We’ve all heard the hype – buy new windows and save 35% on your next heating bill. To put it politely, that’s a bunch of hooey. Unless your windows are old, poorly installed, leaky and missing half the glass, you are not going to save 35% on your heating bills. In fact, there are numerous studies showing that replacing windows is among the least cost effective measures for improving your home’s energy efficiency!

That said, windows are among the worst performing parts of your home when it comes to energy efficiency. Did you know that a single, 3 by 5 foot window can double the energy loss for the wall in which it’s mounted? This is why manufacturers often make such outrageous claims about energy savings. But a home loses energy through more places than its walls. It loses energy through air infiltration, walls, windows, doors, ceilings, the foundation and the slab.

Let’s compare a variety of window styles and their relative energy loss. But first, a definition:

U-value: is a measure of the energy transfer through a window. The higher the U-value, the greater the energy transfer and the worse the insulating ability of the window.

1. Single glazed, clear glass, metal frame. U-value is above 1.0. Metal framed windows are the worst since metal conducts heat so well.
2. Single glazed, clear glass, non-metal frame. U = 0.71 to 0.99
3. Double glazed, clear glass, metal frame. U = 0.71 to 0.99. An old wooden, single glazed window is better than a metal framed double glazed window.
4. Single glazed window with tight storm window. U = 0.50
5. Double glazed, clear glass. Non-metal frame. U = 0.41 to 0.55
6. Double glazed, low-e glass. Non-metal frame. U = 0.26 to 0.40 depending upon frame.
7. Triple glazed, low-e glass. Non-metal frame. U = 0.15 to 0.25

Comparing U-values, we can directly compare the relative energy efficiencies of these different styles of window. For example, if you install a super insulating, triple glazed window with a U-value of 0.20, this will lose 20%-25% as much energy as an old single glazed clear glass window. That is truly substantial. In fact, when I renovated my own home, I went this route. Not because I knew the energy savings will pay off (they won’t) but I was trying to optimize my entire home’s energy efficiency and comfort.

Suppose you have a moderately old wood frame, single glazed window with a tight fitting storm window. This might have a U-value of around 0.50. If you were upgrading to a double-glazed, low-e window, which typically has a U-value of about 0.35, then the new windows would only reduce the energy loss through the window by 30%. Not bad, but not great and probably not worth the investment.

In addition, most new windows are installed poorly. I have seen many instances where a house was less comfortable after installing new windows. Why? Because the installers did not seal around the windows properly and air infiltration is much worse for energy loss than is poor insulation.

If you remove the trim from around a window, you would see something like this. The window unit would be shimmed out and nailed into place. Around the perimeter are big air gaps. Sometimes, you can even see right outdoors.

The problem is, most installers just shove fiberglass in these cracks. Fiberglass is not an air barrier. In fact, when compressed like this, it isn’t even a good insulator!

Please see my website for more detailed information on proper window installation.

For all these reasons, if your windows are in good shape and don’t seem drafty now, then I usually don’t recommend replacing them. Instead, start with some high-quality window treatments.

It’s amazing how much of a difference cellular shades or window quilts can make. At a fraction of the cost and disturbance of new windows, properly installed shades or window quilts can reduce energy loss by anywhere from 50% to 80%, making your home more comfortable and energy efficient.

Practically speaking, usually I recommend that people outfit one room with these initially to see if they yield the desired improvement. However, you really can’t go wrong with these unless you’re planning on renovating anyway and will be upgrading the windows. In that case, I suggest holding off on the window treatments until you get the new windows because often the new windows will be a different size and the treatments might not fit then ew windows.

For more detailed information, please see the links below.

Other links:

Dept. of Energy – Energy Performance ratings for Windows

Dept. of Energy – Energy Saver Tips for Windows

Efficient Windows Collaborative for more technical information on windows.

Florida Solar Energy Center – Windows

Grace-Vycor – Contractor’s Guide to Window Installation

National Fenestration Rating Council – General website