Understanding R-value and U-factor in Windows

Why the R-value and U-factor Matter

If you live in an old home, you know that old windows are not very efficient. Sure, they provide a view outside, and it’s really hard to beat that historic old window look, but in the cold winter months, you can certainly feel that they aren’t blocking the cold air. The result is that you have a drafty home and high energy bills. If you’re nodding in agreement, and you’re tired of being cold in the winter, and baking through the summer, then you’re probably considering upgrading to better windows for your home.

When it comes time to upgrade to a better, and more efficient, window, you will notice that like insulation, the windows are given an R-value and a U-factor. But what exactly does that even mean? Obviously, a higher R-value is better, but is that all you need to consider? And what exactly is the U-factor?

When it comes to window systems, here is what you should know about these concepts to choose the most efficient replacement windows for your home.

What is R-value in Windows?

There are a variety of different ways to measure the efficiency of the windows. They all depend on how the glass is created, what the frame is made of, and other factors. But a good place to start is with the R-value.

R-value measures the resistance to passing heat. A lower R-value means the resistance is lower, and more heat passes through the windows (coming in from outside during the summer or going out from inside during the winter). That lower value means your home gets hotter in the summer and colder in the winter.

When you look at the windows, look for those with a higher R-value. So if two windows that appear identical, but one has an R-value of 2 and the other has an R-value of 4, you can quickly see that the second window will be twice as efficient as the first (and you can see why the price is higher for the second window).

What is the U-factor in Windows?

Similar to the R-value, the U-factor has to do with how much heat can transfer through the glass and out into the open air. Where the R refers to resistance, the U-factor measures the rate at which heat transfers through the glass. This means that while a high R-value is good, it’s the opposite for the U-factor.

When looking for an efficient window, consider those with a low U-factor. The lower the number means that it is slower to pass heat through the glass. You want the lowest number possible in order to keep the heat outside in the summer, and inside during the winter. If you live in a climate where you are heating the home more often than you are cooling (such as most states in the northern part of the country), then this is a very important factor to take into consideration. For those living further south, it’s not as high of importance, but in an effort to conserve as much energy as possible, you might as well get the most efficient windows as possible.

Single pane windows are going to be some of the highest U-factors you can find. When you move to a double pane (such as nearly every modern replacement window on the market), you can get them with U-factors around .3 or a little lower. To drop it even further, a triple pane is necessary and will get your factor to around .15.

U-factor and R-value are inversely related. If one isn’t listed, simply divide 1 by the value listed. So a window with a U-factor of .25 is a window with an R-value of 4 (1/.25 = 4; 1/4 = .25).

Perks of Higher R-value and lower U-factor Windows

When you’re replacing the windows on your home, you likely want to go with the most affordable option. The materials used (wood, vinyl, fiberglass, etc.) will determine the bulk of the cost. But two windows, made from the same materials, may vary in values and factors. To understand if the higher R-value (and thus higher cost window) and if the cost increase is worthwhile, consider the following that will be provided with the more efficient windows:

• Lower environmental impact. Less pollution is always a good thing; let’s leave this world cleaner than we found it.
• Lower utility bills. Over time the costs will diminish with savings on your bills; who doesn’t love saving money?
• A more comfortable home. The cold drafty areas will be gone; no more extra blankets on one side of the room.
• A more valuable home. Someday, when you sell, you will recoup some of the costs of the windows; plus appeal to more buyers.

Windows Go Beyond R-value and U-factor

R-values are usually referring to insulation and less about replacement windows. This is due to the fact that windows conduct heat differently because they are transparent. It’s not just about blocking the heat, but you have to take radiant heat into account as well (that is, the light that comes through and heats up the room); thus taking the U-factor into consideration as well. Before you make a decision based solely on R-value or the U-factor, learn about the other considerations of energy efficiency in windows. Then keep in mind that there are other aspects that play an important role when you’re installing new windows in your home: such as aesthetics.

Not long ago, if you wanted a window that was energy efficient, you would have to sacrifice a bit of style. The real efficient ones were a bit clunky, they weren’t very nice to look at, and they could be described as just plain. Today, however, you can get beautiful windows, in just about any style that you desire, that will enhance your home stylistically and economically.

When it comes time to upgrade your windows, find some the ones that look great, are energy efficient, and will ultimately make your home more comfortable and more valuable by taking everything into consideration before you purchase.