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What is a U-Value?

When comparing energy efficient windows, one number that really matters is a U-Value. But knowing what the U-Value actually means makes the difference when selecting the right energy efficient window for your home.

If I walked into your home and offered you $1 million house tokens to buy it from you, the first thing you’d ask is, “how much is $1 million house tokens worth in dollars?” It’s the same with a U-Value. If I tell you my window has a U-Value of .25 that number means nothing to you until you understand what a U-Value is.

What’s the Value of “U”?

For starters, a U-Value measures how well a window insulates heat. That is, how efficiently does it keep heat out of the home in the summer and keep heat in the house in the winter to put it more simply. The reason a lower U-Value is better than a high one is because lower numbers mean that a particular window is allowing heat to transfer at a lower rate.

Heat is always active and will escape or enter your home regardless of how well your windows are insulated. But technology in replacement windows and efforts by manufacturers to lower the rate of heat transfer have given windows great insulation value.

Putting It into Perspective

You may have heard of “R-Values” before. The higher this number the better. R-Values apply to things such as fiberglass insulation (or new spray in foams and other methods of insulating a home), walls, and doors.

For example:

  • Your insulated walls may have an R-Value somewhere around R 10-18.

  • Your floors may be insulated well enough to achieve a R 20-25.

  • Your attic space may have an R-Value up to R 39-49.

So with higher being better you can get an idea of what the actual value of these numbers are in terms of “R-Value”.

Now back to U-Values. To understand their value think of the U-Value as the opposite of the R. Since R-Values measure how well solid materials insulate heat you can’t apply them to glass. Glass in windows is designed to see through and open and no matter how well it is insulated it will let heat out at some point. So in order to understand how well a window is insulated we have to understand how well it is slowing heat down NOT keeping it in like we would want to know with an R-Value. That’s why a U-Value is the inverse, or opposite, of a R-Values.

So, to keep the math easy…

  • a window with a .25 U-Value has an insulation value equivalent to a R Value of 4.

  • a window with a .10 U-Value has an insulation value equivalent to a R Value of 10.

Now it’s not safe to assume that R Values and U Values are quite as interchangeable as I’ve just demonstrated but the purpose is to give you an idea of the value of each “U” number you get as you look at thermal double paned windows.

To sum up, lower U Value means a lower rate of heat transference. The lower the U Value the better the insulation of the window. We use U Values rather than R Values when considering windows because they’re really more accurate for judging how well a window is insulated.

Also, keep in mind, when looking at a U Value make sure you know whether or not this number is for the whole window or just the center of glass. Center of glass numbers are always going to be better than the overall U of a window and to really get an accurate comparison you want the overall U Value.

Check NFRC.org to look up insulation ratings on windows that are thermally certified or give us a call at Atlanta Area Window and Door Co. to find out about which thermal windows we offer.