Honestly, the name is a bit of a lie. You’ve got a "window" unit, but you don't want it in the window. Maybe the view is too good to block, or perhaps you're tired of the security risk that comes with a piece of plastic and some accordion fins being the only thing between a burglar and your living room. Whatever the reason, installing a window AC unit in a wall is a project that sits right on the edge of "I can do this Saturday" and "I should have called a contractor."
It's totally doable. But it isn't just about cutting a hole and shoving the machine through.
If you mess this up, you aren't just looking at a drafty room. You’re looking at structural failure or, even worse, mold growth inside your wall because the unit wasn't pitched correctly to drain the condensation. Let's get into the weeds of how to actually do this without ruining your house.
The Reality of Wall Sleeves and Airflow
Most people think they can just take any old window unit and slap it into a hole in the siding. You can't. Well, you can, but it’ll probably die in two seasons. Window units are designed to breathe through the sides. When you slide a side-venting unit into a thick wall, those vents get blocked. The compressor overheats. It struggles. It dies.
For a through-the-wall installation, you specifically want a "Rear-Venting" unit or a "Universal" model designed for a wall sleeve. If you’re dead set on using a standard side-venting window unit, your wall needs to be thin, or you need to build a custom frame that leaves those side louvers completely exposed to the outside air.
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You also need a wall sleeve. Think of this as a metal "garage" for your AC. It stays in the wall permanently, and the AC unit slides in and out for maintenance. Without a sleeve, the weight of the unit will eventually cause your drywall to crumble and your siding to sag.
Preparation: Finding the Sweet Spot
Before you even touch a saw, you need to know what’s behind the paint. Use a high-quality stud finder—not the $10 one that beeps at everything—to locate your studs. You're looking for a space between two studs, usually 16 inches on center. If your AC unit is wider than 14.5 inches (and most powerful ones are), you’re going to have to cut a stud.
This is where people get nervous.
Cutting a stud means you have to build a "header." It’s basically a horizontal bridge that redistributes the weight of the wall to the neighboring studs. If this is a load-bearing wall, do not skip this. If you’re unsure if it’s load-bearing, look at which way your ceiling joists run. If they’re perpendicular to the wall, that wall is holding up your house.
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Tools You’ll Actually Need
- A reciprocating saw (Sawzall) for the rough cut.
- A circular saw for cleaner lines on the exterior siding.
- Level (this is non-negotiable).
- Galvanized flashing to prevent water intrusion.
- Expanding spray foam (Low-expansion type, unless you want to buckle your frame).
- Silicone caulk. High-quality stuff, like GE Silicone II.
The Scary Part: Cutting the Hole
Measure your wall sleeve. Now measure it again. Add about 1/4 inch to the height and width for "wiggle room" and shimming. Mark your lines on the interior wall first.
Drill "pilot holes" through the four corners of your markings all the way through to the outside. This tells you exactly where the hole will appear on the exterior of your house. Go outside and check. Is there a downspout in the way? An electrical meter? A rose bush you actually like? Better to find out now than after you’ve got a gaping hole in your living room.
When you cut the drywall, keep a vacuum running. Drywall dust is the absolute worst. It gets into everything—your lungs, your carpet, the AC unit itself.
Once the drywall is out, you’ll see the insulation and the studs. Remove the insulation. If you see wires, stop. Electrical lines usually run horizontally through studs at about outlet height. If you hit a wire with a Sawzall, your Saturday just got a lot more expensive. You’ll need to reroute any wiring around the new opening using junction boxes, which, honestly, might be the point where you call an electrician.
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Framing the Opening
You’ve got a hole. Now you need to make it strong. You’re building a "box" inside the wall using 2x4 or 2x6 lumber (matching the depth of your studs).
- The Sill: The bottom horizontal piece. This needs to be doubled up if the AC is heavy.
- The Header: The top horizontal piece.
- The Jack Studs: Short vertical pieces that support the header.
Pro tip from HVAC pros like those at This Old House: The unit must have a slight tilt toward the outside. We’re talking maybe 1/4 inch to 1/2 inch total. If the unit is perfectly level, the condensation (the water pulled from your humid air) will pool in the base pan and eventually overflow into your house. If it tilts too far, the oil in the compressor might not lubricate the parts correctly.
Waterproofing: Where Most DIYers Fail
Water is your enemy. When installing a window AC unit in a wall, the most common point of failure is the interface between the sleeve and the siding.
You need to install flashing. This is a thin strip of metal or flexible membrane that sits under the siding above the unit and laps over the top of the sleeve. This way, when rain runs down your house, it hits the flashing and jumps over the sleeve instead of trickling behind it.
Apply a generous bead of silicone caulk around the entire perimeter where the sleeve meets the exterior wall. Don't be shy. Use your finger (wear a glove) to smooth it out and ensure there are no gaps.
Sliding the Unit In
Once the sleeve is secure and the caulk is dry, it’s time for the heavy lifting. Slide the AC unit into the sleeve. It should fit snugly. Most sleeves have a locking mechanism or a few screws that secure the unit's chassis to the sleeve so it doesn't vibrate or slide out.
Inside the house, you’ll have a gap between the sleeve and your drywall. Do not leave this open. Use backer rod (foam rope) and then cover it with decorative wood trim or the plastic molding that often comes with the unit. This isn't just for looks; it stops conditioned air from escaping and prevents bugs from thinking your wall is their new luxury apartment.
Critical Checklist for Success
- Check the Amperage: A large AC unit can pull 12-15 amps. If you’re on a 15-amp circuit and you have a TV and a vacuum running on that same line, you’re going to trip the breaker constantly. A dedicated 20-amp circuit for the AC is the gold standard.
- The "V" Test: Once installed, pour a small cup of water into the front of the unit's base pan (carefully). Go outside. Does it drip out the back? If it stays in the pan or drips inside, your pitch is wrong. Fix it now.
- Insulate the Gaps: Don't just rely on the trim. Use low-expansion spray foam in the cavity around the sleeve. Standard spray foam expands too aggressively and can actually crush the metal sleeve, making it impossible to slide the AC out later.
Final Steps for a Long Life
Now that it's in, maintenance is your best friend. Since through-the-wall units are harder to swap out than window units, you want this one to last 10+ years. Clean the filter every two weeks during the summer. Once a year, go outside and use a soft brush to clear any debris from the rear coils.
If you live in a cold climate, buy a specialized "winter cover" for the exterior. Even with the unit off, that metal box is a massive heat sink that will pull warmth out of your house all winter long. A good insulated cover pays for itself in one heating season.
Next, you'll want to verify the structural integrity of your header. If you noticed any sagging in the top plate of the wall when you cut the stud, you might need to add an extra king stud on either side of the opening to transfer the load more effectively to the floor joists below. Once that is squared away, go ahead and finish the exterior with J-channel if you have vinyl siding, as this creates a professional, water-tight "picture frame" look that actually adds value to the home rather than looking like an afterthought.