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budedm 04-15-2011 01:07 PM

A/C thru a wall
I have a big 18,000 BTU a/c that I plan on putting thru a wall in my workshop, instead of a window. I think I've got the jist of it but I'm a little foggy on a few things. It's a 2 x 4 wall with 7/16" OSB on the inside and 7/16" OSB + cedar lap siding on the outside. My question is how do I support it on the outside? I bought a steel brace at Lowes but I don't think it will work. I think it's too short. So, my thought is to run two treated 2 x 4's outward on each side of the a/c and then angle brace each from the end - down and toward the garage. I will cut out the wall, (it's a non load bearing wall), and run a 2 x 4 sill accross. But how would I attatch the brace to the sill? I'm thinking I may need a double sill. The bottom would be all the way accross, and the top one would be in the middle of the opening leaving 3 1/2" shy on each side for the braces, which would go outward under the unit. Does that make sense? And also, maybe I should use 2 x 6's for the sill so they would cover both inside and outside OSB and siding. Any ideas would be appreciated. Thanks

oberkc 04-15-2011 01:32 PM

I would probably trim the exterior of the opening as if it were a window...including caulk and seal. I would tend to avoid running the brace as you describe it (integrated into a double-sill) for fear that water may get absorbed and make its way into the wall structure. I would attach the brace to the exterior wall sheathing and, if possible, on top of a wall stud.

I would remove some of the cedar siding, sized to expose the sheathing to match a piece of trim attached vertially, below the AC opening, where the metal brace could be attached. Fill the new opening with the cedar trim board, caulk the gaps, and attach the metal brace through the cedar and into the sheathing (and stud, if possible).

BTW, if you don't have one of those neat little reciprocating saws, this would be a great excuse to treat yourself. This would be the perfect tool to make the cut on your siding.

budedm 04-15-2011 01:40 PM

Ok. I gottcha on the brace. Good idea. And you'd put brickmold around the opening? Would you use a 2 x 4 or 2 x 6 for the sill?

oberkc 04-15-2011 03:17 PM

I don't consider brickmold to be a good match to lap siding, but if that is consistent with the existing trim, then that may be your best choice. My assumptions were to simply use 1x4 or 1x6 cedar boards as a window/AC-opening trim. So long that it is weather proof, pick one that looks consistent with the style of the rest of the structure.

As far as a sill, 2x6 sounds good to me. I would put a water barrier between it and the rest of the structure. Perhaps something like an ice and water shield membrane would work.

Ron6519 04-15-2011 04:02 PM

The large A/C's usually have brackets for the exterior that integrates into the chassis runners on the bottom.

When you do the opening, make sure you frame the opening enough to get the slope you need for drainage to the exterior. I usually make the framed opening about 3/8" larger then the chassis the A/C sits in.

dtsman 04-15-2011 10:26 PM

Create your own bracket using angle iron or steel. A small single piece of this centered on the unit will be nice and clean. If not, this gives you some idea of a path to consider for supporting.

Cut and frame your opening to fit the the shell of the AC with maybe a 1/4" gap all around, tops maybe less. Brickmold or cedar or whatever looks good to trim it out. Slide the unit in last and seal with silicone to the trim. Use treated lumber for the seal plate. It will stay damp because of the metal sweating when it runs and the cold water that sits in the bottom.


If the women don't find you handsome,
they should at least find you handy.
(Red Green)

Gary in WA 04-15-2011 11:08 PM

Use a waterproof sealer on the sill wood whichever type you use, p.t. is treated against fungus and bugs, not water.


dtsman 04-16-2011 09:52 AM

I wish I had time to quote irrelevant documentation.

The initial question in all the words was, quote, "My question is how do I support it on the outside?
My suggestion being a single piece of metal vs framing with 2x wood.

I installed 56 AC units into a hotel. Of course they weren't the same size as the originals. Plans called for PT plate for it to sit on, whether it touched concrete or not, and NOT because of #2 fungus or the #3 bug resistance, but the #1 reason being rot (exposer to water/moisture).

I don't understand GBR why you challanged the statement to use a rot resistant wood in an area that will be subject to moisture.


If the women don't find you handsome,
they should at least find you handy.
(Red Green)

Tizzer 04-16-2011 12:52 PM

This bracket may do the trick, and adjusts for proper angle.

I think GBR meant while a PT sill won't rot, but it's still porous and over time will wet the wood under it unless it's sealed.

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