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Old 03-11-2021, 10:16 PM   #1
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What to use on floor insulation?

We are in the works of purchasing a skoolie and starting to plan things out! We have decided to use closed cell spray foam for insulation. I noticed in build videos people who go this route use the rigid foam for the floor. Is there a reason or advantage to using the rigid foam on the floor rather than doing the whole thing with spray foam?
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Old 03-11-2021, 10:28 PM   #2
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I think the primary reason is that you want your floor to be flat...and the foam boards come that way. Plus, they're cheaper per square foot than spray foam. If you had to fill joist bays, spray foam is the way to go. But on a flat floor the boards are just easier and cheaper.
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Old 03-12-2021, 03:07 AM   #3
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The rigid xps boards are flat, square, cheap, easy, and effective. If you decide to add radiant floor heat they make that easy as well. They also hold up to pressure better, as in if you have something heavy on it, it won't dent inwards
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Old 03-15-2021, 05:45 AM   #4
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We went with closed cell for the walls and ceiling, we had a company come and do it, it is some pretty tough stuff. I had the wheel wells foamed, but the floor was done in xps with 2x3'' framing. The foam 2'' boards fit with a small gap to the floor. I would not see any issues if you framed the floor out and sprayed closed cell foam in there, trimmed and laid the subflooring on top. I Might imagine a professional company charging a little more to do it. I think if I was to do it again, I would have the floors sprayed
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Old 04-06-2021, 07:36 PM   #5
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Approximately how many inches do you lose vertically after insulating the floor and ceiling with spray foam above and xps board below?
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Old 04-06-2021, 08:57 PM   #6
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Approximately how many inches do you lose vertically after insulating the floor and ceiling with spray foam above and xps board below?
It's up to you how much you lose. If you spray foam the ceiling in between the ribs and then attach your ceiling material directly to the ribs, you lose only the thickness of your ceiling material (3/4" t&g, 5mm underlayment etc.), but you still have the steel ribs creating a thermal bridge. If you attach furring strips to the ribs and spray foam out flush with them, you will have better insulation but you will lose an extra 3/4" of headroom.

Your floor is basically going to be foam board plus plywood plus whatever flooring you choose. If you want 2" of foam board insulation, you'll lose about 3" of headroom (2" foam + 3/4" plywood + 1/4" flooring). If you only want 1" of floor insulation, you will only lose 2" of headroom etc. etc.
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Old 04-07-2021, 05:02 AM   #7
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It's up to you how much you lose. If you spray foam the ceiling in between the ribs and then attach your ceiling material directly to the ribs, you lose only the thickness of your ceiling material (3/4" t&g, 5mm underlayment etc.), but you still have the steel ribs creating a thermal bridge. If you attach furring strips to the ribs and spray foam out flush with them, you will have better insulation but you will lose an extra 3/4" of headroom.

Your floor is basically going to be foam board plus plywood plus whatever flooring you choose. If you want 2" of foam board insulation, you'll lose about 3" of headroom (2" foam + 3/4" plywood + 1/4" flooring). If you only want 1" of floor insulation, you will only lose 2" of headroom etc. etc.
Biggest concern for headspace is my shower, so my best option may be to cut the floor and then put in a commercial sink and drop the ceiling by around 12”
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Old 04-07-2021, 05:25 AM   #8
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Biggest concern for headspace is my shower, so my best option may be to cut the floor and then put in a commercial sink and drop the ceiling by around 12
I did this in my bus. My step tub projects about 8" below the bottom of the floor, and with a projection on the roof to hold my Maxxair vent fan level, the headroom in my shower is about 6'9" (without a roof raise).
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Old 04-07-2021, 06:40 AM   #9
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Im assuming you welded your bracket slightly oversized to account for insulating around the tub?

How did you determine where you wanted to cut through the floor?
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Old 04-07-2021, 10:38 AM   #10
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Im assuming you welded your bracket slightly oversized to account for insulating around the tub?

How did you determine where you wanted to cut through the floor?
It wasn't so much determining where I wanted to cut through the floor as it was determining where not to rebuild it: https://imgur.com/a/Vp4Xpv6.

The location of my tub was constrained by the wheel well in front of it and the fuel tank filler pipe behind it, and the chassis rail on the inside of it. And the projecting box to fit it is 2" wider all around to accommodate the XPS foam insulation surrounding it.

My build thread shows all of this, although not in one coherent spot since it was part of the overall (long) process.
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Old 04-07-2021, 11:09 AM   #11
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It wasn't so much determining where I wanted to cut through the floor as it was determining where not to rebuild it: https://imgur.com/a/Vp4Xpv6.

The location of my tub was constrained by the wheel well in front of it and the fuel tank filler pipe behind it, and the chassis rail on the inside of it. And the projecting box to fit it is 2" wider all around to accommodate the XPS foam insulation surrounding it.

My build thread shows all of this, although not in one coherent spot since it was part of the overall (long) process.

I was going through your pics, what size sink did you use? Building shower walls and framing them in yourself?
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Old 04-07-2021, 11:30 AM   #12
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I was going through your pics, what size sink did you use? Building shower walls and framing them in yourself?
I'm using an actual RV step tub for mine: https://www.campingworld.com/replace...in-115582.html

although my space for it is 24"x30" so I'm cutting 6" out of the flat part to get it to fit. I've built the shower "walls" already (only one is full-height, the other two walls are partial height) and the shower will have a full-surround curtain to keep the water off everything.
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Old 04-13-2021, 09:59 PM   #13
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I insulated my floor last summer and then lived in my bus through a Wisconsin winter. I installed a continuous layer of 1 1/2 inch rigid insulating board and then screwed some 1” x 2-1/2” sleepers down on top of it. As I installed the sleepers I laid down strips of 1 inch insulating board to fit in between.On top of that I installed 5/8” plywood decking. I also had a total of 3 inches of insulation in the ceiling in the walls, 2 inches of spray foam +1 inches of rigid board, screwed to the steel framing of the bus. In the race to get closed in for winter I walled off the rear most 14 feet of the bus with the intention of creating an insulated room because I knew I would never be able to heat the entire bus with a mere 40,000 BTU (36K after 20% of the heat goes up the flu pipe) of propane fired forced air heating. It was a good thing that I did. As it turned out it was barely enough. During the coldest month it cost me about $300 in propane to keep my nighttime temperature at 55 F. I allowed myself to turn the temperature up for a few hours in the early evening before bed to 65 F. I also have removed about four of the 54 inch long windows and converted that space into insulated wall. That left me with two smaller windows on each side, and the rear window which I covered over with one and a half inches of rigid insulation board.

Now back to your question, the floor insulation, I would say the floor was pretty freaking cold with only 2 1/2 inches of insulation. Of course no one should be surprised. Building code for residential construction would call for R30, which would be 6 inches of foam insulation, in a floor above an unheated area in this climate zone.A bus is not a house though, And because there’s so much less space to heat we can probably afford to have less insulation, but how much less is the question. I think when I redo it this summer I am going to add another inch of foam board to the floor.





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