When considering new windows, don’t neglect to consider the energy efficiency of the framing.
While a window’s energy efficiency is dominated by its glazing, the framing contributes 10% to 30% of the unit’s weather resistance. You can negate a lot of the efficiency of highly insulating glazing by inserting it into a frame that allows rapid heat transfer.
A window's rate of heat loss is expressed as its U-factor. The lower a window's U-factor, the better its resistance to heat flow, and the better its insulating abilities. Low U-factors are most important in colder climates.
Heat admitted through a window is expressed by the SHGC (solar heat gain coefficient). The lower the number, the less solar heat is being transmitted. Low SHGCs are more important in hotter climates. The U.S. Department of Energy's ENERGY STAR program provides optimum U-factors and SHGCs by region of the country.
Here's a guide to the five basic types of window framing material:
Aluminum/Metal—At the bottom of the scale in terms of price, metal frames are strong and pretty much maintenance free, but poor in terms of energy efficiency, since they conduct heat rapidly. To counteract the heat flow, metal frames should have a thermal break—usually a plastic strip inserted in the assembly between the window’s interior and exterior frame to reduce temperature transfer. The addition of thermal breaks can cut an aluminum frame's heat transfer in half, from a U-factor of 2.0 to 1.0.
Wood—It’s hard to beat a wood window for beauty, but they are pricy. Wood is a good insulator with frame U-factors at 0.3 to 0.5, but it expands and contracts in response to prevailing weather conditions. It is susceptible to depredation by insects and molds, though chemically modified woods (heat is used to destroy the material’s appeal as a food source without damaging its structural integrity) can alleviate that problem. Wood also requires regular maintenance; however, aluminum or vinyl cladding can reduce maintenance costs.
Composites—Proprietary mixtures of plastic and wood, aimed at reducing the maintenance associated with wood. Very stable, these composites have the same or better structural and thermal properties as conventional wood.
Vinyl—On the lower end of the scale price-wise, vinyl window frames are welded hollow units typically composed of polyvinyl chloride. Because it’s a welded product, you end up with a continuous material, unlike wood or other materials, which have to be machined for corner joinery, explains Jeff Lowinski, vice president of technical services for the Chicago-based Window and Door Manufacturers Association.
When the hollow frame is filled with insulating material, a vinyl window unit offers very good energy efficiency, comparable to wood. Ultraviolet light stabilizers are added to prevent sunlight from degrading the material and causing cracking. Vinyl can become brittle in extreme cold. Vinyl windows don’t require painting and offer good moisture resistance.
Fiberglass—A resin combined with a reinforcing glass fiber, fiberglass is extensively used in the boat building industry, and is also replacing steel in highway and bridge construction.
Fiberglass is well-suited for windows in any climate. It has the strength of steel and the thermal efficiency of wood, and resists corrosion, mold, and mildew. The product is also thermally stable, unlike wood, and moves very little as the climate changes. Price-wise, it's at the high-end of the scale, comparable to wood.
By combining an energy-efficient frame with the appropriate glazing type for their climate, homeowners should be able to obtain the best high-efficiency window for their needs.
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