Have you ever found yourself picking a sticker off a building product or material from the store, and wondering, why did they put the sticker here? I have often had this thought with everything from stovepipe to plumbing fittings, but the classic example in the building world is probably windows.

The glass part of the window is for looking through, right? Then why does every window manufacturer put two are three large stickers right in that area? Sometimes these come off easily--sometimes they are a real pain to remove. Sometimes they don't come off at all: I have seen window stickers, or remnants of them, still in place years after construction.

The next time you encounter one of those stickers--in your house, on the construction site, or at the showroom, take a close look. The different values that appear on them can mean a world of difference in terms of how those windows will perform in your building.

NFRC gives us apples-to-apples comparisons

To enable apples-to-apples comparisons among windows, industry leaders formed the National Fenestration Rating Council (NFRC) in 1989 to create standard metrics for comparing performance of windows. NFRC has set the format for those stickers for most of the time since then. It gives us four main numbers to pay attention to. First is U-factor. Americans have gotten pretty comfortable with R-value, used to measure insulation, so it's a little unfortunate that we have to learn a new metric for windows, that is basically measuring the same thing. R-value measures resistance to heat flow; U-factor is the mathematical inverse (1 divided by R-value gives you U-factor), and measures the rate of that heat flow.

In a cold climate a good U-factor for a window is between 0.17 and 0.39. (That's between R-6 and R-2.5). Lower is better with U-factor--the opposite of R-value, when higher is better. The low end of that range is only achievable with higher-quality triple-glazed windows--windows with three layers of glass. These are worth considering for high-performance houses, but they do cost more.

In a hot climate, the only difference with U-factor is that it's better to look for an even lower figure, for reasons I'll explain in a minute. I would look for a U-factor range in a hot climate of between 0.17 and 0.30, and again, lower is better. 

Letting in solar heat--or not

The next key number is solar heat gain coefficient (SHGC). This is a number between 0 and 1, and it compares amount of solar heat that reaches the window with the amount that gets through to the inside. In a hot climate, or in a cold climate on west-facing windows with rooms that tend to overheat in low-angle afternoon sun, less solar heat gain is generally good, and this number should be low.

In cold climates on south-facing walls, a higher solar-heat-gain number is better--this supports "passive solar" heating. In this situation, an SHGC value of 0.42 to 0.63 is desirable, and higher is better. (In hot climates, look for values as low as 0.25.)

Unfortunately, due to the different low-e coatings used, higher SHGC values generally also come with somewhat higher U-factors. In other words, to get more solar heat gain you have to give up some insulation value. That's okay, but usually is only worth doing on south-facing walls. On north-facing walls, you want the lowest U-factor possible.

"Southern" windows may not work well up north

To further complicate matters, most windows made in the U.S. cater to the larger southern market, where both low U-factor and low SHGC are sought after. Window stickers in the Northeast attest to the fact that a lot of homes are stuck with windows that don't match their "passive solar" designs: they insulate well, but they reject too much solar heat.

The "30/30" rule is common: U-factors of under 0.30 (which is good) are often paired with SHGC's of under 0.30. That's a good target for the South. For colder climates, I would propose for discussion a "30/40" rule.

Pella, Marvin, and other major U.S. companies are increasingly offering products sensitive to northern buyers, but I have seen a number of area homeowners buy windows from Canadian companies, which are generally ahead of the game.

If you're in the South, then good news--most windows that are for sale already cater to your needs. Just look for the highest-performers possible.

When is a window not a window?

We install windows to let in light and enjoy the outdoors, so it's important that we don't lose sight of this in our search for highly insulating windows. A third number on that sticker, visible transmittance (VT) is a value from 0 to 1 that indicates the amount of visible light that makes it through, taking into consideration light blocked by the frame and muntins.

Most double- and triple-glazed windows have VT values between 0.30 and 0.70. Higher VT is desirable, but lower VT values are often delivered along with lower U-factors. If you get a really high-performing window, beware of VT values less than 0.40. Below that point, your view may start to have a grayish cast.

The missing number: air leakage

Tiny cracks in the window assembly can add up to significant heat loss and gain, so it's worth looking for air leakage numbers . NFRC labels sometimes provide an air leakage rating (AL), although it is not required, and manufacturers often omit it. AL is expressed as cubic feet of air passing through a square foot of window area. The lower the AL, the less air will pass through cracks in the window assembly. Select windows with an AL of 0.30 or less. Lower is better.

For a full discussion of how to find the best windows, check out Choosing Windows: Looking Through the Options, in Environmental Building News.

Photo: An NFRC label for a double-glazed south-facing window in a northern climate shows high SHGC for good solar gain. Kirstin Edelglass