Explaining the mysterious U-value

If you’ve ever been shopping for a window, you’ve no doubt encountered a term that you’re probably not all that familiar with – U-value. U-values are a rating of energy efficiency, and they are used to rate and compare windows, exterior doors, skylights and certain other exterior building components, including exterior walls. U-values are also the standard used in the building codes for specifying the minimum energy efficiency values for all of these components.

Most people are much more familiar with R-values, which are used to rate how well a material such as insulation resists the flow of heat through it. The higher an R-value is, the more heat loss it resists, and therefore the more energy efficient it is. U-values differ from R-values in two important ways, and when you’re shopping for energy-efficient components for your house, it helps to understand the difference.

First of all, where the R-value is used to rate the energy efficiency of a single component – a batt of insulation for example – U-values rate the energy efficiency of the combined materials in a building component or section. For example, in a typical insulated window you have the two panes of glass, the dead air between the glass, the frame material and perhaps some weatherstripping or sealants that hold everything together. Because each of these components works together as a unit, the U-value is used to indicate the energy efficiency of this entire assembly.

The second distinction is that U-values rate how much heat is conducted through this combination of materials. So, where R-values rate how much heat loss the material resists from passing through it, U-values rate how much the heat the component allows to pass through it. Therefore, while a high R-value indicates good energy efficiency – R-19 is better than R-11, for example – while a low U-value indicates good energy efficiency – U-0.35 would be better than U-0.43.

CONVERTING FROM U-VALUES TO R-VALUES

You may have noticed that U-values tend to be very small numbers. Because R-values rate heat loss resistance and U-values rate actual heat loss, they are the opposite – or reciprocal – of each other. Therefore, you can convert from one to the other using the following formulas:

1/R-value = U-value, or 1/U-value = R-value.

For example, if you are looking at a window with a U-value of 0.35 – which would be a good window from an energy standpoint – you could figure out what the equivalent R-value would be by using the above formula. In this case, the R-value would be approximately R-2.86 (1 / 0.35 = 2.86).

Using these values, you can also readily see the impact that lots of windows can have on the energy efficiency of a house. If you have exterior walls that are rated at R-21, every window you install at R-2.86 leaves a pretty good hole for heat to escape through. Even more dramatic is the impact of a skylight – a good skylight will have a U-value of around 0.45, which means that every skylight you install leaves an R-2.2 hole in your R-38 attic!

WHAT MEETS CODE?

From the above examples, it’s easy to see that windows, doors and skylights lose energy. Because of that, most building codes – especially those in colder climates – have set minimum standards for the energy efficiency of these components. When you’re doing your shopping, it’s good to know what will meet the codes.

For Oregon, the Revised Energy Code calls for windows to be rated at U-0.40, and skylights at U-0.50. Exterior doors other than the main entry door have a requirement of U-0.20, which is typically a foam-insulated door. Because of their decorative nature, some exceptions are made for the main entry door, and the acceptable minimum is reduced to U-0.54.

Remember that there are other factors involved in selecting doors, windows and skylights to meet the energy codes, including the type of construction being used for the house. Always check with your local building department for the specific requirements for your particular home.