Question about mounting window AC in plywood

[hush]

The room I'm setting up has crank-style casement windows, and I need to install a window AC unit. I have already removed the vertical aluminum brace that went right down the middle of the window opening (where the swing out windows would latch closed), and now I have an open space that can accommodate the small 5000 btu unit. But I obviously have to mount the unit inside of a sheet of plywood that will be cut to fit the window opening, and then glued/caulked into place.

The sheet of wood needs to be only 25 inches by 36 inches. I went to Home Depot today and saw that 3/4" pressure treated plywood comes in 4' x 8' sheets, and is pretty expensive, at like $45 a sheet. I was told that here in my very humid climate I really should go with pressure treated wood, but I couldn't help noticing how much cheaper regular old plywood is.

My question is this: Do I really need pressure treated plywood, or could I just somehow seal a regular sheet, making it impermeable to the weather?

A follow up question is, are there any alternatives to wood that would be effective for this purpose? Like aluminum sheeting or something? (Not that I know the price of that stuff or anything... I'm just looking for cost-effective alternatives).

Thank you!

 

[burns1n209]

ive been using 3/4 plywood for awhile and have no problems and dont forsee any either, homedepot has a spot where you can buy half a sheet or even smaller pieces.

 

[hush]

Thanks for the response. So are you saying you are using regular plywood, and not the PT kind? If so, what's the humidity like in your area? It gets up to 85% frequently where I am!

The ghetto home depot I went to didn't have half sheets. They did have a cart with scrapped smaller pieces but none were the size I need (they were all too small in one of the dimensions). Tomorrow I'm going to check out a super nice yuppy home depot in the rich part of town and see if their selection is any better.

 

[LEDfoot]

I don't know about Home Depot but any decent lumber place will cut panels on a jig for so much per cut.

I think I'd primer and paint some ACX or CDX plywood. I used to think the X meant exterior but no, from ezwoodshop.com :

"CDX Plywood - What Does the "X" Really Mean?
The letter "X" in CDX refers to the type of glue used in the factory to bond the plywood veneers. Some mistakenly think the "X" stands for "exterior"—which is not true, exactly. The letter stands for exposure, which means the plywood is tough enough to withstand a little moisture—but for only a short time.
For example, let's say you're in the middle of building a garage, and you've just finished putting up CDX plywood as the sheathing for the walls, which will eventually be covered with siding. But now you've discovered that your siding won't be delivered till the end of the week. Can you leave your CDX plywood exposed for a couple days? According to grading standards for CDX plywood - you can leave it exposed temporarily without worrying about water damage. In fact this is one reason why CDX plywood is used so extensively in outdoor construction. The glues and adhesives are made to be water resistant (long enough for to wait for a late delivery), but they're not strong enough to withstand rain and snow indefinitely.
CDX plywood is used under shingles and roofing felt, on walls (just behind the siding and insulation), and as a sub floor (just under the carpet pad or a tiled floor's durock). Although it's easy to find at your local home center, it's probably not the best choice for smaller projects, like a bookshelf or other furniture around the house. However, I've used it for creating rough shelving in a basement or garage, and it works fairly well for those kinds of projects where function is more important than appearance.
The letters "C" and "D" represent the quality of the surfaces—front and back. If we consider that the letter "A" is the best level of plywood veneer you can buy (that means plywood without any noticeable knots, holes, or repaired defects), then letters C and D certainly indicate a lower grade of quality when talking about plywood in general—and the price will reflect this.
CDX plywood is fairly easy to spot on the shelf. You'll see plenty of football-shaped patches and shallow holes and indentations on both sides."

 

[LEDfoot]

From that same site:

Hardware for Pressure-Treated Plywood
Keep in mind that pressure-treated plywood has a tendency to corrode and destroy metal—up to five times faster than untreated wood. That means you’ll need to use hardware and fasteners designed especially for pressure-treated wood. Look for fasteners labeled “exterior” or “galvanized” or “stainless steel.”

 

[hush]

Thanks for that information... Very helpful! I'm going to try calling a few lumber liquidator kind of places tomorrow too, and see if they have any scraps I can buy.

 

[LEDfoot]

If you need primer, you might pick up a quart of Glidden Gripper at Home Depot. I just used a gallon of it to seal an old particle board floor (two coats).

 

[hush]

Okay so I went to the rich folks' Home Depot today and they had half-sheets (4' x 4') of what they had labeled as "CD Plywood" for $19, but it didn't say CDX, or mention exterior or anything. But... since I plan on immediately sealing and painting it, it seems it doesn't need to be exterior grade... right? I think I will just go ahead and pick up one of those on my way home from work this evening.

Thanks for your help LEDfoot!

 

[Jnugg]

You're doing what I'm having to do (made a thread last week).

Do you plan on using 2x4's as support?

 

[hush]

Do you mean under the unit itself, to keep it from "pulling" on the plywood?

If so, I honestly don't know yet. My brother in law (a union carpenter, who knows nothing about why I'm installing the A/C!) told me that with the minimal weight of the particular 5000 btu unit I'm using, it might not be a big deal, and I might not have to brace it at all, especially if I use 3/4" plywood. He did say that it's possible if I use 1/2", but even then not necessarily so.

I'm kind of playing it by ear. The good thing is that there is a substantial window ledge that I will be able to rest the unit on, or wedge 2x4's underneath if necessary. But my brother in law told me that if I can get a tight fit in the wood for the unit itself, I probably won't have to worry about it. I plan on putting the unit into the plywood and caulking it with heavy duty stuff on both inside and outside, then also caulking the plywood to the concrete block on the inside.

Hopefully with all that, if I can get things to fit tight enough, I won't need to get any braces or anything obnoxious to hold it up with. The unit I bought is pretty light, I must say. I am able to pick it up over my head without groaning, and I'm a fairly skinny dude, lol. The room I'm putting it in is only 6' x 8' with a low ceiling.

 

[Jnugg]

Yeah under the plywood,building a 2x4 frame for structural support.Within that frame us a smaller frame of 2x4's just big enough fir the unit to slide through.Use plenty of weather stripping and she should be vibration & leak free.

 

[LEDfoot]

Okay so I went to the rich folks' Home Depot today and they had half-sheets (4' x 4') of what they had labeled as "CD Plywood" for $19, but it didn't say CDX, or mention exterior or anything. But... since I plan on immediately sealing and painting it, it seems it doesn't need to be exterior grade... right? I think I will just go ahead and pick up one of those on my way home from work this evening.

Thanks for your help LEDfoot!

As far as I know, all general purpose plywood sold these days uses the X type resin, meaning OK for short term eXposure. If you bought 3/4" (nominal), that's some strong stuff and you probably won't need extra bracing.

 

[hush]

Okay so I called Lowe's and they have 4x4 sheets of 3/4" for $24. That's better than the 1/2" at HD for $19. So that's what I'll be picking up!

Oh boy... now I get to try out this "plunge cut" technique my bro in law told me about... I'm a little nervous about that! But all I have is a circular saw... no table saws or jig saws or anything like that...

 

[LEDfoot]

Wear some eye protection and I like ear protection as well.

 

[Lammy]

hi hush.
I don't know if it's your house or if you're renting an apartment or what your deal is. obviously since I don't know you.

a few thoughts that I have because I removed a window to put a big ass AC through the wall.

you'll probably take this out during the winter but if not you want a layer of insulation in there to keep condensation from forming on the inside.

oil-based primer will block out moisture far better than latex. you definitely want to coat all the cut edges very well and with several coats because any moisture getting in there will destroy the plywood pretty quick.

for security purposes and to help get a better airtight seal. I would consider removing the trim around the window then putting a piece of plywood up that's far bigger than the window and screwing it into the stud that are located on either side of your window. still put a cut piece or something in there where you're planning on but I think about bigger piece flush on the wall. that will help keep light and co2 and smell inside and thieves outside and can also hold the weight of your AC.

Lammy

 

[Wendull C.]

Get a split a/c if you can afford it.

 

[hush]

I definitely cannot afford one of those, plus I don't think it would be worth it for such a tiny room.

hi hush.
I don't know if it's your house or if you're renting an apartment or what your deal is. obviously since I don't know you.

a few thoughts that I have because I removed a window to put a big ass AC through the wall.

you'll probably take this out during the winter but if not you want a layer of insulation in there to keep condensation from forming on the inside.

oil-based primer will block out moisture far better than latex. you definitely want to coat all the cut edges very well and with several coats because any moisture getting in there will destroy the plywood pretty quick.

for security purposes and to help get a better airtight seal. I would consider removing the trim around the window then putting a piece of plywood up that's far bigger than the window and screwing it into the stud that are located on either side of your window. still put a cut piece or something in there where you're planning on but I think about bigger piece flush on the wall. that will help keep light and co2 and smell inside and thieves outside and can also hold the weight of your AC.

Lammy

Hey Lammy thanks for the response!

So first of all, this is my house, I am not renting. Second of all, I should mention that this is not a room inside my house... it's actually a utility/laundry room that is attached to the house, but it is technically separate. It's built out of concrete block, so it's relatively well-insulated as it is right now. But yeah, this is not like an internal room in the house, with drywall and studs and stuff. It is entirely concrete block. The window frame was actually mortared into the block. That's why I left the frame.

About the insulation you mentioned, during winter time... can you explain that a bit more to me? I actually don't plan on taking it out during winter, because the "winter" I get here is laughable to most people, lol. We get a few cold fronts here and there, but most of the time where I live people have to run the air conditioner 12 months out of the year. So, knowing that once I put this thing in there I don't plan on taking it out (unless I have to to replace it or something, down the road), what exactly are you suggesting? That I insulate the plywood I'm mounting the unit in? Or insulating the walls on the inside? I should also mention that, if the AC unit doesn't do a good enough job dehumidifying, I plan on purchasing a dehumidifier to do that job.

I'm all ears, my friend. I'm a noob when it comes to growing outside of a small tent inside my closet in the already-air-conditioned house.

 

[hush]

I have another quick question... since someone brought this up to me in a reputation comment...

The AC unit I have does not have a drain hole. It's one of those kind that uses the condensate to help cool the radiator... so, since it's typically recommended that window units are installed at a slight angle so that the water will drain outside, do I need to drill a drain hole into the thing?

 

[Lammy]

I definitely cannot afford one of those, plus I don't think it would be worth it for such a tiny room.
Hey Lammy thanks for the response!

So first of all, this is my house, I am not renting. Second of all, I should mention that this is not a room inside my house... it's actually a utility/laundry room that is attached to the house, but it is technically separate. It's built out of concrete block, so it's relatively well-insulated as it is right now. But yeah, this is not like an internal room in the house, with drywall and studs and stuff. It is entirely concrete block. The window frame was actually mortared into the block. That's why I left the frame.

About the insulation you mentioned, during winter time... can you explain that a bit more to me? I actually don't plan on taking it out during winter, because the "winter" I get here is laughable to most people, lol. We get a few cold fronts here and there, but most of the time where I live people have to run the air conditioner 12 months out of the year. So, knowing that once I put this thing in there I don't plan on taking it out (unless I have to to replace it or something, down the road), what exactly are you suggesting? That I insulate the plywood I'm mounting the unit in? Or insulating the walls on the inside? I should also mention that, if the AC unit doesn't do a good enough job dehumidifying, I plan on purchasing a dehumidifier to do that job.

I'm all ears, my friend. I'm a noob when it comes to growing outside of a small tent inside my closet in the already-air-conditioned house.

well I hope I understand you right that your air conditioner is going to be mounted in an exterior wall? that is made of concrete block.

well I live in a colder area and it's often around 30 degrees in the winter. if it gets cold enough outside the AC doesn't want to work right if for no other reason than the metal body of it gets really cold because most of the metal body is hanging outside in the freezing weather and that screws with the reading on the temperature probe which although it is inside still reads way way colder than the room. so the air conditioner never wants to kick on. I made mine slightly better by pulling the temperature probe out of the grill and hanging it outside of the air conditioner but it doesn't help that much. plus they say you need a low ambient temperature kit installed on the window AC to use it when it gets to low of temperature. I don't remember what that is. but aside from the false readings mine seems to work quite well as low as the temperature ever got except for when condensation would form and freeze on the fan.

but it sounds like you won't have too many of those problems. but if the temperature is low enough outside it will still cool the body of the AC and could cause condensation inside your structure. I doubt it'd be that much of an issue for you. it's driving me nuts.

as far as insulation what I was talking about: for instance, when I cover up a window. I like to cover the entire window with one fancy insulating foam board. when I just use a single piece of plastic I get condensation on the inside the plastic. it's also harder to stop light leaks with just the plastic just one single little pin hole in now light has gotten through that layer of Defense. and then there is the issue of getting robbed which is why a piece of plywood is nice. the plywood has more insulating value than a piece of plastic and condensation won't form as easy.

anyway I think a really good solution considering light leaks, air leaks, holding the weight of your air conditioner preventing break ins. I would get the foam board with the aluminum foil backing. cut it exactly the same size as your plywood except for a tighter fit around the air conditioner. place the foam board up against the wall and then the plywood over top of that. get those really nice screws for anchoring wood into concrete. put the screws into the plywood through the foam board and into the concrete. when you tighten down the screws, that should create a pretty decent seal between the foam board and the block wall. what I was just talking about I would make larger than the window opening. I would still put something else in the window opening to prevent water and weather from getting in. depending on whether not anybody can see it I might just be really lazy and cut it out of that foam board considering you can cut it with a razor knife and its weatherproof. you can cut it so that you can get a tight fit in areas and it'll pretty much stay in by itself

 

[Lammy]

let it hang at an angle