When shopping for new windows, homeowners often feel overwhelmed by the numbers and terms used to market those windows. One value you’ll come across is the U value, also known as the U factor. Different windows can have very different U values. Should you choose windows with a higher U value or windows with a lower one? Is there a particular U value you should look for?
These questions are easier to answer if you have a good baseline knowledge of U values and how to interpret them. Keep reading for a thorough overview.
The U-Value: Defined
The variable “U” stands for a value called thermal transmittance. It is an indicator of how much heat is transferred through a surface. A high U value means a lot of heat passes through the surface. A low U value means less heat passes through the surface.
Another way to state this would be to say a material with a low U value is a good insulator. A material with a high U value is a poor insulator. So, in general, you want to look for windows and materials with lower U values. They’ll do a better job of insulating your home, keeping it at a comfortable temperature, and maintaining your energy efficiency.
U Values Versus R Values
If you’re familiar with R values, then the description of U values above probably sounded familiar. You may be wondering whether a U value is the same thing as an R value. The short answer is “kind of.” The R value is also an indicator of how much heat passes through a material. However, the R value is the inverse of the U value.
Without going too deep into the math, what this means is that a higher R value indicates a material is a better insulator. A lower R value means it is a poor insulator. With the U value, it’s the opposite.
For the more mathematically inclined, you can divide 1 by the U value and get the R value. For example, if you have a material with a U value of 0.5, you can divide 1/0.5, yielding a result of 2. To say a material has an R value of 2 is the same as saying it has a U value of 0.5.
R values and U values are used in different circumstances. Builders and contractors use R values when they speak about materials used to form a building’s envelope, such as siding and fiberglass insulation. U values are used when discussing glass, specifically.
The Best U Value for Your Climate
Most windows have a U value between 0.2 and 1.20. Remember, the lower the U value, the less heat you’ll lose through the window. So what is a good bench-line number to look for?
In Northern climates, including the climate of the Twin Cities area, Energy Stargenerally recommends windows with a U value of 0.30 or lower. However, if you can find windows with a high Solar Heat Gain Coefficient (SHGC) of 0.4 or higher, windows with a U value up to 0.32 are acceptable. A high SHGC means much the sun’s heat will go through the window. This can “cancel out” some heat loss that comes with a slightly higher U value.
As you search for new home windows, keep this guideline in mind. Narrow down your options to windows with a U value under 0.30. Then, while also weighing other factors, choose the windows with the lowest U value you can comfortably afford.
There are a few ways window manufacturers can increase the U value of their windows. They can fill the space between the window panes with an insulating gas like argon. Or, they can coat the windows with a low-e substance which reflects more heat waves. Windows with these features take longer to manufacture, which is why windows with lower U values tend to cost more.
Reducing Heat Loss Through Windows
Regardless of which windows you choose, there are a few steps you can take to slow heat loss through those windows. First of all, make sure you have your windows installed professionally. Even windows with the lowest U values will be inefficient if they are not installed well.
You can also reduce heat loss through your windows with a good set of insulating drapes or blinds. In Northern climates, these are helpful, even if your windows meet Energy Star’s U-value recommendations.
If you need new windows and want to learn more about your options, contact Midwest Roofing, Siding, and Windows. Our knowledgeable technicians work on homes throughout the Twin Cities, and they’ll be happy to discuss U values and other window characteristics with you.