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I have an air conditioner that looks similar to this...

Air conditioner

...except mine doesn't have the flaps on the sides, leaving a large open gap between the air conditioner and the window frame. What's the best way to fill that gap?

The air conditioner gets taken down in winter, so permanent solutions are no good.


The ideal solution would:

  1. Be easy to make
  2. Be easy to put up/take down
  3. Insulate well
  4. Keep rainy weather out, and not be ruined by rain
  5. Keep nosy cats inside

We used to shove blankets in there, but the cats got out. I also tried buying flaps similar to the image, but they failed all 5 tests. I've heard suggestions of cutting out plywood or rigid foam, but I'm curious if there's a simpler and/or better solution.

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I think the simplest thing would be to buy a sheet of foam insulation from the home store. It comes in large sheets so you'll probably have to cut into pieces to get it home, but its easy to work with.

It ticks all your boxes:

  1. Cuts with a razor knife or hand saw
  2. Very lightweight
  3. It literally is insulation
  4. Water proof foam
  5. Well, a determined cat could scratch it up or eat a hole in it, but they would have to really hate the foam. It's pretty durable.

The downside: One style is pink. The other style is usually yellow, but has a white printed side and a shiny foil side. They can both be painted with exterior house paint (just buy a color sample size of paint), but the paint might not be super durable on that surface.

For installation, duct tape will work well, and duct tape comes in a variety of colors. If you don't want duct tape to leave residue on your window, you can put down a layer of blue masking tape to protect the surface and then duct tape to that.

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  • Does it need to be painted? I don't care about the color, but IIRC that stuff deteriorates in sunlight? Jul 22, 2019 at 20:15
  • Here is a tech bulletin from one manufacturer. It claims that you don't have to worry about issues until after 60 days and even then: " Beneath the thin UV damaged layer of cells on the surface are millions of fresh cells that are undamaged." So, the sunny side will get a powdery residue on it, but that's about it. Foil backed insulation is totally impervious to the sun.
    – JPhi1618
    Jul 22, 2019 at 20:22
  • Direct answer - no, it doesn't need to be painted. Some people would scoff at the idea of having pink insulation in their windows, but if no one else can see it, doesn't matter.
    – JPhi1618
    Jul 22, 2019 at 20:26
  • Hrm. What about health concerns? My experience with insulation is that it's not something you generally want to be handling once it's put it, but I'd need to move this in/out of the window at least twice a year. And "powdery insulation residue" sounds exactly like something I wouldn't want to be in the air. Jul 22, 2019 at 20:29
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    Concern about insulation is normally directed at fiberglass insulation and blown in cellulose insulation (not as bad as fiberglass) because of the amount of dust they have. Fiberglass is very itchy to handle and if it causes dust, that's not good at all to breath. The small surface area of foam we're talking about isn't going to generate much dust over the course of a season, and if you're that worried about it, just paint the outside. If you've never handled it, it is a smooth foam like a foam disposable plate or take-out container. It's not the messy type of foam that is used in shipping.
    – JPhi1618
    Jul 22, 2019 at 20:33
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another good holding suggestion is gaffer's tape - it's legit designed to not leave sticky residue, and if you get the good stuff, can hold up to a lot.

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    How are you recommending applying the tape? Just fill in the openings on either side of the window? If so, I don't think gaffer tape will hold up all that well in the rain. Also, it will leave a residue if it's left on long enough, especially to warm things... don't ask how I know. :(
    – FreeMan
    Jul 29, 2022 at 12:14
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    This doesn't really sound like it's easy to put up and take down. you'd have to re tape each time.
    – JACK
    Jul 29, 2022 at 12:34
  • Welcome to DiY, and thanks for contributing! Seconded on the leaving residue bit though. The adhesive layer will after a few months (faster if exposed to heat), turn into a hard yellowish layer that's difficult to remove (organic solvents work, but they may affect the surface on which the tape was applied). This also means that any composite surface you've taped together from it will start falling apart.
    – MiG
    Aug 4, 2022 at 8:16
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It may be obvious, but please note (clear in that picture) the side panels, with the wood screws installed are also playing a role in holding the mostly open bottom (inside) sash lowered and in place. That bottom sash is the predominant (only?!) part of holding the A/C unit in place. Be sure you have the brackets installed that prevent that bottom sash from opening - thereby letting your A/C unit fall out!

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Consider using cardboard. Just simple cardboard from a box. The flaps from appliance boxes, that are coated in plastic for printing, are perfect.

I used cardboard for years. Just this year I bought a new window AC with the frame and flaps, and I miss the cardboard - it was so much easier and doesn’t require drilling holes!

  • You probably already have some around
  • It’s easy to adjust
  • Can be layered
  • It’s pretty rigid and sturdy
  • It removes easily
  • Scrap cardboard is easy to fold up and stuff underneath to angle to ac out the window for drainage
  • It never seemed to get rain on it, I was able to reuse the same cardboard.
  • It has held up to high winds, storms, and nosy cats (my own included).
  • I don’t know the insulation value of cardboard, but I also don’t know the insulation value of the cheap vinyl accordion flaps that come with window ac kits.

I’ve used tyvac tape to seal the edges in the past. Surprisingly left no residue at all, unlike every other tape I’ve tried. I’ve also stuffed and sealed the edges with putty.

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