How to Install a Window Air Conditioner

Add a little cold comfort where you need it most by safely installing a window air conditioner sized to suit your needs. We have step-by-step instructions and helpful DIY tips to guide you through your window air conditioner installation.

Few Hours
Less Than $500

What You Need for This Project

Before You Buy Your Home Air Conditioner

  • Determine the window where you’ll install your air conditioner. If there’s no three-prong outlet nearby, you’ll need an extension cord labelled for air conditioner use.
  • Check the load on the circuit you plan to use. If it’s also powering other devices, even simple ones such as a hair dryer, you may want to choose another window near a different circuit, add a dedicated circuit or move those other appliances somewhere else to avoid a tripped breaker or blown fuse.
  • Not every air conditioner fits every type of window. Determine whether your window is a slider, a double-hung, or a casement style. Most units work only in double-hung windows with a lower sash you can raise and lower. Also, measure the width and the height of the window opening to confirm whether an air conditioner will fit.
  • Decide how much additional insulation and rain protection you’ll want to add around the window (beyond the air conditioner’s side curtains). For example, you may want to add pieces of white expanded foam insulation along the sides or within the gap between the upper and lower window sashes.
  • How’s the weather outside the window? A west-facing window could catch enough sun to force the air conditioner to work harder. Does the wind typically whip rain against that side of the house? Consider adding additional barriers to keep the water outside.
  • If you have curtains or drapes at the window, consider adding a blind or shade. These options won’t block the air conditioner and can be adjusted to give you privacy.

Prepare the Window for Your Window Air Conditioner Installation

Inspect the window to make sure it’s in good repair. After installation, you probably won’t open that window for months until it’s time to remove the unit, so now’s a good time to wash the glass inside and out. You’ll also want to clear the area around the window and leave yourself plenty of working space with no tripping hazards.

Check for obstructions in the window opening, such as storm window frames or screens that might interfere with the overhanging part of the air conditioner on the outside. Raise or remove the screen and either remove the storm window (and frame) or fasten a spacer block to the windowsill to lift the air conditioner case away from the storm window frame.

Note: There’s a reason to fasten spacers in place instead of leaving them loose. Weather, vibration and the challenge of lifting a heavy air conditioner onto a windowsill can knock an unsecured spacer out of position, allowing the unit to tilt or even fall.

Remember to: 

Follow the manufacturer’s instructions to prepare the air conditioner for mounting. This may include attaching brackets and the accordion-style side curtains, for example. Double-check the air conditioner’s measurements and compare them to the window opening.

How to Install a Window Air Conditioner

  1. 1

    Secure the Air Conditioner Unit

    Enlist a helper to place the air conditioner. With the bottom window sash up as far as it will go, centre and insert the air conditioner in the window. Keep it balanced as you slide the flange on the bottom of the case against the outside edge of the windowsill. Then lower the bottom window sash until it rests on the air conditioner and securely against the top flange.

    Tip: Follow the manufacturer's instructions for positioning the unit to ensure it allows condensation to drain properly.

  2. Steps:

  1. 2

    Drill Holes into Upper Flange & Attach L Clips

    After balancing your air conditioner on the windowsill, it’s easy to see why you need that top sash to stay put and hold the unit in place. To prevent the sash from being accidentally raised, insert at least two screws through the upper flange on the air conditioner case into the window sash. The screws could possibly split the wood if you don’t drill pilot holes first. Use a bit that’s slightly smaller than the core of the screw. Then wrap a piece of masking tape around the bit so that it limits your drilling depth to the length of the screw.

    Then do one of the following: Attach the L-shaped clips that connect the bottom sash frame to the top sash frame or cut spacers from 2-in. by 2-in. stock and wedge them between the bottom sash and the top of the window frame.

    Tip: Not all windows allow you to use L-shaped clips to hold the bottom sash in place. If your window is a vinyl window, you can make a replacement for clips by cutting strips of wood just long enough to wedge snugly between the sash and the top of the window frame. We cut these from paint stirring sticks.

  2. Steps:

  1. 3

    Seal Around the Air Conditioner Unit

    Pull the side curtains out to the edges of the window frame to keep bugs and moisture and hot air from entering the room. Check the manufacturer’s instructions for directions on screwing or clipping the curtains in place.

    Expanding curtains help seal your room, but they’re no substitute for insulation. Consider cutting white foam insulation board to fit snugly around the opening from the outside if your window receives lots of sun. You can also buy accordion-style insulating panels and cut them to fit the opening.

  2. Steps:

  1. 4

    Fill the Gap

    That gap between the frame of the lower sash and the window on the upper sash needs to be filled. If your air conditioner comes with a foam seal, you can cut it to size and slip it in place. If your foam filler disappeared during the winter while the air conditioner was in storage, you can buy them separately.

  2. Steps:

  1. 5

    Turn it On & Cool Down!

    Plug it in (use an air conditioner extension cord, if needed). That was probably hot work, so pour yourself a cold glass of water while you wait for your newly air-conditioned room to cool off.

  2. Steps:

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