Volume 39, Issue 5, May 2004
Optimizing Window Thermal Performance
by Ric Jackson
Window thermal performance is a hot issue among manufacturers and consumers alike. According to the American Architectural Manufacturers Association and Window & Door Manufacturers Association 2001 U.S. Industry Market Study of Fenestration Products, only 6 percent of residential windows do not feature a thermal performance rating.
Performed by Ducker Research Co., the study says 75 percent and 35 percent of residential windows feature National Fenestration Rating Council labels and Energy Star® performance ratings, respectively, indicating various levels of energy efficiency. These ratings vary depending on the assortment of components used in the window.
Each component affects a window’s ultimate thermal performance, which is reflected in a minimized total U-value. The trick is finding a combination of low U-value components that provide optimum thermal efficiency. Component choices include:
A typical window’s surface area is more than 80 percent glass, providing a large area for thermal transfer. In single-pane windows cold and heat travel directly through the glass. However, insulating glass units (IGUs) that feature more than one lite of glass sealed together provide a buffer of air space that absorbs cold and heat. The Ducker study shows IGUs in 91 percent of residential windows, making them a popular component in achieving thermal efficiency.
IGUs require spacers sealed to the perimeter of the glass to maintain their structural stability. The majority of heat transfer through window assemblies occurs at the spacer (through conduction), as this area is in contact with both inside and outside surfaces. Warm-edge spacers, which feature stainless steel, limited-metal or no-metal constructions, minimize thermal conduction and provide lower U-values than aluminum spacers. According to the Ducker study, warm-edge usage surpassed aluminum spacer usage in 1995 and now represents 88 percent of all residential windows. The study also states that of all spacers used, 29 percent are non-rigid—or flexible—spacers.
Another important consideration in choosing spacers is their width. Adding air space increases the amount of gas through which heat and cold must travel. In general, U-values drop when moving from 0.125-inch to 0.5-inch air spaces; however, they begin to rise again between 0.5-inch and 0.625-inch due to the offsetting effects of conduction versus convection. In gaps greater than 0.5 inch, enough space exists for a second ary flow of convection to increase movement of air within the IGU, leading to increased heat transfer.
Air and other gases sealed in the IGU air space reduce heat transfer through the center of the IGU. While air provides natural insulating properties, supplemental gases such as argon, carbon dioxide, sulfur hexaflouride (SF6), krypton and argon-krypton mixtures, reduce convection as they are more viscous than air. However, each gas presents differing conduction and convection rates. For example, SF6 has lower conduction than argon but higher convection, making it more suited to smaller air spaces (i.e. 0.25 inch).
Vinyl, fiberglass and wood frames typically offer U-values significantly lower than those found in aluminum frames. To bring U-values closer to other framing materials, some aluminum frames feature a thermal break of polyurethane sealant embedded in the frame. According to the Ducker study, vinyl windows represent the majority of the residential window market with a 44-percent market share, and have increased from 8.2 million units in 1991 to 25.5 million units in 2001. This growth trend is expected to continue, as the study states “the aluminum and wood markets will both feel pressure from vinyl usage.”
Low-E coatings consist of microscopically thin metal or metallic oxide layers deposited on glass. The coatings curb the passage of heat radiation, thereby lowering a window’s total U-value. Low-E coatings continue to increase in the residential market, as the Ducker study shows a jump from 40-percent market penetration in 1999 to 47 percent in 2001.
Creating the most thermally efficient window is a challenge that requires careful consideration of individual components and total U-values. When choosing components, remember to factor in their long-term durability and the end unit price. Thermal efficiency sells windows, but so do durability and cost factors. If a component jeopardizes the unit’s lifespan or provides only a marginal thermal improvement over alternatives, but costs significantly more, it may not be the best choice.
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