After years of somewhat janky solutions, I'd like to figure out the correct way of installing a window air conditioner in the window of my office. The windows are probably not original to the house. They have a plastic frame. From the side, the profile would look something like this:

enter image description here

That is, on the inside, the plastic frame extends up about an inch above the wood sill. There's a second thin strip of plastic that seals the window on the outside when it's closed, and it's a chunk shorter. Beyond that, the plastic frame extends beyond the wall (so there's not really a solid surface into which things can be screwed on the outside).

What's the correct device for mounting a (small-ish) air conditioner in this window? A lot of them seem to assume you can lay a flat surface across the bottom of the window from the inside to the outside, but that's not the case here.

I've tried something that looks like this, but it never really seemed to work: it was fiddly to install, and never really seemed secure. We haven't had the a/c fall out or anything, but I'd like to figure out how to do this right.

  • With a similar-but-different issue between wooden window frames and external aluminum storm windows not allowing a standard A/C to sit correctly, I have used styrofoam (where only support was needed) and wood (where a screw was needed to hold) strips to fill in the odd-shaped bits of the bottom of the window to allow use of a window air conditioner. Removable and store with the A/C over winter.
    – Ecnerwal
    May 28, 2021 at 17:50
  • Put in a strip of wood that's at least as tall as the thin plastic on the inside of your office. The weight of the entire A/C should rest on this wood, not the thin plastic.
    – MonkeyZeus
    May 28, 2021 at 19:40
  • The block of wood on the sill is tertiary. A block of wood jammed in there vertically that keeps the lower sash from raising is paramount. The four screws you're probably not willing to damage the window by inserting are secondary; do them anyway. Drill new holes if you have to. If anything, a block on the sill just helps you do those two things. Unless you're a shim master, when it's all said and done (right) it's probably not sitting on it anymore anyway.
    – Mazura
    May 28, 2021 at 23:14
  • what about one like this? amazon.com/dp/B07SXDYL4R/ref=syn_sd_onsite_desktop_55
    – Jasen
    May 29, 2021 at 2:33

1 Answer 1


Based on your diagram, more or less what I have done. Jankiness level is variable depending on level of craftsmanship employed, or not.

The wood strip may need to be higher than the "foam or wood" strip as many air conditioners expect to find an edge to "push" against there. That's also why you might need screws to hold it in place (the A/C wants to rotate out at the top and in at the bottom. You want it to be held by those forces, not to give into them and fall.)

Modified sketch

  • Make the wood sill the full width of the window to protect the thin plastic. In most cases you won't need the foam channel filler. May 29, 2021 at 0:42
  • The filler helps to reduce greatly the odds of dropping the thing while installing/removing it, by shifting the pivot point of the device when it's free of the constraint of the window sash at the top. It also helps (in this case) to prevent crushing the outer plastic lip of the window frame while wrestling the thing in or out. I highly recommend using it.
    – Ecnerwal
    May 29, 2021 at 1:54

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