What Do Window Performance Specs Mean?

Wednesday, November 18th, 2015 by Ben Kershner

NRFC Rating Labels show how your new window will performWhen it’s time to replace your windows, it’s time to replace your windows.  New modern windows are easy to clean, easy to maintain, easy to operate, and add value to your home — hopefully.  They are also better insulated and less drafty than older windows — hopefully.  You can get all of this and more if you pick out the right windows, and the right installer.  Here’s how.

Once upon a time, windows were made out of dense hardwood like oak and mahogany, and then painted (to preserve the wood).  If they’re well maintained, they would last for generations.  Modern wood windows are made out of softer pine wood.  Many people like the appearance of natural wood, although it doesn’t insulate as well as a vinyl window can.  Look at the “U-Factor” to know the insulating ability of the entire window assembly.

The U-Factor measures how easily heat gets through, so lower numbers are better.  Manufacturers achieve lower U-Factors by building in double or triple pane glass packages that contain inert gases such as argon, krypton, or xenon.  They also may add different types of foam insulation to the hollow frame that holds the glass.   In Northern climate zones, ENERGY STAR qualified windows have a U-Factor of 0.30 or lower.  Cheap, low-end windows may be almost twice that — stay away from anything with anything with a high U-Factor.  Pricy, high-end windows may have as low as U-0.15 — this may be a situation where the cost of the window doesn’t justify the insulating performance.

How drafty a window is depends upon two factors:  the window itself, and the quality of the installation.  Of course, you want to use a contractor that you trust.  The perimeter of the window assembly should be sealed with foam.  Shoving bits of fiberglass into the perimeter gap does nothing.  A blower-door test can help identify the source of drafts that need to be sealed.

Most window manufacturers don’t advertise their Air-Leakage ratings, and they will vary quite a bit based on the home they’re installed in.  The best way to make a window more air-tight is to build it with more stringent tolerances.  If you have access to a sample window, try to wiggle the sash — see how much play there is — how much does it move when it’s not supposed to.  When the wind blows, that extra space turns into a draft.  Also look at the weatherstripping.  Most windows have a white fibrous weatherstrip made of mylar.  A double or triple row of weatherstripping helps even more.

What you WILL find on the window label next to the U-Factor is the SHGC, or Solar Heat Gain Coefficient.  In southern climates, this can make a huge difference in comfort.  A high SHGC means that your window transmits lots of radiant solar heat through the glass — it can be quite warm sitting in a patch of sunlight.  A low SHGC blocks the heat gain like a tinted window.

Compare the U-Factor and SHGC to the “Visible Transmittance” number — that’s how CLEAR the glass is.  A dark tinted window would have a very low VT.  An opaque wall has a VT of zero.  “Perfectly clear” glass would have a VT of ONE.

Now, there’s lots more to look for in a window besides the performance specs, and that’s probably what most salesmen will spend their time talking about — and there’s lots of important things to talk about.  I’ll get into that in another post.  But before you sign on a new set of windows, make sure you know the specs.

About the author

Ben Kershner

Ben Kershner is a home energy advisor at True Energy Solutions, and is a regular contributor to our website. He lives with his wife and family in Greece, NY.

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