The Ultimate Installation Guide for Through-the-Wall AC Units: DIY or Call a Pro?

👋 Hey there, it’s Mark again.

Installing a through-the-wall air conditioner is like doing your own appendectomy with a butter knife—if you skip the prep, tools, or safety. 

But seriously, with the right planning (and this guide), you can absolutely tackle this job. The trick is knowing when to DIY and when to throw in the towel and call a pro. So let’s dive into:

• The full step-by-step process

• Required tools and materials

• How much it costs (time and money)

• When to bring in help

• Energy and safety tips that pros swear by

🔧 Step 1: Choose Your Location Wisely

Start indoors. Ask yourself:

• Is this an exterior wall?

• Are there electrical wires, plumbing, or studs in the way?

• Will it get good airflow and reach the whole room?

Avoid load-bearing walls unless you know your framing. Check the outlet: most through-the-wall ACs run on 115V, but some high-powered models need 220V.

💡 Pro Tip: South- or west-facing walls can get more sun, which forces your AC to work harder. North-facing walls often make for more efficient cooling.

📏 Step 2: Measure Everything—Twice

Your through-the-wall unit likely requires a universal wall sleeve (unless you’ve got a slide-out chassis style). Standard sleeve size is around 26" W x 15" H x 16–20" D, but always follow the exact manufacturer spec.

You’ll also want to:

• Check the unit depth—it shouldn’t stick out or sit too deep

• Align it above the floor by at least 3 inches

• Leave room around the unit for proper ventilation

A laser level, stud finder, and measuring tape are your best friends here.

🧱 Step 3: Cut the Wall (Gulp)

This is the moment where people get nervous—and rightly so.

Here’s how to do it:

1. Mark your cut lines from inside using painter's tape and a level.

2. Drill pilot holes in each corner through the wall to the outside.

3. Go outside and connect the pilot holes using chalk lines.

4. Cut from the outside in using a reciprocating saw (with a long blade) or a circular saw set to the wall thickness.

5. Frame the opening with 2x4s, forming a clean box that matches your sleeve size.

Don’t forget insulation and caulking—especially in colder climates.

🔩 Step 4: Install the Sleeve

Slide the metal sleeve into the opening. Make sure it’s:

• Slightly sloped downward toward the outside (to prevent water from draining inside)

• Centered and level

• Securely screwed into your framing

Seal any gaps using expanding foam or caulk, especially around the edges. Outside, use weatherproof flashing tape or silicone to keep out wind and rain.

Don’t skip this! Improper sealing means energy loss, water damage, and even mold in the long run. The EPA's indoor air quality portal has some solid background on why sealing is so critical.

⚡ Step 5: Electrical Setup (The “Call a Pro” Point)

Some through-the-wall ACs plug into a standard outlet, but many require a dedicated 220V circuit with a 20A breaker.

If you’re not familiar with breaker boxes, GFCI outlets, or voltage load, this is where you should absolutely:

👉 Call a licensed electrician.

Not only is it safer, but an incorrect install could void your warranty—or worse, cause a fire hazard.

Typically, you'll pay $150–$600 depending on circuit access and outlet type.

🧊 Step 6: Slide in the AC Unit

Once the sleeve is secure and the wiring is ready:

• Remove any temporary weather panels

• Slide the AC into the sleeve gently

• Lock the unit into place with the built-in brackets or screws

• Attach the interior trim panel (some are screw-on, others snap)

Plug it in and power it up!

Check the airflow, test the remote, and look for vibrations. If something sounds off, turn it off and recheck alignment. Often, it’s just a small mounting bracket issue or fan rubbing during startup.

💨 Step 7: Seal, Caulk, and Finish the Interior

Inside the room:

• Use silicone caulk around trim edges to block air leaks

• Patch and paint any gaps

• Clean dust or debris from the install area

Outside:

• Add a rain guard or louver hood, especially in climates with heavy storms

• Double-check the drainage angle so condensation flows outside

Want a weatherproofing deep-dive? Building America Solution Center has resources trusted by pros.

📉 Cost Breakdown: DIY vs. Professional Install