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

Picture of 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.

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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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.


buradd made it! (author)2014-05-17

YAY! I have a window AC in my sliding casement window now! Thank you so much for the instructable, very easy to follow! The only thing I had to get differently was a more powerful drill because the cheapo I had wouldn't even go thru the wood without the battery dying. I used 1x1's for the frame with a 2x4 base standing upright (to clear the window frame) It's soooooo coool in my room now! Love it! Thanks again!

starshipminivan (author)buradd2014-07-20

Great job! It looks good. I hope you enjoy your cool sanctuary!

Tinker Tom made it! (author)2012-05-28

Great instructable. I moved into a highrise last year with side sliders and wasn't sure how I was going to mount an AC.

I picked up the materials at my Local Home Depot and had the cuts made there as well. The only tools required were a drill to pre-drill the holes and a screwdriver. The entire project cost about $70, $45 of which went toward two sheets of plexiglass and mini blinds. The second sheet of plexi diverts drips from above.

Thanks for the great idea.

Photos attached.

What a beautiful thing! $45 for such a nice finished product is great! I love the miniblinds and curtains.

Thanks. I have a southern exposure so the blinds and curtains help block the midday sun.

nevroth (author)2010-08-26

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 (author)nevroth2010-08-26

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 (author)nevroth2010-08-27

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!

BrandyM16 (author)2017-09-04

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 (author)2017-08-18

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 (author)2017-08-01

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?

SuziQ22 made it! (author)2017-06-19

Thank you so much for creating this post. Thanks to you I was able to do this project all by myself and now have a/c just in time for this week's 110+ heatwave. Of course I had to tweak the details for the particularities of my window and a/c/ unit (which had large 'feet' on the back preventing it from sliding into a pre-assembled frame. I had to assemble sections of the frame and then fasten the sections together during the installation process (that was 'fun' .. LoL) For this season, I focused on structural support and my results, I admit, are neither pretty nor perfect. In October I will remove the unit and use the frame as a template to make a nicer one for next summer :-) Again, Thank you so very much!!

SlotsF (author)2017-06-12

We ended up using a stiff flat bar on the upper window bracket in order to "extend it to the side so it sits against the window frame on one side and the sliding window on the other. We took some thin plywood and used it to sandwich some insulation board between it for our filler panel. We then just caulked to seal. I have vinyl windows and the channel was 1.5" so I had a good bit of room to work with.

SandiJ4 (author)2017-06-05

I am hoping for some advice. This tutorial is the closest to what I need. My window is a double-hung type, but my AC has no louvers to adjust for the sides. Would I need some plywood? Or just some 2*4 type material? Window is vinyl, frame is wood. Advice appreciated!

DIY-LOL (author)2016-08-07

You are a huge help, thanks for the instruct, this is how I am going to mount my 2nd AC I just got for my bedroom, and I'm also going to redo the front room as well, as it is set up with a typical rigged outside support base made of wood that is big and bulky. This will clear up space and look professional as well. I had bought 2 sheets of thick plexi to rig in to a standard jimmy rigged install but now they will go with this instruct as part of a nice clean pro looking install, painted and all. Can't wait to get crackin! Thanks again!

DavidH599 (author)2016-06-22

Excellent idea! I made this for my 3 year old daughter's room as it can get fairly toasty in the evening being on the south side of the house. I was able to add the screen and door instead of the simple panel as suggested. I used 1x2's as panelling to secure the screen to the outside of the frame, which gives a nice polished look from the outside of the house. Thanks so much for the wonderful idea!

chimochimo (author)2016-05-02

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

chimochimo (author)2016-05-02

Really late to the party...but what am I missing. I understand how the design prevents the AC from falling outside or inside. And I understand how the design prevents the frame from falling outside. But what is preventing the entire frame from falling inward?

The weight of the air conditioner (well, its compressor) is what helps with this. They are designed to be heavier toward the back to help them angle downward slightly when installed properly. This is important because it helps the units drain water that accumulates on the evaporator coil through normal use. Water drips from them outside, not inside, because of the angle created through the design/weight distribution.
If you want, you can secure them to your window opening, either with angle brackets or by screwing through the a/c frame into the stud that frames the window.

MikeS68 (author)2015-07-23

I tried hard to get someone--anyone!--to install a standard a/c unit in a slider window in our living room but no luck. One guy who came over said I needed a mini-split system (not a piddly window unit) and gave me a quote of around $4,000. I wish I could afford that, but that is definitely not in the budget. Which is why this post is a godsend.

I'm about the furthest from a carpenter a guy could possibly be. Still, these instructions looked, well, doable, even for me. Spent about an hour walking around the local big box hardware store looking for lumber and plywood and deck screws. I also had to buy a cordless drill and a cool trim saw attachment I used for the plywood panel. Wife and dog are happy now. Guess that makes me happy too. Mahalo starshipminivan!

alex.melodymaker made it! (author)2015-05-30

Took about four weekends + several visits to the hardware store at different phases of the build to make two of these for our apartment, but it was well worth it. Thanks to starshipminivan for this Instructable, and also to Tinker Tom for the pics of his modified version that gave me the idea to take this project a step further.

jackel_III made it! (author)2015-03-27

Thanks for the instructions, I added Plexiglas rather than wood. It was fairly easy to make

Riorank. made it! (author)2015-03-20

All done.

john.d.watkins (author)2014-08-29

yes great instructable, I wish I would have seen this last year.

anna.brightmeadow (author)2014-07-28

i am wanting to do this in a pinch. I pulled a table up to window , stacked books, to prop ac up to level of sill. Taped a painting in window and cardboard to fill gaps. But, the water wouldn't drip out window because it was tilted toward the sill , inside. Hence not platform.... Now i have dismantled. Suggested to me to use siding inside of painting which is framed. Does siding come in slabs rather that strips, i wonder. Any suggestions. I'll watch the video in How To steps you provided. I want to see how to sit my ac onto the sill. I dont see a platform here in pic, and that's usually how it's done.

starshipminivan (author)2014-07-20

I'm so glad you got good results! Thanks for posting the picture. I hope it works for years to come!

seaweed009 (author)2011-07-19

Hi, My window space is 17 3/8. Do you have any suggestions for me? I am desperate. It is very hot this week and Friday will be 101, maybe higher too due to humidity. I am going to buy a 5000 btu unit and am hoping to somehow get this in this space. There is not enough space to frame it. I could place a wood frame above the unit but how to hold the unit to this frame is the question. Plywood would of course cover the frame.

starshipminivan do you have any suggestions? Thanks for your help or anyone else who has any ideas.

Oh dear, I see that I did not respond to this comment when it was originally posted. I hope you found some relief. In that situation, where width is tight, I'd simply build a bracket to mount to the house which would support the unit, if possible.

jenga-kun (author)2011-07-18

How do you screw together the 2x4's when they are perpendicular to each other?

You screw through the outside piece (the vertical ones as shown in the picture) into the horizontal piece. Deck screws, which I probably should have pointed out by giving their dimensions, are very long and sturdy and should have no problem securing the two pieces in this way.

What I meant was how do you hold the wood together to screw them? Screwing together two 2x4's when they are perpendicular to each other is kinda hard. Do you put them in a vice? Do you glue them beforehand then screw them? Do you get other people to hold the wood while you screw them?

Also do you have to drill pilot holes beforehand?

I can see my reply is coming very, very late. I used my husband to hold it together but 90 degree angle clamps are extremely helpful here. I did drill pilot holes.

stubbsonic (author)2011-07-24

I just finished this project. Thanks for the instructions.

It was a little more challenging because I only had warped 2x4's. Perhaps I got lucky because everything fit ok. Needed a little tweaking here & there.

My cross-piece in the center had to stick out a little toward the front to accommodate the A.C. unit. Stapling the screen to the back worked fine. I worked with some bracket steel and made little holders that the cover board slid into on the bottom, and fashioned some twist latches on the top corners to make installing and removing the board easier.

Thanks again. It is SO hot. The swamp cooler just doesn't work anymore. Now our bedroom is our little oasis.

jsanchez17 (author)2011-07-05

This may be a silly question but can I just use plastic to fill in the extra room around my air conditioner hose? I have a standing unit and a side ways opening window, so instead of using wood to fill in the top I just used a big sheet of plastic, will this work?

For my portable air conditioner--the kind with the hose--I have a piece of plexiglass that's made to fit my window opening with the hole for the hose. So, yes. The plexi was cut and framed to replace the window screen during the summer months. It was done at the local glass shop. Since most of my windows have the same size screen, it can be moved around between each of the windows.

debbiedoo (author)2011-06-29

Thank you again for the diagram confirming installation. : )

marquisk2 (author)2011-05-29

Will something like this work if the window only opens roughly 15 inches? I don't think I can find a a/c that's small enough!

I don't think it would simply because, you're right, the standard air conditioners are larger than this, even the tiny little 5,000BTU units are something like 16" wide. The 2x4 frame actually narrows the width of the window opening (depending on the window framing itself) but you could probably build a metal frame that would not narrow down the opening of the window any more. However, the size of the air conditioners are still the problem here.

traycerb (author)2009-08-31

Thanks for the great instructable. I had the same dilemma, and some websites mentioned a solution was to build an "interior frame" but I wasn't clear on what that entailed. Your detailed diagrams helped me finish building this yesterday, and I had my first pleasant night in 2 months. I'll be putting up plexi, (got a temporary cover over the top right now), and I still need to weatherstrip the (sizeable) gaps, but all-in-all, it turned out great.

CANDACE SMALL (author)traycerb2011-05-25


bfarm (author)2010-07-12

This would go well with an upholstered couch on the front porch and lots of external cable TV wires. Some old tires on the front lawn (or blacktop) would complete the look.

canida (author)bfarm2010-08-25

Hey, be nice!

Few a/c hacks are pretty, but when it's hot you do what you need to do to stay cool.

starshipminivan (author)bfarm2010-07-12

I can live with it for two months out of the year and I think a lot of other people feel that way too. I've been in beautiful houses that are hot hellholes in the summer. My guests would be more offended if I didn't have air conditioning because my 2nd story gets quite hot without it. Once built, I can install and uninstall it in a few minutes which is its beauty.

LauraBS (author)2010-05-31

I think that looks great.
Sure, the Plexiglas looks nicer but I'm sure the cost is high on that.

I have the window brackets to hold my air conditioner in, but don't have a clue how to install properly. No instructions, purchased used.

Is there any way you could show how to do that?

I would really appreciate.

I can't even figure out how to remove the air conditioner from the case to install to the brackets first and then put the conditioner back into the case.

That is what a neighbour said they thought I should do.

I have searched for hours online looking for some kind of instruction with no luck.

Thanks for all your suggestions.

starshipminivan (author)LauraBS2010-07-11

I'm sorry you had problems with your air conditioner. I see it's been over a month since you posted. I hope you managed to install it and enjoy it.

LauraBS (author)starshipminivan2010-07-12

Thanks for your concern. I did get it set in the window, but not exactly the way I was looking to do it. Keeping cool though. :)

rayterrill (author)2010-07-09

Excellent work, starshipminivan. I used this Instructable last night, and had my AC unit installed in my casement window in just over 1 hour. Well done! :D

It is so neat that people are actually using these instructions! Stay cool!

As soon as I saw how much more expensive the sliding/casement-style AC units were (3-4 times as expensive as the ones intended for double-hung windows!), I thought 'someone has already solved this problem'.

I also mentioned it in my little blog -

Thanks for the help!

Thanks for the link! That cost issue is precisely why I built these frames. My air conditioners worked just fine, so why buy replacements when you can fit a square peg into a round hole with a little bit of lumber and time? Enjoy your air conditioner.

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Bio: I'm a Renaissance woman. I love to create things with a fantasy, medieval, or geeky edge. I'm also a math/science nerd. I ... More »
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