I have a brand new AC in the box which was meant for another room, but recently had a baby and her room is too hot.

I want to use the AC I already purchased, but the max window width is 36" and the window in her room is 48"

Is it possible\safe to use plywood or some other material to act as the AC curtain to make up for the remaining 12" of space? Cant seem to find anything on the web. (perhaps I am searching for the wrong keywords)

  • This Answer might be helpful.
    – Tester101
    Commented Jul 16, 2012 at 11:43

2 Answers 2


You could use exterior grade plywood, but you should still seal it on both sides with paint or clear coating suitible for exterior use. It still may be somewhat unsightly on the inside.

As an alternative, I have regularly used plexiglas type plastic to fill in the spaces along the sides of air conditioners. This avoids weatherproofing and appearance issues. It also give you a bit of a view to the side of the air conditioner (this was critical in my New York apartment where the peek-a-boo river view was in the only window that could take the air conditioner).

The easiest way to use this system is to remove the expandable plastic filler frame altogether. The accordian plastic is prone to leaking air anyway. While the filler sides do not provide much support, they do provide a bit, so it is essential to make sure the air conditioner is well attached behind the upper sash, prefereably with screws, and that the upper sash is blocked from opening with a bracket of some sort.

Cut the plexi to fit. You do need to seal the plexi on all four sides. One way to to this is to create a molding one side of which will press against the face of the plexi (on the inside surface) and the other side against the side of the window frame, the sill and the upper sash and against the air conditioner itself. Silicone sealant or similar latex glues can be used to glue the plexi and the molding. These can generally be removed fairly easily with a razor blade if needed.

  • Great advice - thanks! I didnt even think about plexigalss. One question - how does one cut plexi? Commented Jul 14, 2012 at 21:20
  • There is a scoring tool to cut it. It is a small hand tool with a sharpened hooked end that you pull along a straight edge on the surface of the plastic. Make several repeated scores in the same line. Place the scored line of the plastic sticking out slightly over a firm edge and press down. The plastic should crack neatly along the scored line. It sometimes helps to begin the pressing nearer one ned rather than in the middle.
    – bib
    Commented Jul 14, 2012 at 22:11
  • 1
    If the sash is part of a vinyl window I would support the a/c unit with a metal bracket. A 48" vinyl sash may bow or deflect enough for a/c unit to fall out.
    – mikes
    Commented Jul 14, 2012 at 22:58
  • 1
    mikes has a great point. if you use an L shaped metal bracket (aluminum stock, e.g.) across the top of the air conditioner, you can set it up so that the AC is firmly attached to the bracket and the window frame without relying on the sash. Then, in milder weather, you can turn off the AC and open the sash without removing the AC unit.
    – bib
    Commented Jul 15, 2012 at 0:14

I had the same problem AC fit a 36 inch window but the bedroom window is 52. I happened to find a piece of already cut plexiglass. couple of things your going to want to paint the plexiglass regardless if you have blinds because they cant block off the open window area .If you have curtains then it will be fine. Otherwise treated plywood will do the trick just as well.

Paint it before you put it in the window and put it in the window before you install the AC unit that way you are installing the ac unit into area with the dimensions appropriate for your unit. Also when you buy the plexi or plywood pick up a piece of foam to cover it as well to help with insulating the unit as well kind of like the heat shields that come with most new window units.

  • I had the accordian pieces from an old, dead AC. Pop riveted those to the end of the pleats on the new AC, and got enogh length to seal the window properly. Commented Jun 27, 2018 at 23:11

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