How to Install Window and Wall ACs: Sizing, Sealing, and Safe Wiring in 2025

Key Takeaways

• Size it right—too big or small wastes energy and comfort.

• Tilt window units slightly; TTWs need level, drained sleeves.

• Seal all gaps to prevent up to 30% cooling loss.

• Use a dedicated breaker for 230V TTW units.

• Clean filters monthly and coils seasonally to boost efficiency.

Introduction

If you’ve ever felt a hot breeze sneaking around a window AC or heard a TTW unit rattling in its sleeve, you already know that installation quality can make—or break—summer comfort. In the United States, room air conditioners account for about 9 % of all home electricity use. A poorly sealed or badly sized unit throws away much of that energy and can even damage your wall or window frame.

This guide walks you through the entire process of installing windows and through-the-wall air conditioners. You’ll learn how to measure correctly, pick the right mounting hardware, seal out leaks, and wire safely—using plain language a middle-schooler can follow but with depth an HVAC pro will appreciate. 

Compare Window vs TTW Units

Why Correct Sizing Comes First

Before the first screw is turned, confirm the unit’s cooling capacity. The sweet spot is big enough to pull heat and humidity out of the room but not so big that it shuts off before finishing the job.

Find Right-Sized Units

• BTU basics: One ton of cooling equals 12 000 BTU per hour. Living rooms (350–550 ft²) generally need 8 000–12 000 BTU, while small bedrooms (<150 ft²) may only require 5 000–6 000 BTU.
  

• Adjust for real-world factors: Add 10 % BTU for kitchens, subtract 10 % for shaded rooms, and add 600 BTU per person beyond two people who regularly occupy the space.
  

• Why it matters: An oversized compressor cycles off quickly, leaving humidity behind and shortening component life. An undersized unit runs flat-out, hiking your bill and wearing itself out in half the time.
  

For a deeper dive, bookmark our Air Conditioner Sizing Guide—it includes printable charts and a quick BTU calculator.

Mounting Options: Window vs. Wall

Window Units

• Drop-in convenience—no major carpentry required.
  

• Rest on the sill with L-brackets or wood cleats underneath.
  

• Must tilt ¼ in. per foot toward the outdoors so condensate drips outside, not on your parquet.
  

• Side accordion panels fill gaps; weather-strip seals the sash.

See DIY vs. Pro Install Tips

Through-the-Wall Units

• Clean, permanent look—ideal for rooms without good window placement.
  

• Require a steel or heavy-duty plastic sleeve anchored into a framed wall opening.
  

• Sleeve keeps the building envelope intact, supports weight, and directs drainage away from studs.
  

• Typically higher capacity and 230 V; plan for a dedicated circuit.
  

Unsure which path fits your project? Our DIY vs. Pro AC Installation guide compares tool costs, permits, and warranty fine print.

Preparing Your Workspace and Tools

Accurate prep prevents mid-job surprises:

1. Measure twice: Width, height, and depth of the window opening or proposed wall cut-out must match the manufacturer’s spec.
   

2. Check structure: For TTW jobs, pick a non-load-bearing section or brace headers properly. Avoid cutting electrical or plumbing lines behind the drywall.
   

3. Gather tools: Tape measure, torpedo level, drill/driver, masonry bit (if stucco or brick), screwdriver set, pry bar, foam gun, and exterior-grade silicone.
   

4. Safety gear: Gloves, eye protection, and a helper—most room ACs weigh 50–120 lb.
   

5. Power check: Verify outlet voltage and breaker size match the nameplate rating.

Need Wiring Help? Read Our Help Center

Installing a Window AC: Step-by-Step

1. Weather-strip first: Apply foam strip to the sill and sash contact points.
   

2. Set the chassis: Lift the unit onto the sill; center it; lower the sash behind the top flange.
   

3. Angle for drainage: Slide a thin shim under the exterior edge until the bubble in your level just touches the rear line.
   

4. Secure brackets: Screw L-brackets into the sill or side jambs; attach the top window lock to keep the sash from lifting.
   

5. Expand side panels: Pull accordion panels to each jamb; fasten with small screws.
   

6. Seal gaps: Fill any daylight cracks with foam rope or low-expansion spray. Finish with a bead of clear silicone outside.
   

Proper tilt and foam seal can save up to 15 % on cooling costs over a “just-drop-it-in” install. Shop Top Window AC Models

Installing a Through-the-Wall AC: Step-by-Step

1. Frame the opening: Follow the cut-out template; install a header if you’re not between studs.
   

2. Insert the sleeve: Level front-to-back but allow a ¼ in. slope outward for drainage; anchor through side flanges into framing.
   

3. Insulate and seal: Spray foam around sleeve perimeter; trim flush after curing; caulk exterior seams.
   

4. Slide in the unit: Two people lift the chassis straight into the sleeve until it seats; secure with manufacturer screws.
   

5. Add trim kit: Snap or screw the bezel to hide rough edges and block drafts.
   

6. Test run: Plug in, set mode to cool, listen for smooth compressor start, and verify warm air exhaust outside.
   

For long-term performance tips, see Maintenance Tips to Maximize Performance of Through-the-Wall Air Conditioners.

Sealing Techniques for Draft-Free Comfort

Even a pencil-thin gap around a room AC can leak as much air as a 4-in. hole in your wall. Stop those losses:

• Window units: Foam rope in large side gaps, adhesive weather-strip on sash, and a continuous silicone bead outside.
  

• TTW units: Low-expansion spray foam around sleeve, compressible seal strip between chassis and sleeve, and UV-proof exterior caulk on the bezel perimeter.
  

• Check annually: Sun and vibration shrink seals. Reseal any cracks before the cooling season.
  

Tight installs not only cut bills but also keep pollen and insects out—especially important for allergy sufferers.

Shop Weatherproofing Accessories

Wiring and Safety Checks

• Voltage and amperage: 5 000–12 000 BTU window units usually run on 115 V, 15 A; larger TTW units may need 230 V, 20 A or more—verify before purchase.
  

• Dedicated circuit: NEC recommends a separate breaker for fixed TTW units; window units can share if load permits but avoid high-draw devices on the same branch.
  
  

• No extension cords: They overheat and void warranties. Install a new outlet if the factory cord can’t reach.
  

• GFCI/AFCI protection: Required in many local codes for window and wall-mounted appliances near wet areas.
  

• Test run: Measure current draw with a clamp meter; compare to nameplate amps. An over-amp draw signals undersized wiring or compressor trouble.
  

Ready to install it right the first time? Find perfectly sized, ENERGY STAR® rated ACs and the accessories to match.

Frequently Asked Questions

Q: Do I really need to tilt a window AC?
A: Yes—about ¼ inch per foot toward the exterior so condensate drains outside.

Q: Can I install a TTW unit in a brick wall?
A: Absolutely. Use a masonry bit for the opening, add pressure-treated framing, then insert a corrosion-resistant sleeve.

Q: Is a sleeve optional?
A: No. The sleeve supports weight, seals the wall cavity, and channels water safely out.

Q: How do I stop drafts around my unit?
A: Fill gaps with foam rope, weather-strip, and exterior caulk; inspect yearly.

Q: Do I need a pro electrician?
A: Hire one if the unit requires a new 230 V circuit or if you’re not comfortable calculating load on an existing breaker.