Is there full transparency in the window industry?
Windows are a most noticeable part of our homes. Allowing us see the world outside and letting light inside. Unfortunately they are also gaping holes in the thermal envelope. A significant amount of energy is lost through our windows because glass is an extremely poor insulator. Due to this characteristic windows have evolved into the multi-pane types today. But is replacing your “old” windows with a modern, thermally superior variety going to make a huge difference in your utility costs? The short answer, very likely no.
I have had clients relate claims by window installation companies of energy savings of as high as 40 percent. That number may very well have been derived from the chart from the Efficient Window Collaborative. The problem I see with the chart is the base line window is not a very realistic example. An aluminum frame, single pane window is not common. In fact I can never recall ever seeing one in the many years I have been inspecting, although I have seen numerous double pane aluminum windows. The baseline example in the chart would be the absolute worst thermally performing window possible. A more realistic and accurate example would have been a wood framed single pane window or the second green bar.
Now let’s say your house has thermal double pane windows. They are old, worn and drafty. If you replace the old your old thermal pane windows with new double low-E windows the expected savings is about 5 percent according to the chart. That is the difference between double clear (green bar) and double clear with low-E (orange bar).
Let’s take another scenario; a single pane window outfitted with storm windows, which is conspicuously absent from the chart. It is also a very common window type I find still in use. That is essentially a double pane window, but not thermally sealed. In the first infrared image, the window on the left is not equipped with a storm window, while the right window has it’s storm in place. What is very apparent is the temperature difference is fairly significant, approximately 15 º F. What is also noticeable is the storm window also decreases drafts. The left window has a cloth draft stop along the center seam. That is also the coldest part of the window. This is not to say a newer double pane window is the similar or even inferior. It simply demonstrates the differences between the two window types may not be as great as stated.
The windows in the first IR image are original to this 1914 built home. One may believe that replacing the windows in this home would be very beneficial. The truth is through the assistance of a complete energy audit the windows were found to be a poor improvement. There were 27 windows of this general size in the home. Replacement cost was estimated at $9500, more than a bit conservative in my judgement. The estimated yearly savings was said to be $320. The time to payoff the windows through realized energy savings was 30 years. The average functional life expectancy of a window, 20 – 25 years., although as we can see here, it can certainly be much longer.
One parameter that can not be well estimated is the actual draftiness of the windows. Air infiltration through leaky old windows can be an immense factor not just in comfort, but energy costs. Therefore it can be reasonably inferred that the figures just given may be considered conservative. That due to the elimination of drafts, the energy savings and more importantly the comfort level of the home would be greatly improved. And what is the value of comfort?
Here’s another example. The window in the second IR image at is about 12 years old. It is a newer, double pane window. It’s a low quality, “spec” window that is substantially leaking air, thus undoubtedly causing energy loss and discomfort. At the bottom of the image is a baseboard heater. The low quality of the window being such that the heat from the baseboard has little effect on the glass.
The last IR image is of a good quality replacement thermal double pane window. Notice the glass is about the same temperature as the walls around the window. One condition here that is worth mentioning is the location of the forced air heat supply. The register is located well above and to the right of the windows. The cold area seen at the base of these windows is actually aluminum foil the homeowner placed there, probably due to a perceived draft. The foil is reflective to IR as well as conductive, thus appearing “cold”.
What is evident is that replacing windows to gain energy savings is a complex process. Changing windows can be in some instances a bad energy saving choice, but can typically improve comfort.
The National Fenestration Rating Council window tag should be present on any window anyone is considering purchasing. Manufacturers voluntarily submit their products to NFRC for certification. However in order to meet the ENERGY STAR® criteria a product must be NFRC certified.
A detailed explanation of each criteria found on the NFRC Label can be found on their web site @ www.nfrc.org/label.aspx
When considering replacing windows, an independent energy audit would be highly recommended. Through the audit process the cost effectiveness and return on investment of a window replacement can be sensibly evaluated against other improvements found for the house. If the audit determines a window replacement makes economic sense, using the quality parameters of the NFRC should aid in selecting the highest performing window for the improvement budget.