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Old 06-10-2020, 09:38 PM   #1
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Flooring question (compressive strength of unframed foam boards)

Hello,

Based on learnings from this forum, I'm planning to do an unframed foam panel base with plywood overlay for my floor. I'm really trying to save weight by using 1/2 plywood. Also, I'm already very close to crouching in this thing and I have a bad back, so I'm trying to keep every bit of head room.

I'm hoping to use these panels linked below as they're almost 2 whole R-value higher than the standard pink boards. The compressive strength of these is "> 16psi" (vs 25 psi with pink stuff). Is that sufficient with standard 3/4" T&G plywood? How about with 1/2"? The center walkway for this bus will only be 7' x 40".

Thanks!

https://hw.menardc.com/main/items/me...alSpecs_19.pdf

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Old 06-10-2020, 10:03 PM   #2
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I can't speak on the exact specs, but I put down 2" insulation with no framing. Screwed my plywood directly threw it and into the floor of the bus. no issues at all 2 years later...
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Old 06-10-2020, 10:26 PM   #3
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I did 1" pink board (which I thought was 15 psi) topped by 3/4" tongue-in-groove plywood. No framing, just plywood on the polystyrene, did sink some screws here and there. I believe, with 1/2" plywood you'd have to land the plywood on a sleeper.

I think the 1" 15 psi foamboard topped with the 3/4" plywood is a great way to go, at least for me in Western Washington. I put sleepers where I was going to put my water tanks, but I changed my plans and the water tanks are now going over the rear wheels.
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Old 06-10-2020, 11:38 PM   #4
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Polyiso insulation (the stuff you linked) is usually rated at R-6 or R-6.5 per inch, vs. R-5 per inch for XPS foam. However, the R-value of polyiso actually declines in lower temperatures, which is unusual since most types of insulation have lower R-values in higher temperatures. In cold weather climates, polyiso is usually rated at or below XPS foam (in the neighborhood of R-4 to R-5).

16 PSI is probably fine for an unframed floating floor, even with 1/2" plywood. If you weigh 200 pounds, you could stand on a 4"x4" square of plywood (16 sq. inches) on top of the foam and you would exert a pressure of 12.5 PSI. Plywood is stiff enough to spread the load over a larger area than that.
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Old 06-10-2020, 11:55 PM   #5
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If a person stands about as small as they can, which I'd say is 12"x12", they could weigh a ton, at a 15 psi foamboard loading. Of course it would then depend on the plywood to spread that ton.
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Old 06-11-2020, 12:49 AM   #6
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Thanks, all. Based on the feedback, I think I've settled on the polyiso foam board, unframed, with a 5/8" tongue & groove Sturd-I-Floor. I was not aware that the insulation loses some properties in cold weather, but if it drops to something similar to the pink stuff, I don't think I'm any worse off for it. The bus will primarily be used in the desert southwest for the next few years.

Foam Board Link

5/8" Sturd-I-Floor

I'm still on the fence on foam thickness (1"=R-6, 1.5"=R-9), but leaning toward the 1", then seeing how that goes. With ripping out the stock floor, that will have a minimal impact on headroom (also using 1" in walls, with 5/32" beadboard over it, with stub-outs off the chair rail for anchoring furniture).
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Old 06-11-2020, 09:06 AM   #7
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FWIW, I used 2 inch foam and I think my flooring was 23/32 or 3/4. used 3 inch decking screws to secure it all the way through the floor of the bus. 2 years since install. now issues at all. Cheers
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Old 06-11-2020, 10:52 AM   #8
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Quote:
Originally Posted by FatBoySTL View Post
Thanks, all. Based on the feedback, I think I've settled on the polyiso foam board, unframed, with a 5/8" tongue & groove Sturd-I-Floor. I was not aware that the insulation loses some properties in cold weather, but if it drops to something similar to the pink stuff, I don't think I'm any worse off for it. The bus will primarily be used in the desert southwest for the next few years.

Foam Board Link

5/8" Sturd-I-Floor

I'm still on the fence on foam thickness (1"=R-6, 1.5"=R-9), but leaning toward the 1", then seeing how that goes. With ripping out the stock floor, that will have a minimal impact on headroom (also using 1" in walls, with 5/32" beadboard over it, with stub-outs off the chair rail for anchoring furniture).
The plywood you've picked is interior rated ply. It will not do well with any water leakage...
If you know your bus is well sealed I guess this is less an issue. I would still paint both sides and edges of the board (unless the top side is for show with no additional flooring) with any good house paint to seal the wood. Doesn't have to be exterior because no UV is involved -- oops-discount bathroom/kitchen paint would be a good deal...
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Old 06-11-2020, 08:22 PM   #9
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Quote:
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The plywood you've picked is interior rated ply. It will not do well with any water leakage...
If you know your bus is well sealed I guess this is less an issue. I would still paint both sides and edges of the board (unless the top side is for show with no additional flooring) with any good house paint to seal the wood. Doesn't have to be exterior because no UV is involved -- oops-discount bathroom/kitchen paint would be a good deal...
Thanks for the advice. I was under the impression that exterior rated (i.e. pressure treated) plywood was a no-no for interior applications. Is that no longer true?
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Old 06-11-2020, 09:19 PM   #10
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Quote:
Originally Posted by FatBoySTL View Post
Thanks for the advice. I was under the impression that exterior rated (i.e. pressure treated) plywood was a no-no for interior applications. Is that no longer true?
Modern PT lumber uses copper-based compounds so it tends to promote corrosion in steel. Older style PT tended to promote cancer and death so it's probably a good trade-off.
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Old 06-11-2020, 10:31 PM   #11
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Thanks for the advice. I was under the impression that exterior rated (i.e. pressure treated) plywood was a no-no for interior applications. Is that no longer true?
Pressure treated and exterior grades are two very different things --

So, correct, you DON'T want pressure treated.

(I think) you DO want exterior grade which means it was made with a water proof glue and will handle moisture, condensation, leaks ect w/out buckling...

Pressue treated means you can put it directly in contact with dirt.
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Old 06-11-2020, 11:23 PM   #12
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I used 1/2 Polyiso and 1/2 Baltic Birch Plywood because I was in real height saving mode. I was concerned about compression so I did run some strips where heavy weight was going to land- perimeter of cabinets, and a strip down the middle of my isle. I used the Baltic Birch (basically cabinet grade ply) because it has no voids and more plys making it stiffer that the pathetic stuff I could find at HD. It will be my finish flooring- stained and sealed with polyurethane. The things I do to save 1/8" of height. All glue, no screw.
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Old 06-12-2020, 07:53 AM   #13
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Quote:
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I used 1/2 Polyiso and 1/2 Baltic Birch Plywood because I was in real height saving mode. I was concerned about compression so I did run some strips where heavy weight was going to land- perimeter of cabinets, and a strip down the middle of my isle. I used the Baltic Birch (basically cabinet grade ply) because it has no voids and more plys making it stiffer that the pathetic stuff I could find at HD. It will be my finish flooring- stained and sealed with polyurethane. The things I do to save 1/8" of height. All glue, no screw.
That will be a sexy floor!

I like the idea of saving weight -- even if it's just a game in my mind...
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Old 06-12-2020, 08:21 AM   #14
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Quote:
Originally Posted by peakbus View Post
I used 1/2 Polyiso and 1/2 Baltic Birch Plywood because I was in real height saving mode. I was concerned about compression so I did run some strips where heavy weight was going to land- perimeter of cabinets, and a strip down the middle of my isle. I used the Baltic Birch (basically cabinet grade ply) because it has no voids and more plys making it stiffer that the pathetic stuff I could find at HD. It will be my finish flooring- stained and sealed with polyurethane. The things I do to save 1/8" of height. All glue, no screw.
I used a piece of 3/4" 4x8 baltic birch plywood for my sunken floor section and that stuff is rock-solid and smooth. Expensive to do the whole floor that way, though, since it's $55 a sheet at Lowe's right now. I got my piece for $25 because the edges (which I cut off anyway) were bunged up.
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Old 06-12-2020, 05:34 PM   #15
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I used a piece of 3/4" 4x8 baltic birch plywood for my sunken floor section and that stuff is rock-solid and smooth. Expensive to do the whole floor that way, though, since it's $55 a sheet at Lowe's right now. I got my piece for $25 because the edges (which I cut off anyway) were bunged up.
Yeah, I got lucky as it was only $4 more than a standard sheet of HD's "Sande" wood. Oh, well I did have to buy an extra sheet since they aren't the same sq footage, but it's just bus money
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Old 06-13-2020, 12:29 PM   #16
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I used the polyiso you had listed and bought from menards also. I believe I went with 3/4 or 1" for my floor ( I can't remember! Would have to read back through my build! Lol) and 3/4" plywood over it. I used self tapping deck screws (for a trailer) and screwed down through the sheet metal floor to attach plywood sandwiching the foam. I used no framing underneath.

With the 1" polyiso I can walk on it (size 10.5 shoe 250 lbs) and it'll hold it's shape no indentions. But if I kneel down on a knee (smaller footprint increasing psi) I would create and indention in it. So plywood spreads out the weight and I've had no issues so far. Installed my sub floor over a year ago. Good luck!!
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Old 06-17-2020, 03:56 PM   #17
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I wouldn't

A few years ago I built a refrigerated trailer out of DIY SIPs consisting of two 3" EPS panels sandwiched between 3/4" OSB. Everything was glued with foam glue so as to form homogeneous structures. I deeply regret not using 3/4" ply for the walking surface. The OSB floor panel has sagged in a few places where interior fixtures rest on it. Maybe 1/2" ply will be sufficient, but I would not use less the 3/4".
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Old 06-17-2020, 04:35 PM   #18
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Quote:
Originally Posted by Rovobay View Post
I can't speak on the exact specs, but I put down 2" insulation with no framing. Screwed my plywood directly threw it and into the floor of the bus. no issues at all 2 years later...
+1

except I did it for a portable storage building with 1” poly iso & 3/4 plywood. No negligible compression noticed... I think with the cross layered lamination of plywood, direct force per inch is distributed very well.
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Old 06-17-2020, 04:40 PM   #19
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Made my panels after fitting. Glued 1/2 x1-1/2 hardwood sleepers to the ply and fit the foam board to shape in between. Only where the Isle was and cabinets/appliances went. Used 3m 5200 to set them in and no creaks or squeaks. Sikaflex makes similar adhesive and both stay pliable for years.
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Old 06-23-2020, 01:28 AM   #20
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If you are going to put flooring down over the plywood, please read the flooring information sheets. You may find that the top surfacce of the plywood needs to be UNPAINTED to work with your flooring. This was the case with the flooring we installed ... Alure ISO-Core (now called LifeProof).
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