My office is on the 3rd floor and it's pretty hot in the summer. I wanted to get a window AC unit, but the windows are dormer windows, so there is no outer vertical surface on which to mount a support bracket (diagram below). Basically, I'm having the same problem as this person.

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I want to get a portable AC unit, but they all claim to need 20"+ of clearance on each side. Can I set the portable AC unit back in the dormer, closer to the window, and let it blow down the "hall" of the dormer? Will that give it enough airflow on all sides?

For reference, below is a picture of the attic dormer. Here is the AC unit I am looking at, and here is the doc (page 6) where it talks about the space requirements.

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  • yes you can blow down the "hall". You might want/need to add a small pedestal or box or table fan to assist with moving the air if you discover hot pockets in the larger bedroom.
    – dandavis
    May 12 at 20:43
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    Nowhere that I've had a window unit, have I had a support bracket. Perhaps I've just had smaller units, but they have been supported by the window frame itself. They usually include a small 'L' bracket to screw between the window sash and the frame, to prevent accidentally opening the window - which is necessary to keep the AC from falling out.
    – Glen Yates
    May 12 at 21:01
  • @GlenYates I should add that my window sill (when I open the window) isn't flat for the AC unit to rest on. There are plastic tabs/tracks that the window sits down in when closed.
    – Logan M.
    May 12 at 21:06
  • Bad idea. See this. (feel free to skip from 1:06 to 4:50). youtube.com/watch?v=_-mBeYC2KGc May 13 at 2:37
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    @FreeMan - I've been talked out of a portable unit and did as you suggested and built up both window sills using some scrap pieces. If you want to submit your comment as an answer, I'll accept it.
    – Logan M.
    May 17 at 2:46

3 Answers 3


I've seen many AC units with outside support brackets, but I've seen many more without them.

Since your window sounds like a replacement window where the tracks encroach on the opening instead of being placed fully within the framing, I'd suggest that a piece of 2x4" on the inside sill, and possibly another on the outside sill, plus a piece of foam between them to insulate/keep creepy-crawlys out will give you a nice surface to sit the AC on without an external support bracket.

You might still need something to keep the window sash tight against the top of the AC unit to keep it from pushing the sash up and tipping in or out.


I've had a similar problem: the window was too tiny to fit a window AC unit.

The problem with portable AC units is they have only one pipe to the outside. A normal window AC unit has one airflow through the condenser, which is outside the room, and another airflow through the evaporator, which is inside. But the portable ones suck air from inside the room, blow half of it through the condenser and the other half through the evaporator, and throw away the hot air from the condenser outside. New hot air from outside has to seep into the room to replace the air that's blown out. So the AC unit blows cold air in your face, while hot air seeps under the door, and it's really inefficient.

So I got a portable AC unit that's roughly the width of my window, put it on a support so it would be at the proper height, and built a small frame stuck to its back so the evaporator fan sucks air from outside and blows it back outside.

enter image description here

This increases the efficiency to about the same level as a window AC unit, which is much better.

Oh, I forgot to answer the question. The reason they tell you to leave space around the device is to prevent it from blowing out cool air, sucking it back in, and blowing it outside. With the above, it doesn't happen.

  • 1
    The problem is, this varies by model. Some portables use the same inlet for both the process air (ejected outside) and the conditioned air (circulated inside). I've seen a Youtube video of someone trying to hack a portable to not suck, and the ducting required was considerable... Of course this varies by model, and you can just buy a 2-hose model in the first place. But that would be more money, so no one will do it, even though the money will come back to them in reduced utility bills in the first year... May 13 at 2:36
  • Yeah, you have to check the inlets, and pick a model that has different inlets for conditioned air and processed air... Couldn't find a 2-hose model at the time.
    – bobflux
    May 13 at 6:31

trim (or adjust) the legs so that they reach your roof, if you roof is some fragile material (most are). trim them further and put a horizontal stick of pressure treated timber at the bottom to spread the load.

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