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

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

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.

YOU NEED:
2X4s *(2 or less but the amount varies)
12 deck screws (or other long wood screws)
Drill
Saw
Screwdriver
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.

Comments

[nevroth]

THANK YOU! My boyfriend and I used this guide to make a frame to fit in my windows for an A/C. Super cheap and easy. I had spent $32 on a proper "mounting frame" but it still seemed unsteady so we checked online and found your guide! The 2x4s only cost $6 and it took MAYBE an hour from first cut to last screw drilled in place. The best part is that I can take it with me when I move, and can help friends to make their own. To "pretty it up," I'm going to cover the wood with a painting/drawing of mine. I thought about painting it to match the walls, but it doesn't look bad at all really- and it WORKS!!

[nevroth]

Ack, I just wanted to mention, that to get an angle for any water to drip out, once the sides and bottom were done, I held the A/C with the back tilted downwards a bit, and my boyfriend put the middle bar right on top and drilled it in place (from outside the frame in, of course). So instead of being straight, the middle bar is already at an angle. Also, we drilled the frame into 3 sides of the window sill - that sucker is going NOWHERE! (I mention it because I didn't quite understand your directions, although my boyfriend knew what you were talking about, lol) THANKS again.

[starshipminivan]

The way you screwed in the center bar makes sense. Yeah, I can see that it might need to go in at an angle to keep the unit angled. I'm also glad that you guys figured out how to secure the frame. I'll bet that thing is solid! It makes me very happy that people are getting some use out of these instructions. I have crank windows now and don't have my pics of this project because I didn't even think of documenting it as an instructable when I was making my frames for the old windows. So I have to rely on my words, some drawings, and old pictures. My husband thinks I might have over-engineered the frames a little but I figure safety first!

[yellowkoala57]

hi there! juuuust in case you're still checking in on these comments and are able to help...
I have both a standard-sized window AC unit for my bedroom and a casement window AC unit for my living room (which is suuuuper heavy - I think it weighs about 75lbs). Is this frame secure enough to hold that heavy of a unit? I can't drill it to the window frame because I'm in a rental.

[Amy3112]

Hi I was wondering what is stopping if from falling in.
I have kids and was wondering if the pushed the unit from the outside would it push it in

[DarthJason]

Most AC units have side and top vents on the outside area and depending on how you measure and piece everything together it makes it nearly impossible for someone to push the AC in. On my system the vents would stop it, and the wood frame is drilled into place in 3 spots so it can’t be moved either.

[DarthJason]

I used this as a base for my window AC install. I changed the wood covering at the top to plexiglass and bought some trim wood to place on both sides of the plexiglass along w a small amount of caulking for a tight bugproof seal as plexiglass tends to have irregularity more so than glass. I cut the plexiglass so it could sit in the middle of the upper part between the wood trim. It all looks great and I even added mini blinds! Thank you very much for the idea!

[Lhay89]

Just wanted to drop a comment here and thank starshipminivan for these instructions. Made a bracket today (Jan 2021) and it worked perfectly - super stable and no need for any further bracing or drilling into the window sash. Under two hours from cutting the timber to completing the installation.

[vanessaguycaamano]

This is a lovely way to do it, but are there other options perhaps?

[littlezentz]

in case of very skinny window frame I included the metal bar you see in many videos about this installlation. I much prefer a solid frame Hard explain when this would be needed. But a copper ground rod ran on the outside against the framing rail gave us security. We also had to adjust for a crooked window install. I hacked your hack but basically couldn't have done it without you,

[chimochimo]

Late to the party.....I understand how the frame and ac won't fall outside. But what is keeping the frame from falling inside?

[AprilJ26]

This.

[Wiafe]

If my a/c unit is too wide for my window.
Can l mount it sideway

[Bedwinnjamin]

Unfortunately, air conditioners are not meant to be installed in any other orientation. The compressor and it's coolant have to be upright, so by turning an AC unit 90 degrees and turning it on would most likely break the unit.

[BrianK253]

Awesome plans! Thank you to everyone commenting with ideas and mods. I’m going to give this one a try this weekend with some old pallet wood I have. I’ll post when it’s done.

[RiverarrowPs46]

Thank you for publishing, I found it really helpful. We are preparing to put an air conditioner into a super ancient casement window (so old it's not even crank!) and we can't quite do what you've done here, but it has helped me come up with ideas and formulate our own plan. My question is, which way did you lay the width 2x4''s - the 4" side deep and the 2" side vertical, or the 2" side deep and the 4" side vertical?

[Landbier]

Just wanted to say thanks. Good design, easy to build. Saved the day since I never considered A/C units were window style specific until we got it home and started reading the instructions. Robust and less shooglie than the design I came up with.
Cheers!
p.s. Pretty sure Rangda is happy now!

[BrandyM16]

Anyone installed an air conditioner like this - on top side of a casement window, so the cold air doesn't blow directly on people sitting in front of the window?

[BrandyM16]

Love the idea, but I have casement windows with a 15" width and 54" height, and want to move to a different location in my living room, so cockatiels (9) are not subjected to the cold air so much. The spot I want has a sofa right a the bottom of where the A/C would go. So my question: would there be any reason not to build the wood frame so the A/C is on the top portion of the window, so you don't get frozen sitting in front of it? Seems to me it would work, but what precautions should I take to make sure it doesn't fall in or out, or would it be a problem at all?

[JasonP257]

I know it's been awhile but maybe you're still reading comment emails.

I was getting ready to start putting this together when I noticed in the A/C removal instructions.

"Raise the sash and tilt the air conditioner backward draining any condensate. If you don't, it will run out while you carry the unit making a mess..."

Do you have a solution to that problem? or is it not really an issue?