Introduction: Mounting a Standard Air Conditioner in a Sliding Window (From the Inside, Without a Bracket)

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Many people have inexpensive air conditioners that are intended to fit into a double-hung (vertically sliding sash) windows and want to use them in horizontally sliding windows. If you already have some of these air conditioners or are attracted to them because their price tags are significantly lower than that of casement units, portable units, or central air, it is possible to make a frame to adapt them to sliding windows.

These solid, simple frames require no exterior brace.  This is perfect for apartment dwellers and those who only need an air conditioner a few months of the year.  It takes about an hour and some basic lumber and tools to construct these frames.  Once built, your air conditioner can be installed and uninstalled in minutes.  (Please note that window air conditioning units aren't intended to be permanently installed.  These frames also make it simpler to remove them at the end of the warm season.)

The units pictured include unfinished plywood.  However, this can be finished, painted or replaced with plexiglass or other materials, as the sheet lumber simply covers an opening and is not structural.

2X4s *(2 or less but the amount varies)
12 deck screws (or other long wood screws)
Plywood Scrap

  • These window sills in the picture are fairly deep, allowing for the use of 2X4s. You may be able to substitute other lumber for shallower sills.

Step 1: Take Measurements

Measure the width of your air conditioner. Be sure to include any vents that stick out on the side in this measurement. (My first attempt to build this did not account for the vents and the air conditioner couldn't slide in. I considered building the frame around it but it was simply too awkward to maneuver this frame + air conditioner into place.)

Measure the height of your air conditioner. This height should be the box height and not include the lips on the bottom or the top.

Measure the height of the window opening. Take this measurement right up against the window because it will give the shortest height of this opening. (Again, mistakes reminded me that sills can angle away from the window, giving a height that is too long.)

Step 2: Cut the Frame Lumber

Cut your 2x4 as follows:
3 - width (including vents!) of air conditioner
2 - height of the window frame

Step 3: Starting the Frame

Lay a shorter piece of 2X4 down and screw the ends of the longer pieces to the ends of this shorter one, as shown. Because of the depth of the lumber, I used deck screws, 2 at each connection point.

Create a rectangle by screwing another short piece of 2X4 between the long pieces at the other end, as shown. Make sure the corners are square.

At this point, it's a wise idea to put this rectangle into the window opening to make sure it fits. It should slide in and sit against the window framing. (If it's a little loose, you can use shims to help wedge it in place during installation.)

Step 4: Finishing the Frame

Next you will want to dry-fit your air conditioner. Set the A/C into the frame with the bottom lip against the outside of the frame. You'll notice that the top lip is probably U-shaped with the back side being shorter. The last piece of 2X4 will sit across the top of this shorter leg of the U (or the top of the A/C if there isn't a second leg). But don't screw it in yet.

To figure out the placement of this piece of 2X4, lean the A/C so that the front vertical piece of the top lip lines up with the front edge of the wood frame. This is how the A/C will be oriented in the frame when complete, tipping the unit back.

While you're holding the A/C in place, take a look at the whole configuration to make sure it leans back sufficiently. Air conditioners must drain and are built to tip away from the window. If this lean appears to be too little (it really shouldn't be if the top and bottom lip are properly aligned with the frame), then you might want to rip this last piece of lumber, making it less than 3.5 wide, before screwing it in place.

If everything looks o.k., lay this piece of lumber in place and mark its position on the frame. Also, mark the top edge of the front lip where it will hit this piece of framing.  (This mark is so you can measure for the piece of plywood covering the hole in the top of the frame.)

Remove the air conditioner from the frame and screw this piece of lumber in place. It would be good, at this point, to dry-fit the A/C into the frame just to make sure you are on-track.

Step 5: Covering the Top of the Frame

Now you will want to measure the opening on the top of this frame for the plywood that will cover the hole. Measure this opening keeping in mind that the piece of plywood cannot be mounted below the long horizontal mark you made on the last piece of 2X4.

Screw the plywood covering in place.

Optional Consideration: I realized after I made the frame that it would be really nice to be able to open the window for a breeze to come through on cooler summer days instead of running the A/C. Consider stapling screening material over the side of the frame that faces outside. Then you can hinge the piece of plywood over the opening and install a latch for it so you can open it up when you want. Maybe add some weather-stripping to ensure a good seal with the door is closed.

Step 6: Installation

To install this frame, simply open the window and slide it inside up against the window framing. If you want to add extra stability to the installation and/or prevent it from leaning directly against the window frame, screw it into place by running long screws through the wooden frame and into the framing around the window opening. You can also use angle brackets for the side that you cant screw-in.

Slide the A/C into place and secure by running a screw or two through the lip. The weight of the unit should keep it in place but this is extra insurance. If the frame seems a little too loose to you, you can always use shims to wedge it tightly in place.

Fill any gaps around the opening. You can use a removable caulk, trim narrow pieces of foam insulation, or other ways to fill the gaps.