window performance question

renbelleOctober 6, 2010

We installed a couple of new windows this summer and we're planning to replace the rest of the windows in the house. Now that the weather has cooled, I was surprised how cold the window glass gets on the interior. Anyone know if the following temperature measurements seem reasonable for a double pane, argon filled, low-e coated window?

* Exterior temperature approx. 55 degrees

* Temperature of exterior glass approx. 50 degrees

* Temperature of interior glass: 66 - 67 degrees

* Temperature of interior window trim: 69 degrees

* Temperature of interior glass of existing SINGLE PANE window: 61 degrees (note: there is a storm window covering about 3/4 of this window, but the measurement was taken where the storm window is not covering the window glass)

I realize that double pane windows will get pretty cool in the depth of winter (as I understand it, the interior glass in a window like ours will get down to about 55 degrees when the exterior temperature is 0 degrees). I was surprised they got so cool already, and that the interior glass temperature is so similar to a single pane window. Just want to make sure there is nothing wrong with the windows before we complete our investment.

Thanks for the help.


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Windows on Washington Ltd

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The interior surface temperature of a single lite of glass, when the temperature outside is 0 degrees F and the inside air temperature is 70 degrees, will be about 16 degrees.
Add a storm window on the outside and the surface temperature of the inside lite jumps up to about 43 degrees � a huge improvement.

But these are center-of-glass readings and not the temperature readings at the edge of the window where condensation usually forms. A typical clear glass dual pane window is going to have center-of-glass temperature reading pretty much the same as a single pane with a storm.

However, if that dual pane has a LowE coating and an argon gas infill then the center-of-glass temperature will be about 57 degrees � a 14 degree improvement over a clear glass dual pane or a single pane with storm window � but again, and more importantly, there will be a comparable edge of glass improvement as well, particularly if the IGU (Insulating Glass Unit) was manufactured using a warm edge spacer system........

The numbers above are laboratory numbers and are verified by repeatable conditions. There are some variables at play in your situation that are slightly skewing the data.

You can see that the realistic temperature differential should be between single pane, single pane/storm combo, and double pane insulated.

    Bookmark   October 6, 2010 at 10:01AM
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Thanks, this is very helpful. It seems that all of the test data is based on 0 degrees exterior temperature. Do you know (or anyone else know) what the data looks like as the outside temperature changes? Is the relationship linear (so the differential between single, single w storm, double, and double with low-e remain constant)? Is it curved?

I'd love to know so I can evaluate our experience given the current weather conditions. In our current weather, the dual pane, low-e window is only 6 degrees better than single pane...

By the way, the spec's on our new windows are:

* double pane
* argon gas
* warm edge spacer
* U value = .27 (NFRC total unit), .24 (center of glass)
* SHGC = .19 (NFRC total unit), .27 (center of glass)

    Bookmark   October 6, 2010 at 2:44PM
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