Whether you’re buying windows for a new house or to replace some leaking clunkers past their prime, look beyond extravagant sales claims to a small label in the corner. It lists several ratings (on skylights and glazed doors too) administered by the non-profit National Fenestration Rating Council (nfrc.org).
Beyond personal choices about the type and style of window, the ratings help you evaluate insulating properties for summer and winter, air leakage and overall energy efficiency. All major suppliers, such as Andersen, Hurd, Jeld-Wen and Pella, subscribe to the system. The NFRC website lists hundreds of other companies that do as well, offering more than 100,000 windows in their Certified Products Directory. And the ratings are not for the glass only, for example, single versus double glazing. They rate the entire package, glass and frame.
But the products of many small companies, contractors, home centers and others who fabricate mainly replacement windows do not carry the label. That doesn’t mean their windows aren’t up to current energy standards. It means that there’s no way for you to be sure, or to comparison shop.
Before you delve into U-factors and solar coefficients, bear in mind that no label can cover the installation, even though it’s just as important. High-efficiency glazing isn’t much help if the unit isn’t flashed and caulked properly, leaks and rots the supporting sill. That’s why it’s a good idea to read the installation directions, even if you hire a contractor. Find out what’s supposed to happen. If there will be several units going in, be there to keep track of the first installation.
What follows is a guide to deciphering the NFRC labels.
Label basics
In addition to technical information, the NFRC label lists the manufacturer and model, and the type of operation, such as double-hung or casement. There’s also a brief description of the frame (like vinyl-clad wood frame) and the glazing (double glazing, argon fill, Low-E).
Cladding refers to a covering on the exterior frame that provides more protection and requires less maintenance than paint.
Double glazing is simply two panes of glass filled with a gas like argon that provides an insulating barrier.
Low-E is shorthand for low emissivity, and takes the form of a microscopically thin metallic coating that retards heat transfer.
That glazing combination (double, argon, Low-E) usually carries the most efficient energy rating, aside from triple glazing. But windows with three layers of glass are very costly, very heavy and generally take a long time to return energy savings on the extra investment.
U-factor
Like R-values for insulation, this is the most important rating because it measures energy efficiency for the glass and frame — the window overall. Technical explanations and formulas are mind-numbing, so the point to remember is that unlike R-values where higher numbers offer more insulating value, with U-factors it’s the opposite. The lower the number, generally between 1.2 and 0.2, the better. Windows with lower numbers provide more insulating value.
Solar heat gain coefficient
The SHGC is how much solar heat is absorbed by a window and transferred into the house. It’s represented as a number between 1 and 0, the lower the better, particularly where air-conditioning costs loom larger than heating costs.
Visible transmittance
The VT rating measures how much light comes through the glass, written as a number between 0 and 1, the higher the better. It’s the least important number because without nearly superhuman vision, guess what: Glass looks clear.
Air leakage
You also want a lower number in this category, which rates the cubic feet of air passing through a square foot of window area by way of cracks in the window assembly. Aside from an energy rating, it’s also a kind of draftiness rating. Bear in mind, it evaluates the window only and can’t account for gaps between the frame and siding.
Condensation resistance
An optional rating only some manufacturers include. Expressed as a number between 0 and 100, the higher the better, it rates the ability of the interior surface to resist the formation of condensation. But dripping that collects and damages sills is more a byproduct of the ambient humidity inside that’s controlled by the house heating and cooling system.
