How to Install a Window AC in a Sliding Window?

Installing a window AC in a sliding window requires a slider kit, foam seals, and a secure mount, and it differs from a standard drop-in install.

Finding the right way to handle a window AC install when your windows slide sideways instead of up is a common snag. Most standard window air conditioners are built for double-hung windows that move vertically, so a sliding sash calls for a different approach. The fix is a mounting kit designed to fill the wide opening, paired with foam insulation and a few screws to lock everything down.

Step-By-Step Install For A Sliding Window

Start by measuring the opening’s width and height before you buy anything. Slider kits have an adjustable range, and your window must fall within it. A few manufacturer manuals note that the kit needs shortening if the window is narrower than roughly 20 to 26 inches, so check the specs for your specific unit.

Once you have a compatible AC and kit, set aside about an hour and gather a measuring tape, screwdriver, level, and the foam seals that ship with the unit. Here is the order that works:

1. Cut the adhesive foam strips to match the width of your window sash and frame, then press them into place along the surfaces the AC will touch.
2. Place the slider kit on the window stool—the flat shelf at the bottom of the opening—and extend it to span the full width.
3. Set the AC into the kit, then lower the sliding sash down until it presses firmly against the top of the unit.
4. Seal any open gaps between the inner and outer window tracks with the remaining foam or a weather strip.
5. Use the locking brackets and screws included with the kit to fix the unit in place, then confirm the AC sits level with a slight tilt downward to the outside.

Do You Need A Slider Kit Or A Special AC?

Yes. A standard top-hung window AC usually won’t fit a slider without a kit, and the simplest route is buying a unit designed for this window type. Many manufacturers offer slider/casement-specific models, and the manual will say so on the box.

Another option is a slide-out chassis unit that installs in a window or through a wall, but that demands more work and usually needs a custom frame. For most people, a slider-casement AC with its matching kit is the least hassle.

The Mistakes That Break A Sliding Window Install

Most failed installs trace back to a few predictable slips that are easy to avoid once you know to look for them:

• Treating a regular window AC as a direct drop-in fit for any sliding window—the window type and the chassis must match.
• Leaving out the foam seals, letting hot air and humidity leak around the unit.
• Skipping the screws that secure the kit to the sash, so the whole assembly can shift.
• Forgetting to check that the opening width falls inside the kit’s adjustable range.
• Installing without the outside slope, allowing rainwater to pool on top of the unit.

Follow that pattern if your model includes those brackets, because it is what keeps the unit from collecting water.

Always secure the support platform with screws before setting the AC in place, and make sure the unit sits within reach of a properly rated grounded outlet with nothing blocking the air inlets or outlets. If you would rather see a shortlist of models tested for this exact window type, our roundup of the best ACs for sliding windows breaks down which units fit and which need extra parts.

Frequently Asked Questions