Shedding Some Light on Shading Coefficients, Energy Ratings and Air Circulation

Buying the right windows to suit your needs goes beyond considering just the energy ratings of the glazing system. Other factors to take into account are a window’s shading coefficient, how it handles light and the level of air infiltration.

The window frames also should play an important role in the buying decision, as should the location of the house and where the new windows will be installed. For instance, is there strong sunlight that needs to be blocked?

It’s important to decide what each window should do and then buy the system that best meets those needs.

Here are some points to consider when shopping for new windows:

* Energy ratings. Most manufacturers provide both the center-of-glass and the entire window energy ratings. Obviously, the entire window R value is a better indicator of performance.

Don’t be confused if you see a U value alongside the familiar R rating. They are actually different sides of the same coin. R values measure resistance to heat transfer--the higher the better. U values measure heat transfer --the lower the better.

* Shading coefficient. This rating tells how much solar energy a window captures. A single pane of glass has a shading coefficient of 1. A blocked window would be rated at 0. So, if reducing cooling costs is important, pick a window that has a low shading coefficient.

Tinted windows are good choices, but low shading coefficients can also be provided by clear glass.

* Handling light. Besides letting heat energy pass through, windows also allow light into our homes. Not all windows do this equally. Clear double-glazed windows allow about 77% of the visible light to pass through and a high-R window, around 62%.

Part of the light is in the form of the ultraviolet radiation that’s responsible for the fading of carpets and furniture. You’ll find windows that allow everything from more than 50% of the UV radiation through to those that allow less than 1%.

* Air infiltration. This is important in all climates. Windows should be well constructed and allow a minimum of air infiltration. The design of some types of windows makes them tighter than others. Casement and awning windows are tighter than double-hung windows, for example.

A rating of 0.02 or 0.03 is very tight; a rating of 0.5 is loose. These ratings apply to the window itself, not the installation.

Stopping leaks around a window once it’s in the wall is the responsibility of the installer.