I need to install a window air conditioner in my master bedroom. However, the only suitable window there is a horizontal sliding window. To complicate things more, I need to place it on top, since my king size bed obstructs the lower part, as you can see here:

The bed obstructs the lower part of the window

This is how that window looks from the outside:

The outside of the window

I'm thinking in building a support structure like this one, using 2 by 4 planks:

The outside of the window

And after some thinking I think I can attach the transversal planks to the vertical ones, instead of attaching them to the outer wall, like this:

The outside of the window

And now, I'm thinking I can get rid of the exterior support structure altogether (in red in my last two pictures). The AC unit weights 48 lbs.

What do you think? Is there a simpler/better way to do it? Also, I'm renting, so I'd lean towards a solution that does the minimum damage to the window structure.

  • 3
    Great question and write up. I think I would want 1 of the red supports for stability. If you make the elevated sill as wide as the existing it may work without the support. Remember AC units do vibrate when the compressor kicks on and off.
    – Ed Beal
    Apr 22, 2016 at 13:30

1 Answer 1


I did this, almost exactly as you have drawn (in my mothers home when I was a teenager). You will need to support the back end if you cannot support the bottom "lip" of the AC from sliding forward. In other words, the AC will have torquing-force.

enter image description here

The reason I used a shelf for my AC was because I couldn't support the top. But since you are putting the AC at the top of the window, then you need to support the bottom from pushing both down and inward (or set it on a shelf). You will need to build/cut a groove (not just a front lip). A front lip is bad for two reasons: 1. A front-lip-only would mean that you would be able to lift the back of the AC up (when lift it, the bottom lip will slide out of the back of the window). 2. If you plan to unscrew the lip to remove the AC, it may slip forward suddenly (when the last screw comes out).

You need to consider taking the AC out of the window (and putting it in). So leave a little slop/space/room in the groove to lean the AC forward and lift it out. Normally it's not a problem, because you can lift most windows... but you will be at the top of the window. If the bottom lip is 1 inch wide, you will probably need to cut the groove 1 1/4" wide.

Also, I would add one or two more studs under the "ledge", so that it can't fall.

enter image description here


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