Low-E vs Storms on Gothic Window

John_of_MPLSJanuary 21, 2005

We have 1931 Tudor in Minneapolis that has a gorgeous gothic arched window approx 5 ft by 9 ft that over looks a park. It has 26 full-divided panes throughout that curve into the arch. We also have a storm that matches this window. They are a beautiful match and we love the way the two windows look. Unfortunately, but not suprisingly, the exterior storm is rotting away and we must replace it. We have spoken with a fellow how does replacement windows, done at a millwork, like this and he is willing to do it for us, a copy of what we have. We had a very cold spell last week and we noted how much cold was radiating off of the window and began to wonder if we would be better off going with a more modern window that provides more insulation. I called him this morning and asked what he thought. I suprised to hear him say that most low e windows have an R factor of 3 while a single pane window with a tight fitting storm (wood frame for both) has an R rating of 2. If this is the case, we are loosing a lot of beauty and maybe not gaining that much in insulation value. I'd be interested in anyone's 2 cents worth on this question. Thanks much!

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I would agree with him completely, and even add one more thing. Low-e windows (thermo-panes) still use a metal spacer around the edge, so they tend to run cold at the edges compared to the middle. The separate storm and window with wood frames has no metal path and will be much warmer at the edges. This is a good thing, as the bottom edges are where you get the most condensation and ice. When we switched from the separate pane add on storms on our Andersen windows to low-e Heat Mirror glass (R-7) we actually saw an INCREASE in the amount of ice on the bottoms of the windows, at any given relative humidity. To get the best of both worlds, you could do a thin thermo-pane (low-e or not) and still use a separate storm window. You would get a much warmer window and keep your original look.

    Bookmark   January 24, 2005 at 6:28PM
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Just a few thoughts...

R value is not the best measurement of a window's energy performance. U value is a much more accurate measure, but unfortunately most window sales folks still use R value in their presentations because most people have been conditioned to think in R value.

LowE is a metallic coating that is applied to one (or more) lites of an IGU (Insulating Glass Unit) or thermo-pane window. Not all IGU's use LowE coating and not all IGU's use an aluminum (metal) spacer. The top-of-the-line IGU's manufactured today use what is called WET or Warm Edge Technology spacer. There are several different WET spacers in use today, including a couple that are stainless steel based.

LowE coatings substantially increase the energy efficiency of a window. LowE coatings are required by energy code in many parts of the country for new construction.

Having storm windows is better than not having storm windows, but the energy performance of a single pane window with a storm window is not comparable to a modern IGU-equipped window with LowE and argon fill. There really is no comparison in energy performance numbers or actual field measurements.

John, you have classic old windows that might be worth saving for the asthetic value, but you will lose something in energy performance if you do so. I really like older windows, personally, and if the original windows are in good enough shape to save, I would attempt to save them...but that would be a trade off in performance versus new windows. If you can afford it, you can get newer windows that look every bit as good as the originals with much better energy perfomance.

Something over 70% of all LowE coated residential glass in North America is manufactured by Cardinal Glass which happens to be headquartered in Eden Prairie, Minnesota. They also happen to have an R&D lab in Minneapolis, which is a local phone call for you I assume...Cardinal supplies most of the major window companies in North America with LowE glass and also many of the big window companies with IGU's as well.

    Bookmark   January 26, 2005 at 5:16PM
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