Installing an In-Wall Air Conditioner

To install an in-wall air-conditioning unit, you will need to make a hole through the exterior wall of your house - a daunting prospect for some do-it-yourselfers. But if you select a section of wall that is free of pipes and wires, basic carpentry skills and tools will get you through the job. However, unless you have some experience cutting through concrete, brick, or stone, leave installations through solid masonry walls to professional contractors.

When cutting and building an opening for an in-wall unit, always follow the manufacturer's directions that accompany the air conditioner. It's also important to follow the manufacturer's instructions for setting the unit slightly out of level to drain condensation away. Most room units can be plugged into a standard 120-volt outlet but some require 240 volts. Make sure the unit will not overload the circuit and that its cord will be able to reach the outlet. If you're placing the air conditioner in a solid wall, you will have to build a header - a horizontal beam that picks up loads from studs that are cut short and carries them to the sides of the opening. In most cases, you can avoid this step by installing an in-wall unit beneath a window (see instructions below). The space already has a header and should have double studs running down each side all the way to the floor. You may have to make the air-conditioner space smaller than the window, which is easy, but you won't have to worry about supporting loads from above. An in-wall unit also looks better if it's installed under a window. A metal box poking through a clean wall of siding will grab your eye. It's less noticeable under a windowsill, particularly if you trim the exterior of the conditioner the same way the window is trimmed.

Always start work on the inside of the opening before the outside. That way, if you uncover an unexpected obstruction - a gas pipe, for example - you'll have to replace only a section of drywall, and repaint the damaged wall. A pain, perhaps, but a much easier job than adding nailers to support the pieces of siding you've cut.
Project Basics


Estimated Project Time:
Several hours

Start Tips:
Plan the location of the air conditioner unit so that it is near an outlet with sufficient capacity for the unit

Safety Tips:
You might need a helper to assist you in lifting the air conditioner

Do-it-Yourself (hire a pro if installing in masonry and you are not proficient)

Tools and Materials:

In-wall air conditioner unit
Measuring tape
Drywall knife
Utility knife
Work gloves
Safety goggles
Reciprocating saw, crosscut saw, circular saw, and saber saw (optional)
Drywall saw and crosscut saw (optional)
Pry bar

Spirit level
Power drill/driver
2x4s, as needed
3/4-inch plywood, as needed
Taping knives
Drywall nails or screws
Drywall tape
Joint compound
Silicone caulk
Caulking gun
Paint, stain, and varnish, as needed


1) Determine Placement of Unit
Plan a convenient installation. The best choice often is directly under the framing that supports the windowsill where you won't have to reroute plumbing pipes or electrical wiring. In order to avoid any buried cables select a location above the height of wall outlets. Wiring may run in the wall under a window, but it's generally at the same height or lower than the wall outlets. If you install the air conditioner in a wall of full-length studs rather than under a window, check with your local building inspector for the size of the header you'll need to install. The air conditioner's instructions should explain what additional framing (if any) will be required to support your unit.

Fig. 1

2) Open the Interior Wall
Working from the inside of the house, measure and mark the opening carefully. Then cut away a section of drywall to expose the wall framing underneath. Don't cut blindly into a wall cavity with power tools such as a reciprocating saw or saber saw. Instead, trim through surface gypsum by hand using a drywall knife or utility knife. This will produce clean edges that are easily trimmed and will help prevent surprise encounters with mechanical lines or hidden braces buried inside the wall. Use a pry bar to remove the drywall. (Fig. 1)

Fig. 2

3) Frame the Opening
Add framing as specified by the manufacturer to build the rough opening and create support for the unit. Remove any insulation from the area for to be framed (save it for using later in Step 8). To frame under a window, you can cut through the studs to remove them, and add cross timbers top and bottom and new side pieces. Other times it is easier to build a box - for example, from 3/4-inch-thick plywood - to the manufacturer's specifications and then use it to mark and cut away sections of studs. Allow an extra 1/2 inch of length and width so that you can plumb and level the box in the opening. Additional trimmer studs and cripple studs may be required to hold the unit safely in place (Fig. 2)

Fig. 3

4) Transfer Location of Opening to the Exterior Wall
When the frame is complete mark on the exterior wall the opening for the air conditioner. To do this, transfer the perimeter of the opening to the outside wall by drilling small holes precisely at each corner or by driving nails through the corners. (Fig. 3)

Fig. 4

5) Cut through the Siding
Working from the outside, use a straightedge to draw lines connecting the marked points. Remove the nails when no longer needed as markers. Then cut through siding and sheathing using a saber saw or circular saw. (Fig. 4) For greater accuracy, nail a straightedge along the marked lines to serve as a saw guide.

Fig. 5

6) Install the Chassis
For a typical installation, remove the air conditioner from its chassis and then mount only the chassis in the opening. (Fig. 5) Use a level to match the manufacturer's specifications for sloping the chassis slightly downward for proper drainage. Then fasten the chassis to the frame.

Fig. 6

7) Install the Air Conditioner
Slide the air conditioner into position, inside the chassis. (Fig. 6) For heavy units, you may need a helper. Secure the air conditioner to the chassis as necessary.

Fig. 7

8) Finish the Interior Wall
Insulate gaps between framing members, reusing pieces of the old insulation. Then, piece in drywall around the opening. After finishing and sanding the drywall seams to conceal them, paint the wall and then trim the interior to suit and reinstall the baseboard. (Fig. 7)

Fig. 8

9) Caulk the Outside
Caulk around the exterior of the unit to seal the seams between the chassis and the siding. (Fig. 8) Use a silicone caulk that is flexible enough to maintain a seal next to a vibrating machine.