Installing a window air conditioner in a hole in the wall.

Low-Cost A/C:

Window Air Conditioner Through The Wall - Trim Carpentry Details

In This Article:

After the rough opening is made, the siding is cut out and the opening is lined with jamb material. Exterior trim is installed, and pieces of wood are installed to secure the window air conditioner. A plug is made to fill the opening during winter.

Related Articles:
Skill Level: 3-4 (Intermediate to Advanced) Time Taken: About 6 Hours

By , Editor


Earlier, while remodeling this second-floor office, I framed in a rough opening for a window air conditioner.

Since the room had only one window, I wanted the air conditioner to have its own opening, rather than occupying the only source of fresh air (and natural light) in the room.

The black area in the picture is the back side of the old Celotex fiber board that was applied over the studs when this house was built in the early 1960's.

Rough opening for air conditioner.


Cutting fiberboard sheathing with Fein Multimaster. The first thing I did was cut away the Celotex fiber-board. I used a Fein Multimaster to cut through the fiberboard without cutting through the siding.

This could also be done with a sharp utility knife.


After I cut out the fiberboard sheathing, the back of the Masonite siding was visible.

I wanted to leave the siding intact as long as possible, because there were thunderstorms looming on the horizon.

A/C opening, showing back side of house siding.


Measuring the thickness of wall to determine width of jambs. To determine the width of the jambs, I measured from the outside of the fiberboard sheathing to the inside edge of the drywall, which in my case was 4-7/8 inches.

Then I cut my jamb materials to a width just slightly wider than the thickness of the wall.

I used Azek® cellular PVC trim boards for this job. I like cellular PVC because it's stable, won't rot, and holds paint well.


I used a 1/2" spade bit and drilled holes through the siding at each corner of the wall opening. Drilling hole in siding to create starting point for saw blade.


Cutting away existing wood siding with reciprocating saw. Then I stuck a reciprocating saw bit through the corner hole and cut the siding, using the studs as a cutting guide.


After the siding was cut away, I had a nice view, but I also had a liability, since the house was now wide open to the elements. Air conditioner opening after cutting out siding.


Hole cut in wall for window air conditioner. This is what the air conditioner cut-out looked like from the outside.


I installed the bottom jamb first, using 2-inch finish nails.

However...  to prevent water from puddling on the window sill, I installed some shims beneath the bottom jamb. This gave the bottom jamb about 2 degrees of slope. (I nailed through the shims to keep them from moving around.)

Nailing window jambs to framing for A/C opening.


Jamb material installed on all 4 sides of A/C opening. Then I installed the other 3 jambs. I did the top jamb first, followed by the side jambs.

Note that the lower ends of the side jambs were cut to a 2-degree angle to fit the sloped bottom board. I cut these jambs just a hair too long, so they fit tightly against the top and bottom after I tapped them in place with a hammer.


After the jambs were in place there was still a lot of work to do on the outside.

At this point, I laid the "window" casing against the opening and marked the outside edges of the casing, so the siding could be cut away a little more.

A/C opening with jamb material installed.

NOTE: Before laying out the casing, I drew a line on the outer edge of the jambs, about 1/2" back from the inner edge. This gave me 1/2" of "reveal" on the edges of the Azek jambs. Towards the end of this page there is a picture that explains this concept.


A/C opening with siding cut back further to provide place for trim. I cut away the wood siding with a circular saw (set to cut just deep enough to penetrate the siding).

This is not an easy job. The saw spits sawdust in your face, it's difficult to cut across the overlapping part of the siding, and it's tricky to keep your balance while working on a ladder.



Use extreme caution when cutting siding while working on a ladder. Cutting wood siding is tricky enough... the blade can bind or pinch, throwing you momentarily off-balance. When doing this work on a ladder, you need to have a firm footing, a good grip on the circular saw, and be ready for any kickback from the saw. DO NOT reach far beyond the sides of the ladder... take your time and move the ladder so you are working without leaning sideways.


What About Vinyl Siding?

Creating a hole in the wall with vinyl siding should be fairly easy. When I work with vinyl, I usually just remove a few panels of siding around the area, install J-channel next to the trim that surrounds the opening (whether it's a window, door, or A/C opening) and then re-install the siding, cutting notches in the panels where necessary. Under-sill trim may be necessary below the opening, to secure the top edge of the siding when it's notched for the opening.


Making The Exterior Trim From Azek®:

The side trim boards were just simple flat boards cut to about 2˝ inches wide, with the bottoms cut to a 10 degree angle to mate with the angled sill.

The sill can be made two ways... either as two pieces of Azek (nearly) horizontal that are glued together, or two vertical pieces fastened to the wall. On this project I chose to use the vertical double-board approach.

The top trim was made like old-fashioned window trim with a "lookout" board that caps the vertical piece:

I cut a couple of pieces of Azek with a 10-degree bevel.

These will form the top casing trim above the opening.

Two-piece top trim made from Azek PVC lumber.


Applying PVC cement to Azek synthetic lumber. I applied PVC cement to the PVC trim...


...and nailed it together with 2-inch finish nails.

This piece will form the "lookout" trim above the opening. The protruding top board should help keep rain away from the opening.

Gluing and nailing cellular PVC lumber to make exterior trim.


Installing The Exterior Trim:

Hole in wall for window A/C, showing upper and lower trim on outside. I installed the top trim and then the angled sill.


Then I cut the side trim to the exact length and nailed them in place with 2˝ inch finish nails. Exterior trim around window or A/C opening.


Diagram showing 1/2 inch reveal or setback from edge of jamb to exterior trim.

A Wide Reveal Is Needed:

IMPORTANT: All 4 trim components were installed about 1/2 inch back from the inside edge of the jambs (i.e. 1/2 inch reveal).

This large reveal will provide a good surface for the outer cover panel to seat against.

With ordinary window casing, the reveal would normally be a matter of personal taste. I usually make the reveal 3/16" when doing regular trim, but this case is special.


After the trim was nailed in place, I caulked all the gaps with siliconized acrylic latex caulk.


Quick cover for wall opening or window, using piece of plywood and sheet of plastic. I installed this quick temporary cover to keep the rain out.

This was easy... I just cut a piece of high-quality 1/2 inch plywood to fit loosely between the trim boards (against that 1/2" reveal) and then I cut a piece of plastic with about 6 to 8 inches of overhang on all sides. I stapled the plastic to the outside of the cover panel, then I set the panel into the opening and stapled the plastic to the edges of the jambs on the inside.

Note: I cut the cover panel about 1/4 inch smaller than the width and height of the inside dimensions of the exterior trim boards. This created a 1/8 inch gap between the cover and the trim, or about 3/8 inch of overlap between the cover and the jambs.


Installing Blocks Of Wood To Support
The Window Air Conditioner:

Once the opening had been trimmed out with jambs and exterior casing, I still needed to install some pieces of wood to hold the window air conditioner in the opening.

I fastened this block of wood to the sill to support the air conditioner.

The A/C unit I used has a shallow channel in the steel bottom pan, which is about 3 inches wide. So I used a table saw to rip this 2x4 to less than 3 inches wide, and I cut it as long as the width of the air conditioner housing. The idea is to have that channel in the housing sit right on top of this board.

Block of wood fastened to sill to support bottom of window air conditioner.


"L"-shaped wood to support top flange of window air conditioner in opening.

Then I made this "L"-shaped piece of wood, which spans the full width of the opening.

This is just a 2x4 that I ripped narrower on a table saw (arrow 2), which is fastened to a full-width 2x4 (arrow 1) to create the "L"-shaped profile.

The idea here is to simulate the lower sash of a traditional wood-sash window


I set the air conditioner in the opening to test the fit.

That bare unpainted piece of wood is just the right height to catch the flange on top of the window air conditioner.

Test fitting the window air conditioner in hole in wall.

The A/C unit could not fall out, but it would not sit level because the bottom wanted to slide inwards.

Later, a piece of wood fastened to the sill, (below the air conditioner) will secure the unit in place and keep it level.


Outside view of window A/C opening, after painting. Next, I removed the air conditioner and installed a thin piece of plywood on the inside to keep bugs out.

Then I primed and painted all the outdoor parts. Even though all the exterior window trim on my house is painted white, I decided to paint this trim the same green as the house to de-emphasize the A/C opening.


Not Yet Ready For A/C -
Building A Cover For The A/C Opening:

I built a 2-piece cover for the air conditioner opening. I made the outer cover from 1/2" plywood, cut to a size that left a 1/8" gap all around the edges.

From the outside the A/C opening is not overly conspicuous... it almost blends in with the siding.

Cover for hole in wall for window A/C.


Cover for inner part of hole for window A/C. The inner part of the cover panel was made from another piece of 1/2" plywood attached to a frame of 3/4" thick wood and capped with a piece of Lauan plywood, which is less than 1/4" thick.

I filled the cavity with foam insulation, then I primed all the bare wood and applied self-adhesive foam tape to the edges of the box to act as a gasket.


From the inside, the cover panel looks like this. The two sections are held together with 4 long screws, driven from the inside.

Related Article: Building A Cover For The A/C Opening

Hole in wall for air conditioner, with cover installed, inside view.


But Wait... It's Still Summer... What About The Air Conditioner?

After the covers had been built and fitted to the opening, I removed them and returned to the most important task... installing the window air conditioner.

I wanted to complete the covers because I knew that later in the fall, when it's time to remove the A/C, I wouldn't have the inclination to dick around with building the covers. And I would be battling cold weather, since I never seem to get the A/C removed until late October.

Sill board fastened to A/C opening to keep window air conditioner from sliding forward. I set the window air conditioner in the opening and slid the bottom hold-down board under the front of the A/C unit. Then I drove in 2 long deck screws (red arrow) to hold the board in place.

Note how there is a small piece of thin plywood to the left of the air conditioner. This filler block was necessary so I could screw down that plastic accordion-like side screen.

This board is just a piece of 2x4 that I ripped narrower on my table saw. In my case this board was about 2 inches wide. This board basically simulates a window sill. If you really analyze a typical window air conditioner, the weight of the unit pushes out at the top (pushing against the bottom edge of the raised-up window sash) and pushes in at the bottom, (pushing inwards against the window sill).

In this project my approach is essentially to simulate the window sill and the lower edge of the window sash. Normally, when installing a window air conditioner, you raise the window sash high, set the A/C against the sill, and lower the window sash so the A/C's top flange pushes against it. But you can't raise and lower the "simulated sash" when installing an air conditioner in a simple hole in the wall, so a different approach is needed.

Consequently, the simplest way to hold the air conditioner in place is to install a removable sill that the A/C unit can push against.


Since those plastic accordion-type side extensions are pretty useless at sealing air leaks, I cut out a piece of 3/4 inch rigid foam insulation to fit around the case of the air conditioner. This made a big improvement at sealing the unit from air leaks. It might also be a good idea to caulk around the foam insulation with Seal-N-Peel removable caulking. Window air conditioner installed in hole in wall, indoor view.


Window air conditioner installed through wall, outdoor view. From the outside it looks like a window air conditioner without much window. I like this method of installation because it's not too obtrusive.

Being on the second floor, many visitors to my house have not even noticed this A/C unit hanging out the wall, even though it's just above the back door.

More Info:

Tools Used:
  • Basic Carpentry Tools
  • Circular Saw
  • Miter Saw
  • Table Saw
  • Cordless Drill/Driver
  • Cordless Impact Driver (Optional)
  • Pneumatic Finish Nailer
  • Reciprocating Saw
  • Fein® Multi-Master (Optional)
Materials Used:
  • Azek® Cellular PVC Trim Boards
  • PVC Cement
  • Lumber, 2x4, miscellaneous
  • 1/2" Premium Plywood
  • Foam Insulation
  • Caulk
  • Finish Nails
  • Shims
  • Primer
  • Exterior Paint
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Written July 10, 2010