Mounting a window air conditioner on top of a sliding horizontal window

Question

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:

This is how that window looks from the outside:

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

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:

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.

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.

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.

Answer 2

I have a right to left aluminum sliding window and a huge old AC. I use a full sheet of construction foam board ($45 a sheet). I cut the sheet to the overall size of the window and then cut out the bottom of the sheet to fit the AC. I center the AC on the sill of the open window and then slope it to the outside with a block of wood or a book. Once I've fitted the precut sheet into the window, I seal all of the opening with weatherproof tape. I don't use the supplied installation brackets because they do not work with the type of window I have. If I do need a bracket, I'll make one out of wood if necessary. Wood brackets look better, in my opinion. I have never had to brace the AC on top even with the 20mph winds we have although I really think I should.

I'll be installing a second AC using shelf brackets to support the exterior of the unit and filling in the difference of the opening with foam left over from the first nstall.

I've seen some installations where the owner supported the AC dead center from underneath using a 4x4. This could work if the window sill was thick enough and properly supported underneath.

No matter. You have to do something to get out of this heat. Have fun.