Cover for hole in wall used for window air conditioner.

Window A/C Through The Wall:

Building A Wintertime Cover For An Air Conditioner Wall Opening

In This Article:

After jambs and exterior trim are applied to the rough opening, plywood is cut for the outer cover. Once the air conditioner retaining structure is installed, the inner cover is fabricated from plywood and foam insulation.

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Skill Level: 3 (Intermediate) Time Taken: About 2 Hours

By , Editor


In my article about finishing the rough opening for an air conditioner, I showed how I trimmed out this opening and installed the supporting structure for the air conditioner.

At this point I had installed a thin piece of plywood on the inside of the opening to keep out the elements.

Opening in wall for window air conditioner.


Window air conditioner being test-fitted in hole in wall.

Before going any further, I test-fitted the air conditioner in the opening.


The Starting Point For The A/C Hole Cover:


I rigged up this cover from a piece of 1/2" high-quality plywood

The bungee cord and screw-eyes make a convenient handle to hold the cover while setting it in place, but they also allowed me to temporarily hold the plywood in the A/C opening by inserting a piece of wood behind the bungee cord.

Outer cover for air conditioner hole.


Outer cover for A/C hole in wall. Those two unpainted blocks of wood are just pieces of 2x4 that were positioned to lay next to the bottom support block (1) which is mentioned in the companion article about trimming out the opening.

Also, I rigged up a couple of barrel bolts (2) to hold the outer cover panel loosely in place.


When I built the outer cover, those 2x4 blocks were ripped narrower on a table saw so that the face of the 2x4 (A) aligned with the face of the upper board, or false-sash (B). In my case, these blocks ended up being 3-1/8 inches wide.

I wanted the surfaces of the 2x4's and the false-sash to form a flat plane that would serve as a backing for the inner cover panel.

Air conditioner opening, cover panel, surfaces for inner cover to rest against.


Air conditioner opening with outer cover in place. From the outside, the A/C opening is not overly conspicuous... it almost blends in with the siding.


The Inner Part Of The A/C Opening Cover:

The thickness of the inner cover needed to be about the same as the distance from the false sash to the inside face of the wall, which in my case was 1 inches. Distance from wall surface to air conditioner mounting surface.


Portion of inner cover panel for window A/C opening. To create an insulated cover for the inside of the air conditioner opening, I  built this box from 1x lumber and thin Lauan plywood. There will be another piece of heavier plywood covering that blue foam insulation.

I used a piece of 3/4" foam and a piece of 1/2" foam to fill the cavity inside the box.

The side boards were cut to 1 inches wide. When fastened to the Lauan plywood (about 1/4" thick) the assembled box was close to 1 inches thick.

IMPORTANT: I built this box slightly smaller than the opening it fit into, so I could wrap foam tape around the perimeter to act as a gasket. Since the foam tape I used was 1/8 inch thick, I built the box about 3/16" smaller in length and height than the opening. Why 3/16" smaller instead of 1/4" smaller? By creating a slight "interference fit", the foam tape should be compressed slightly, (hopefully) sealing up the gaps around the perimeter of the box.

To make this cover fit snug yet be removable, I used a belt sander to sand down the sides of the box to form a slight taper (red arrow).

Note that the thin plywood (seen at the bottom of the box) will be on the back of this cover when it's completed.

Tapered edges on box made with belt sander.


Testing fit of inner cover panel in A/C opening. To test the fit, I used masking tape to temporarily fasten foam tape around the perimeter of the box.

(This foam tape came in 2-inch widths, so I had to cut it narrower with scissors.)

The foam tape I used is sold for the purpose of insulating water pipes, and may be available at Home Depot, Lowes, or other hardware stores.

After I had sanded the box edges to create the tapered edges, the box fit snug, yet was not difficult to remove from the opening. A/C opening with insulated cover in place.

Note that I also applied some basic window casing around the air conditioner opening. This was just "modern" casing that was installed backwards, with the thick edge towards the inside of the frame.

Fastening heavy plywood to insulated box for cover panel. I put masking tape on the casing to mark the locations of the 1x boards around the perimeter of the box.

Then I set a piece of 1/2" plywood into the opening formed by the backwards window casing, leaving a small gap around the edges.

I drove some short deck screws through the 1/2" plywood to reach into the side boards of the box (which was still sitting in place behind this new plywood).

Why assemble the cover this way? Theoretically, it would be possible to measure the distances from the edges of the outer panel (the 1/2" plywood) to the edges of the box, and fasten the two components together on the workbench, and make the cover fit nicely inside the casing. Yeah... Good luck with that!!!

I have enough experience with carpentry to realize that there are so many small discrepancies and inaccuracies in this type of work to make that "pre-assembly" method doomed to fail. Setting the rear portion in place, then setting the 1/2" thick front panel in the opening and then fastening the two together is the only reliable way I can think of to make this cover fit properly the first time.

(Note that I could've installed that casing after assembling and installing the cover, but then the casing might not look right, with different reveals on every piece.)

Once the two sections were screwed together, I removed the cover and drove some additional fasteners (1" staples) to secure the parts better.

Then I primed all the bare wood and applied the self-adhesive foam tape to the edges of the box.

Inner portion of cover for window A/C hole in wall.


Securing The Inner And Outer Covers:

Then I set the completed inner cover in place and drove some 3-inch screws through the inner cover and into the outer cover (the green-painted cover).

These screws were positioned to align with the 2x4 blocks of wood that I attached to the inner surface of the outer cover.


The completed inner cover for the air conditioner opening.

I used 4 screws to connect the inner cover to the outer cover. Note that these screws will need to be removed every summer when it's time to install the air conditioner.

Air conditioning opening in wall with cover panel installed.

More Info:

Tools Used:
  • Basic Carpentry Tools
  • Miter Saw
  • Table Saw
  • Cordless Drill/Driver
  • Cordless Impact Driver (Optional)
  • Pneumatic Stapler (Optional)
Materials Used:
  • 1/2" Premium Plywood
  • Lumber, 2x4, 1x2
  • Foam Insulation
  • Insulating Foam Tape
  • Deck Screws, Misc.
  • Finish Nails
  • Barrel Bolts
  • Screw Eyes
  • Bungee Cord
  • Masking Tape
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Written July 10, 2010