The window Revolution
The window revolution- Most window markers offered only a few window styles in a limited range of sizes. An experienced builder could frame rough openings for almost every window or door in a home using sizes from memory. Since then, there’s been a tremendous increase in the range of window sizes and styles available. Higher air and moisture infiltration standards and increased window offerings have made window installation more complex.
Flank a set of French doors with matching casement sidelights, then tack on a rectangular transom window over the top, and you have created a virtual wall of glass in a framework that must bear the stress of the roof, weather, and wind. The builder’s challenge will be to transfer those forces to the ground through limited channels without sacrificing energy efficiency or leaving leaky gaps in the home’s shell.
These challenges allow little tolerance for framing, caulking, house wrap, or trim errors. Further, the quality of materials specified for window walls must meet superior engineering guidelines. Framing lumber should contain very little moisture (less than 15 percent). Interior trim must line up appropriately and convey a sense of unity with the windows. When complete, every window and door should function smoothly and quietly.
Window efficiency- The recent trend has moved toward greater use of glass layers. Engineering developments have produced windows much more resistant to air leakage and thermal transmission than ordinary single-glazed units. Consider this: Double or triple glazing (two layers of glass separated by airspace, often called insulating glass) generally offers twice the R-value (resistance to heat transfer) as single glazing. Double-glazed windows with low-E glass (windows treated with a low-emissivity coating, a microscopically thin metallic layer that permits the passage of light while reflecting heat) generally offer twice the R-value of ordinary insulating glass units. Low-E windows, in which the airspace is filled with an inert gas such as Argon or Krypton, offer approximately twice the R-value of ordinary low-E units.
The sash and frame materials also affect resistance to heat flow; wood and vinyl offer the best energy performance, while aluminum and steel windows fall at the lower efficiency end of the spectrum. To set the range of choices in perspective, the most energy-efficient windows available have ratings about R-8; conventional single-glazed units are rated R-1.
A new guideline developed by the National Fenestration Rating Council (NFRC) provides a standard for evaluating and comparing the performance of all windows. Most manufacturers now list the U-value of their windows on a label attached to the window glass. U-value measures the total heat flow through the entire window, taking into account the frame, window divider, and glazing edges. The lower the U-value, the better the window’s performance.
Wood framers still rank as one of the best thermal performers. However, the quality of a wood window depends mainly on the lumber used in its construction. Quality wood windows contain only clear, knot-free lumber. Well-maintained, they will last for decades. They must be repainted every few years unless they are clay with a vinyl or an aluminum shell. Wood has natural insulating qualities and a very low coefficient of thermal expansion, so it can adjust to rapid shifts in temperature without degrading. Vinyl window frames consist of a hollow polyvinyl chloride (PVC) extrusion, sometimes filled with polyurethane foam. The thermal performance of foam-filled vinyl windows often exceeds that of wood windows. PVC, however, has an extremely high coefficient of thermal expansion. Vinyl windows subjected to extreme temperature cycles may crack and lose their integrity over time.
Aluminum or steel framed windows must include thermal breaks to minimize heat gain or loss. In all but the mildest climates, units without breaks suffer from unacceptable thermal bridging, a phenomenon in which heat moves rapidly between the inside and outside of the home. Other less-known but promising new window frame materials include fiberglass and PVC-wood composites. Both materials, such as wood, offer performance, coupling minimal thermal expansion and contraction with low maintenance. High-performance windows and doors are considerably more expensive than standard types, and when making your specifications, you must consider whether the climate requires a high degree of energy efficiency. In extreme climates, you may be able to make up the extra initial cost for premium windows and doors through low energy bills. But in mild or moderate climates, buying super-efficient units may not make financial sense.
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