Understanding Window Efficiency

Advancements in replacement windows over the years

Wood Window Most of us grew up in houses with old-fashioned windows. In the 1920s, it was wood-framed, single-pane, farmhouse-style windows. By the 1940s, the casement window (steel frame and an inside crank for opening) was in vogue. Fifties tract homes commonly had aluminum-frame windows or drafty jalousie (crank-out “slat”) windows. That might be what we were used to, but by today’s standards, they’re all grossly inefficient energy-wasters.

Along with appliances, insulation, air conditioners and building materials and methods, windows have evolved greatly in the last twenty or so years. But the ratings and terminology for modern windows can become a little confusing, so let’s break it down:

These are the materials that are commonly used with replacement windows:

Argon Gas. Sandwiched between panes of glass, argon is a great insulator. Even though it’s been surpassed by other gas compounds, it’s still a familiar choice for many homeowners.
Low E. This is a step up from argon; Low E is a silver compound sprayed on the inside of double-pane windows during the manufacturing process. It does a great job of reflecting heat from outside (important for Houston!), or retaining heat during the winter.
Low E2. It didn’t take too long to discover that a second layer of Low-E did an even better job; Low E2 is simply another layer of Low-E for even better efficiency.

Coupled with a window tint or solar screens, windows with these designs can make a huge difference in your home’s energy use (not to mention protection from fading for your carpets and furniture).

It’s also quite common these days to manufacture replacement windows with double or even triple thicknesses of glass. The extra layers help immensely with outside noise, strength (even if you live next to a Houston golf course!) and efficiency. The “tightness” of your installation is also an important factor, as is the heat conductivity of the frame itself (PVC or vinyl frames have proven to be a big improvement).

Now, as for the energy ratings themselves:

U-value is a measure of heat conductance
R-value is the measure of heat flow resistance.

If your windows have a high R-value, they will lose less heat than a window with a lower R-value. By the same coin, a window with a low U-value is more efficient than one with a low U-value. Of the two measures, the R-value is the more common rating among manufacturers. The numbers to look for are 0.9 to 3.0 for R-value, and 1.1 to 0.3 for U-value.

When comparison shopping, make sure that the numbers represent the entire window and frame and not just the glass itself. Also, make certain that they are for the size and style of replacement window you are specifically looking for.

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