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Old 10-06-2010, 08:18 AM   #1
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window performance question

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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Old 10-06-2010, 10:32 AM   #2
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what brand of windows, and what is/was the specs on them? sadly, not all new windows are created equal. for that matter, depending on the age of your original windows, and their condition, one school of thought is that it is a better investment to add high quality storm windows than to replace the old ones. you can take it even further by going with the interior storm windows, which are supposed to provide the best air seal.


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Old 10-06-2010, 12:42 PM   #3
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Thanks for the quick response. The new windows are:
  • double pane
  • argon gas
  • warm edge spacer
  • U value = .27 (NFRC total unit calculation), .24 (center of glass)
  • SHGC = .19 (NFRC total unit calculation), .27 (center of glass)
Unfortunately, our original windows are a mess. The house is 82 years old and had a recent, lengthy spell in which it was not well maintained. Missing glass panes, gobs of built up paint, weathered sashes, spaces between sash and frame, sash chords busted, storm windows bent, you name it. We considered repairing everything but the cost was too high.

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Old 10-06-2010, 02:06 PM   #4
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The interior temperature of the window (measured on the inside face of the glass) will be approximately the inside temperature of the house. The heat loss through the window will equal the temperature difference between the inside pane of the window and the outside temperature, multiplied by the area of the window, multiplied by the U factor (reciprocal of the R rating). The main difference between a well insulated window and a poorly insulated window is not the temperature differential across the window, but the heat loss, due to the improved U factor of the better window.
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Old 10-06-2010, 02:25 PM   #5
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That makes sense. If I understand correctly: the room temperature (and the temperature of the interior glass) will be held more or less constant by the house's heating system. With a better insulating window, less heat will be lost and the heating system won't work as hard. With a worse insulating window, the heating system works harder. But in both cases (e.g. my double pane and my single pane windows), the temperature of the window glass will be the same.

It seems pretty obvious now :-) Can I ask one clarification? According to the data available from the web, a double pane, low-e window will have an interior glass temperature around 55 degrees when it is 0 degrees outside. I assume this is because the window isn't a perfect insulator -- if it were, the interior glass temperature would match the interior room temperature no matter what is going on outside. So the better the insulating window (the lower the U rating / higher the R rating for the window), the less the interior glass temperature should diverge from the room temperature? In my case this morning, the double pane window's interior glass temperature diverged from room temperature about 1/2 that of the single pane window... giving a sense of the difference in insulating effect.

Is that all hooey, or does that make sense? Thanks again.

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