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I have three old window units in my house and they function as my A/C. I am concerned about the amount of heat seeping into the house and cool air leaking out around the A/C units themselves. They're not fitted to the windows, and we have to use the accordion style panels to prevent drafts, but they're not air tight or very good at insulating.

How can I reduce heat transfer around my window unit air conditioners?

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See my answer to this question giant sliding window portable air conditioner installation. You should be able to get a better seal if you build a custom frame for the units. – Tester101 Jul 30 '11 at 13:13
up vote 5 down vote accepted

Two things come to mind:

  1. Heat loss occurs much more rapidly when airflow is involved. So do whatever you can to seal all those gaps. Buy some compressible foam insulation (about 1" wide) and use it to make a tighter fit around the air conditioners.
  2. Those accordion things don't seem to have much R-value. Buy some rigid foam insulation and cut it to fit snugly behind the accordion. You could even use some of those compressible foam strips to get an even tighter fit.
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Where would I find rigid foam insulation? – Malfist Aug 1 '11 at 17:30
Any home improvement store (Home Depot, Lowes) carries rigid foam insulation. Usually it comes in big panels that you will have to cut to fit (utility knife works well). – msemack Aug 1 '11 at 17:48
Or if you happen to have the materials lying around to make a hot wire cutter, that works even better. (Or you might be able to buy one that would be a bit safer than a homemade one.) I've rigged up something like this in the past out of an old power supply and scrap lumber. – Doresoom Aug 1 '11 at 18:51

Here is a video and pamphlet showing how to do what rifaco suggests, as well as some other things to improve window AC efficiency.

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