window work in concert to resist the forces acting upon the entire unit. Heat is lost and gained through the forces of conduction, convection, radiation and air leakage. The transfer of heat through a window by conduction, convection and radiation is expressed with U-values. An R-value is the mathematical inverse of U. So a U-value of 0.5 is the same as an R-value of 2. 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. Touch a hot skillet and you’ll feel heat conducted from the electric coil trough the pan. Heat flows through the glass, spacers (metal strips that separate the panes of glass), and frame of a window in much the same way. Interrupt the 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-glazed windows do this by trapping low-conductance gas like argon and krypton in the space between the panes of 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 more dense, and drops to the floor. As this stream of air drops 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 recognize this convective movement as a cold draft and raise the thermostat. Unfortunately, each 1-degree increase in a thermostat setting, increases energy use by 2%! Instead, raise the temperature of the glass. Choose multiple glazings with low-conductance gas fillings, warm-edge spacers and thermally-resistant frames. They raise 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. Radiant heat loss is more than a perception: Clear glass absorbs heat and dumps it outdoors. Absorption and emission potentials of glass can be greatly reduced by placing coatings on glass that reflect specific wavelengths of energy. Low-“E” coatings emit less long-wave heat energy. In cold climates, more heat stays in the house. In hot climates, the heat stays outdoors. Low-E coatings improve the insulating value of window roughly the same as adding an additional pane of glass to a unit. So a double-glazed low-E window works as good as a triple-glazed clear window.

Air leakage siphons about 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 weatherstripping and high-quality closing devices that effectively block air leakage. Hinged windows like casements and awnings clamp much more tightly against weatherstripping 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 the technical specifications for windows as cubic feet per minute per square foot of window. Look for windows with certified air-leakage rates of less than 0.30 cfm/ft. 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 is heat (infrared). Shorter wavelengths of energy, beyond purple, is ultra violet (UV). When the sun’ s energy strikes a window, visible light, heat and ultraviolet radiation is 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.

Glazing

There are enough glazing systems on the market to boggle one’s mind. In 1960 the choice was simple: use single-pane clear glass with a storm window. As recently as 1980, fifty percent of the windows sold were single-glazed. Today, more than 90% 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 several dollars per square foot, about 5% of the window unit’ s overall cost. It’s a no-brainer. These improvements boost energy efficiency by nearly 100% 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 (VT) are easy to see through. They admit natural daylight too. 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. Duh! I think that most people understand 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 shady15% for some tinted glass up to 90% for clear glass. Glass with VT values above 60% look clear. Any value below 50% begins to look dark and/or reflective. People have very 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. Get another dealer!

Manufacturers have long used the term shading coefficient (SC) to describe how much solar heat is transmitted by each of their glazing systems. A totally opaque unit scores 0 and a single pane of clear glass scores 1 on this comparative scale. A clear double-pane window scores 0.84 because is allows 84% as much heat to pass as a single pane of glass. However, solar heat gain coefficient (SHGC) is the new more accurate tool being adopted to describe solar heat gain. SHGC is the fraction of available incident solar heat that successfully passes through a window unit. It too uses a scale of 0 (for none) to 1 (for 100% of available). 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% 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. SHGC under 0.40 is recommended for hot climates. In cold climates you want both high visibility and high solar heat gain. SHGC of 0.55 and above is recommended in the chilly north. In swing climates like Washington D.C., choosing a SHGC between 0.40 – 0.55 is reasonable because there is a trade-off between cooling and heating loads.

Windows that block ultra-violet radiation reduce fabric fading. Choose windows that provide high UV protection. Expect to find windows off-the-shelf that block more than 75% of the UV energy. Contrary to conventional wisdom, some visible light fades fabric too. Some manufacturers use both the Krochmann Damage Function and UV transmission values to rate a window’s ability to limit fabric fading potential.

Window manufacturers sometimes boast 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”. 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. No surprise. 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. Condensation affects durability and comfort. It is the number 1 reason for window-related callbacks. Warming the edges reduces the chance for condensation to form.

It is virtually impossible to build a window that doesn’t have a thermal bridge. But the material and shape of the material used to make the spacer can substantially effect 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%. But more importantly, condensation is reduced. These spacers boost the edge temperature by around 5 degrees. There are many trademarked versions like Swiggle Stick, Super Spacer, PPG Intercept, Ultra Edge, etc. 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 3rd. There’s a trickle of alternative materials like wood-resin composites, fiberglass, PVC foam and insulated vinyl dripping into the market stream, but the sum total of these offerings is insignificant. More than 47 million residential windows were sold in 1996. And of that total, 46% were wood (including vinyl- and aluminum-clad), 36% were vinyl, 17% were aluminum, and 1% were made from some other material. Wood dominates new construction, holding a 53% to 27% market edge over vinyl. However, vinyl holds a 45% to 40% edge in the remodeling and replacement market. Vinyl is predicted to be new-construction king within the next 2 years. Durability and performance are the most important issues for builders and homeowners. (SEE FIGURES AT END OF ARTICLE)

About 25% 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. Top quality hardware and weatherstripping should be thoughtfully fastened around the sash opening to limit air leakage. Look closely at this detail. Durability is important. Weatherstripping needs to seal tightly after many hundreds of window closings, rain wettings, sun-dryings and winter-freezings. 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. Pick long-time winners. Let others experiment with a new brand.

Aluminum window sales peaked in the early 1980’s, when they owned 60% of the residential window market. They just passed 17%: heading down. Aluminum windows are very durable, requiring little maintenance. However, they are energy siphons. They can be made to perform reasonably well when a thermal break is included as part of the design. But this is a tricky and very costly detail to manufacture.

Wood windows are typically the most expensive windows. Wood frames are either solid wood, aluminum-clad or vinyl-clad. One of the biggest drawbacks to using solid wood windows is maintenance. Wood rots, shrinks, and swells. Paint fails. Solid wood requires frequent and fussy maintenance. On the other hand, well-maintained wood looks good, is stable and can be recolored easily. Consumers favor clad versions because they are the easiest to maintain.

Alan Campbell, president of National Wood Window and Door Association, reports, “More than 90% of the wood windows sold are clad with either aluminum or vinyl.” Campbell thinks that clad windows provide the best of both worlds: a low-maintenance exterior surface with an attractive interior surface that can be painted, stained or left natural-colored. On the down side, if you get sick of the exterior clad color — too bad. When you choose either a solid or clad version, be sure that the manufacturer has treated its wood frames with water repellent preservative (WRP) to improve durability, paint retention and dimensional stability.

Vinyl (polyvinyl chloride or PVC) windows have been around for 35 years. In the early 1980’s vinyl held an anemic 3% of the residential market, but the popularity of vinyl has grown. Last year its share rose to more than 36 percent of all residential windows sold. Vinyl is energy efficient, durable, rot-proof, insect-proof and weather-resistant. It’s made with chemicals that inhibit UV degradation. Vinyl is colored throughout its cross section and requires no painting. The knock on vinyl is it fades, is unpaintable, gets brittle and is thermally unstable (especially dark colors). It expands and contracts more than wood, aluminum, and even the glass it holds. Vinyl frames have the potential for causing increased air leakage over time because of this differential movement. Richard Walker, Technical Director of the American Architectural Manufacturers Association (AAMA), is quick to say, “Vinyl windows are built with this movement in mind and failures have not been recorded to cause concern.” Good advice is: specify light-colored vinyl windows with heat-welded corners.

The pigments that go into paint are almost identical to those that go into vinyl, but vinyl’s color goes all the way through. Walker claims, “A little rub-down with Soft Scrub or one of the products on our [AAMA] list of recommended cleaners will bring vinyl back to its original brilliance.” AAMA has a vinyl profile certification program where more than 2000 vinyl windows are certified for dimensional stability, heat resistance and outdoor weathering. The outdoor weathering is conducted in Florida, Kentucky and Arizona for a 2-year period after which color readings are taken. I tried the “Soft Scrub” test and was impressed with how much brighter aged vinyl got. Not the original color to be sure, but a marked and acceptable improvement was noted.

Fiberglass framed windows are beginning to show up in a few product lines. Fiberglass is extremely strong and, because it is made from glass fibers, the coefficient of expansion for the frames and the glass are the same. Fiberglass must be painted and is more expensive than vinyl. Owens Corning, Andersen and Marvin are 3 major manufacturers who produce fiberglass windows. Owens Corning is the only manufacturer that makes a fiberglass window with insulated frames. But before you get too excited…. the whole-window U-value for a low-E argon-filled casement window carries the same 0.32 rating for both an uninsulated vinyl and an insulated fiberglass unit.

AAMA and NWWDA have worked for more than 2 years to develop one single standard to cover wood, vinyl and aluminum windows. As of April, 1997, a joint AAMA/NWWDA industry standard officially certifies window performance through independent 3rd party inspection. Window units are randomly yanked from the production line of AAMA and NWWDA member companies and tested for water leakage, air leakage, structural wind-load resistance and forced-entry resistance, but not whole-window energy performance. Windows that pass muster get a AAMA/NWWDA label. Look for this certification.

Rules of Thumb For Window Selection


Cold climate1

Mixed climate2

Hot climate




U-value

<0.33 all climates: low U not quite as important in hot climates




VT

>60%

>50%

>50%




SHGC

>0.55

0.40 – 0.55

<0.40




UV protection

>75%

>75%

>75%




Spacer

warm-edge spacers for all climates




Frame

non-conductive frames for all climates




Air Leakage

<0.30 cfm/ft2 for all climates

1. U-values influence heat loss more in cold climates because the difference between indoor and outdoor temperatures are much greater than in hot climates.
2. Consider trade-offs involving comfort and performance in swing climates.

Verified Energy Performance

Until recently, purchasing a window was a little bit like buying a mattress. Every manufacturer used its own set of standards to promise performance. With mattresses you get cushion-firm, chiro-protector, and posturepedic-plus. With windows you’re promised energy performance with U- and R-values. But what does the advertised R-value mean? And does it mean the same for all windows? Hardly! 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 reported whole-window values.

So in 1989 the National Fenestration Rating Council (NFRC) was formed to level the playing field in the window industry (fenestration is a fancy word for windows and doors). NFRC’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, solar heat gain coefficients, visible light transmittance values, and air leakage rates are now (or will be soon) listed on certified windows. When consumers see an NFRC label on the windows they are considering, they can be sure that they have a reliable tool they can use to compare windows.

As of June 1997, NFRC has over 150 participants with more than 30,000 windows, doors and skylights in the program. “This includes all of the major manufacturers like Jeld-Wen, Andersen, Marvin, Pella, Certainteed, Owens Corning, Peachtree, etc.” says NFRC Administrative Director, Susan Douglas. Douglas estimates that there are a couple of thousand window manufacturers in the country, but most are small local companies responsible for a minority of the overall business.

The NFRC rating system works this way: Window manufacturers who want to be certified hire a NFRC-accredited lab. 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 NFRC-licensed inspection agency reviews the computer simulations and randomly pulls window units from the factory floor as test samples. Physical test values must fall within 10% of the computer predictions for a product line to be validated. If the test results fall outside of the acceptable range the product line fails certification and does not get an NFRC label.

Presently, manufacturers who participate in the NFRC program must include certified U-values on the label. They elect whether to include the solar heat gain coefficient, visible light transmittance and emisivity on the label. NFRC has created a technical procedure to measure air leakage and expects to have the details ironed out by January 1998. Douglas promises, “You can expect air leakage values soon.” Solar heat gain is based solely on computer simulation. While visible transmittance, air leakage and emittance are physically measured values. Currently, NFRC does not evaluate durability, but they have a long-term performance sub-committee in place and expect to consider durability in the future.

Certified products have temporary and permanent markings. The big, temporary label is placed in a highly visible spot on the window. A small permanent NFRC serial code is etched on an inconspicuous part of the window like a spacer or metal strip. Permanent labels are useful. Potential buyers of older homes often ask: “What kind of windows are these?” A phone 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.

The adventuresome can go beyond NFRC window comparisons. RESFEN, a computer program developed by Lawrence Berkeley Laboratory, enables you to minimize energy use, maximize comfort, control glare and maximize natural lighting in your home design. Builders, architects or consumers who want to fine-tune a design and choose specific windows for specific exposures on a house in a particular climate can order RESFEN from the NFRC for about $15. The NFRC Certified Product Directory and RESFEN is available from NFRC, 1300 Spring St., Suite 120, Silver Spring, MD 20910 301-589-6372.

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Last updated: June 4, 2009