How to Install a Window Air Conditioner

A faulty installation can damage your AC—or worse. It can fall out of the window.

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AC in window Photo: Lacey Browne/Consumer Reports

Installing a window air conditioner is a sweaty job, so you might be tempted to simply open the window, boost the air conditioner into place, close the window, and hope for the best. Not a good idea, for a number of reasons.

To help you get the job done right, we asked CR’s resident air conditioner expert, Chris Regan, to walk us through the steps for making sure your AC is safely installed (these things weigh 40 pounds or more) and positioned to save energy and avoid air leaks. He has been installing and testing window air conditioners at CR for 15 years and knows a thing or two.

Quick tip: Consider getting a support bracket, especially if you live several floors above street level in an apartment building. These attach to the outside wall directly under the window for your AC to rest on, and cost around $30 to $100. (For more window AC tips if you live in an apartment, read our AC guide for apartment dwellers.)

One other note: You should still read through the manual for your particular AC model. “Do as we do and always follow the manufacturer’s installation instructions, and use the hardware that comes with the AC,” Regan says.

Steps for Securing a Window AC

Step 1: Make Sure It Works!
When you unbox the AC, make sure to remove any foam blocks, tape, and plastic fasteners. And be careful. “The backside of the unit has very thin metal cooling fins that can easily slice fingers,” Regan says. Place the AC on the floor and plug it in to make sure it works. You don’t want to go through all the installation steps just to find out that the thing is a dud.

Step 2: Attach the Panels
Unplug the AC and screw or slip on the two accordionlike side extensions. (These expand so that you can close up the spaces on the sides of the unit once it’s in the window, to keep the cool air inside and the warm air outside.) Most models also come with a separate metal rail that you’ll need to screw on to the top of the unit, which acts as a guide track for the side panels and helps hold the AC in the window.

More on air conditioners

Step 3: Position the AC in the Window
Raise the window sash, and screen, if you have one. Center the AC on the windowsill. Most of the weight will be hanging out of the window, so be sure to keep a firm grip on the unit. (You may need an extra set of hands.) Pull down the window so that it rests on the top of the AC unit directly behind the metal rail. Keep downward pressure on the window to prevent the unit from tumbling out.

Step 4: Make Sure the Unit Can Drain Properly
When the AC is on it creates water condensation, and that water has to go somewhere. Some newer models have a slanted drain pan built into the unit, and you have to make sure the AC is perfectly level. Others come with a windowsill support for the outside of the AC to make sure it sits at the right angle for water to drain. (To see which models come with a windowsill support, see our air conditioner ratings.) If your AC has neither, you’ll need to tilt it back in the window ever so slightly so that the water drains outside rather than into your apartment or house.

Step 5: Lock It In
Your AC comes with a sash lock, which is an L-shaped bracket that you attach to the top of the lower window sash and the side of the upper sash to prevent the window from being opened either accidentally by someone at home or by an intruder from the outside. Use a cordless drill (or a screwdriver) to fasten it in place. If you have vinyl windows and don’t want to screw into them, Regan suggests cutting a block of wood to fit snugly in the space between the top of the opened window and the top of the window frame to prevent the window from being opened. A wooden curtain rod or thick dowel will also do the trick.

Step 6: Close Up the Gaps
Pull out each extension panel on the sides of the AC to close up the spaces to the left and right of the air conditioner. Screw each one to the window sash to keep them in place. (There should be screw holes to show you exactly where to fasten them.)

To seal out hot air and insects from the outside, stuff the thick foam weather stripping that came with your unit into the gap at the top of the window sash that’s holding your AC in place, between the two panes of glass. If your unit also came with insulating foam panels to place over the side accordion panels to minimize air leaks, cut the panels to size and place them over each extension.

Best Window ACs From CR's Tests

Which ACs are great at keeping you cool all summer long? Check out the top-performers from our extensive lab tests, organized by the room size you want to chill in.

Best ACs for Small Rooms

(100 to 300 square feet)
For a small bedroom, home office, or guest room, look for an air conditioner with a capacity of 5,000 to 6,500 Btu. You’ll want one with good scores for comfort and noise, and might also consider how well each unit performs under brownout conditions involving extreme heat coupled with low voltage. Here are three top picks to consider.

Best ACs for Midsized Rooms

(250 to 400 square feet)
To cool a bigger or busier room, you’ll want to step up to a midsized air conditioner rated at 7,000 to 8,500 Btu. If it’s for a bedroom, heed our noise scores. Here are three top picks.

Best ACs for Large Rooms

(350 to 650 square feet)
For a living room or family room, you’ll want a large air conditioner with 9,800 to 12,500 Btu, especially if you have an open floor plan. Here are three top choices.


Headshot of Perry Santanachote, editor with the Home editorial team at Consumer Reports

Perry Santanachote

I cover the intersection of people, products, and sustainability, and try to provide humorous but useful advice for everyday living. I love to dive deep into how things work, and debunking myths might be my favorite pastime. But what I aim to be above all else is a guiding voice while you're shopping, telling you what's a value, what's a ripoff, and what's just right for you and your family.