Saturday, March 22, 2008

Double-Pane Windows

Yesterday I ordered our new double-paned windows. This is a major investment, but one I expect to pay off in ten years even without factoring in any increase in electricity prices. Selecting the best windows is not easy though. Manufacturers quote U-value, Solar Heat Gain Coefficient (SHGC), Visible Light Transmission (VLT), and sometimes R-value and Ultra-Violet Light Transmission. I will try to explain what they all mean, insofar as I understand them myself.

As far as I can make out, the R-values and U-values are reciprocals of each other, so quoting both is redundant. They measure thermal conductivity, and as a general rule it seems that U-values are used for windows and R-values for walls and roofs. A high R-value (i.e. a low U-value) indicates a good insulator. R-values for roof insulation are typically over 10. However, the interpretation of U- or R-values for windows is problematic because heat transfer through the window is not only due to conduction, and indeed maybe not be primarily due to conduction. I think this depends upon the direction of the heat flow; it hot climates when the house is being air conditioned, a lot of the heat coming in is radiated, whereas in colder climates when the house is being heated most of the heat going out is conducted.

Living in Houston, I decided the SHGC number was the one to concentrate on. This measures the proportion of radiant energy falling on the glass which is transmitted rather than being reflected. Good windows by this measure have SHGC of .18 or .2. There is a trade-off however, in that VLT tends to be lower too. VLT numbers are deceptive however; I saw an installed window with an SHGC of .18 and a VLT of .27. It was noticeably different from plain glass, but it did not seem subjectively that only 27% of the visible light was transmitted. Note however, that even plain glass transmits only about 60% of visible light, and I think these numbers do not match our subjective experience. (A bit like decibels and sound sensations.) The glass manufacturer offers an alternative which looks like it might be a better compromise, with SHGC of .2 and VLT of .44. I have left the final decision on this until I speak to a technical expert at the factory next week.


RosInSheffield said...

Hi Tony,
You don't mention anything about the frames for your windows. When I replaced my old single-pane windows with double-glazing, I chose the usual thing - uPVC coated metal frames. The windows have been excellent as far as insulation is concerned (and are much more secure than the old windows and doors), and have been maintenance free so far (nearly 20 years), but uPVC is a pretty nasty substance as far as it's manufacture, and its eventual disposal, are concerned. If I were doing it again now, I'd want hardwood frames (from sustainable sources, of course!) - but they're much more expensive.

Tony Welsh said...

I must confess I did not pay much attention to the frames, but my understanding is that they are all vinyl, not coated metal.