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Old 01-07-2023, 12:44 PM   #1
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Framing subfloor with spray foam? And thoughts on C channels for plywood

Hello everyone! Iím currently doing the subfloor in my skoolie (40ft flat nose blue bird) and I want a floating subfloor to avoid the thermal bridging. However, there are gaps and whatnot along the walls and where the pink foam sheets meet up with each other (Iím using 2Ē thick polystyrene). Iím planning on using the big gap filler spray foam to hit all those patches. HOWEVER, that got me thinking- what if I laid out the floor like people do who are framing it with wood, and instead of using wood, just used spray foam in those areas so it would be a grid of spray foam and foam board. Has anyone done this? What are yíallís thoughts? The idea would be to have complete coverage and zero thermal bridging from the floor.

Additionally I will be using tongue in groove plywood on top of the foam board. It has been stored on its side for quite a while now and Iím afraid it may have warped. To avoid any additional warping Iím contemplating using C channels on each piece. Especially perpendicular across the tongue and groove connections. Iíve seen table makers use this on slabs of wood to guarantee they donít warp. Has anyone does this? My hesitation would be either the c channel rusting or it just being a waste of time and money. I want the floor as flat as it can be though. Thanks for your time!

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Old 01-07-2023, 02:22 PM   #2
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Spray Foam Floor

Something like this?





Carneiro's thread here:
www.skoolie.net/into-the-mystic
.
Wood in-between the steel floor and steel channel? Tell us more about the C-channel....
.
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Old 01-07-2023, 02:41 PM   #3
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The xps foam can go down and then the subfloor over the top without additional support. When covered by subfloor the foam is strong enough to support any weight you put on it. Then you can fill any gaps or cracks with spray foam. I used formular 250 and it is solid with the subfloor on top of it.

You can use foam compatible construction adhesive to glue the foam to the floor and then the subfloor to the foam to eliminate squeaks. Running a few self tapping screws through the subfloor, foam and bus floor will keep you subfloor flat.

Here is a good tutorial.

https://youtu.be/us4WejejFWE

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Old 01-07-2023, 03:16 PM   #4
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Yes that’s the design I’m picturing EXCEPT replace the spray foam with pink foam board. Then replace the wood with spray foam. Does that make sense? I’m guessing once the foam hardens it’ll be at least as sturdy as wood. But won’t have the thermal bridging that wood has.


The c-channel would be mounted flush with the top surface of the plywood, spanning the width of the boards across the tongue-groove lap.
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Old 01-07-2023, 06:47 PM   #5
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If you're going to be using warped T&G plywood for your flooring, you're going to want studs|joists|furring strips|whatever you want to call them on the floor to provide a solid mechanical connection between the plywood and the steel floor - otherwise your floor is not going to lay flat.

Thermal bridging is a problem but it's not that big of a problem that you need to be worried about having a small amount of wood in your floor. Wood conducts heat about 5X as readily as insulating foam, but since only about 1/10 of the floor is wood instead of foam (assuming your strips are 1.5" wide laid 16" on center) it's really not hugely significant. Houses with wooden floor joists have the same ratio of wood-to-insulation and it's not some sort of crippling situation in cold weather.

Since your foam provides some compression resistance, in fact, you don't even need to have your strips 16" on center. They could be 24" on center, and you could even use 1X instead of 2X so the strips would only be 3/4" wide each (you just need to be more accurate in your screw placement). In fact they don't even need to be full strips to anchor your plywood - for my floor, I welded wood screws upright on the steel floor then spun pieces of oak dowel rod onto them through matching holes in the foam board, then screwed the plywood layer into these dowel pieces. Since you're going to be spray-foaming your floor, you could just attach small wood blocks to your floor in a 16"-on-square grid and then spray foam around them.

Thermal bridging is mainly a problem when you have steel projecting through your insulation layer into your living space, since steel conducts heat about 1500X more readily than foam insulation. This is why people who insulate between the ribs (or "bows") then attach their ceiling material directly to the ribs come to regret it in wintertime.
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Old 01-08-2023, 12:29 PM   #6
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Quote:
Originally Posted by Nick5272 View Post
Hello everyone! Iím currently doing the subfloor in my skoolie (40ft flat nose blue bird) and I want a floating subfloor to avoid the thermal bridging. However, there are gaps and whatnot along the walls and where the pink foam sheets meet up with each other (Iím using 2Ē thick polystyrene). Iím planning on using the big gap filler spray foam to hit all those patches. HOWEVER, that got me thinking- what if I laid out the floor like people do who are framing it with wood, and instead of using wood, just used spray foam in those areas so it would be a grid of spray foam and foam board. Has anyone done this? What are yíallís thoughts? The idea would be to have complete coverage and zero thermal bridging from the floor.

Additionally I will be using tongue in groove plywood on top of the foam board. It has been stored on its side for quite a while now and Iím afraid it may have warped. To avoid any additional warping Iím contemplating using C channels on each piece. Especially perpendicular across the tongue and groove connections. Iíve seen table makers use this on slabs of wood to guarantee they donít warp. Has anyone does this? My hesitation would be either the c channel rusting or it just being a waste of time and money. I want the floor as flat as it can be though. Thanks for your time!
Interesting concept (spray foam "framing") but seems like a lot of extra work with very minimal value in net-r-value-increase. I'm just going with a floating floor on XPS foam and will use some spray-foam-in-a-can around the edges/gaps. Then usual T&G subfloor. The KISS method is usually your friend - Keep It Simple & Stupid

If your subfloor is already warped, in your shoes I'd bite the bullet, sell it at a discount on FB or Craigslist and go buy new rather than try to engineer a way to flatten it.
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Old 01-29-2023, 12:55 PM   #7
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Well said, I’m my opinion.
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