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 (NFRC numbers are U = .27  and SHGC = .19)?

• 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.

Jim

ACCORDING TO CARDINAL CORP (WWW.CARDINALCORP.COM) IF THE OUTSIDE TEMP IS 20 DEGREES THE INSIDE GLASS TEMP OF A CLEAR DOUBLE-PANE INSULATED GLASS SHOULD BE AROUND 49 DEGREES. SO YOUR WINDOWS SEEM TO OPERATING CORRECTLY. IF YOU HAD LOE GLASS IN IT THE TEMPERATURE WOULD INCREASE TO 58 DEGREES.

Jim-

Heat transfer can be complicated, but I would make the assumption that the heat transfer could be approximated as proportional to (Troom - Tinside window surface), where Troom is the room temperature and Tinside window surface is that temperature. You do not give the room temperature, but let us assume that it was 72 degrees F at the time of your other measurements. Then for the double pane window, the heat transfer is proportional to 72 - 66.5 = 5.5, while with the single pane window, the heat transfer is proportional to 72 - 61 = 11. This would mean that the heat transfer rate is cut in half when switching from single pane to double pane windows, which sounds like what I might guess before seeing your measurements, and therefore, is very reasonable.

Your measurements showing exterior glass being 5 degrees F lower than the air temperature sound suspicious, but that is beside the point.

If you are in a mostly heating environment and you have some solar radiation available, then you should investigate higher SHGC windows than the 0.19 ones that you are using, especially for south-facing windows. It should be possible to find ones near 0.50, which, if you have sun to work with, would be well worth the small increase in the U-factor. (See previous discussions at the this site on this topic, and use DOE's RESFEN computer model to quantify.)

Lee

Thanks so much for the help. This is great, and makes a lot of sense. We're in Virginia so we have to manage heating and cooling for the house. There is very little I would say is optimized in this old house, but we're trying to make improvements as best we can.