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I have a window a/c unit (~23"w) I install into a small window (30"w) every summer. What are the most efficient materials to use to maximally seal the large & small gaps to keep the hot outdoor air from entering the room? I removed the a/c fins as they do nothing to insulate. I used towels & duct tape in the past but am looking for better and more efficient solutions. Thanks

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    Thick foam insulation panel, cut to size, is what I use. – Jimmy Fix-it Jun 1 '18 at 19:33
  • The fins do nothing to insulate but they're a 1000x better than cloth at air infiltration. Until you've air-sealed, r-value is meaningless. – Mazura Jun 1 '18 at 19:42
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    I also like the rigid foam it is easy to cut to size. I did see a person that used plastic wrap and injected expanding foam, the plastic kept the spray foam from sticking to the window. – Ed Beal Jun 1 '18 at 19:43
  • @EdBeal That should probably just be an answer – Hari Ganti Jun 1 '18 at 21:07
  • Ah, that's where my mind was heading. I was eyeing this from a big box store...Owens Corning FOAMULAR 150 1 in. x 4 ft. x 8 ft. R-5 Scored Square Edge Insulating Sheathing. What do you recommend I use to fill small air gaps left over? Weatherstripping or caulk? – catjacent Jun 1 '18 at 21:54
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I also like the rigid foam @jimmy fix-it commented. it is easy to cut to size. I did see a person that used plastic wrap and injected expanding foam, the plastic kept the spray foam from sticking to the window. He folded the plastic in 1/2 and used the nozzle to shoot the foam in but not two much the plastic keeps the spray foam from sticking to the foam board and window it also locks everything together once dried and is easy to use a box knife or steak knife to cut the spray foam on 1 side then it pops out the foam board can be used again next year.

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