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Old 05-21-2017, 09:39 AM   #11
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You're going to want to use plywood heavier than 3/8ths for flooring. You won't find 3/8ths with a tung and groove anyway.

You think about this. Insulation isn't made for strength. If you glue down the relatively soft insulation and then glue the plywood to the top of the insulation, where exactly are you getting enough strength from to hold it all in place?

If it was a vertical, wall instead of a floor, you'd never consider gluing plywood over the top of insulation that's glued to the wall. You'd screw it to the wall.

You're over thinking this. There are many methods for accomplishing the same goal. Use your basic building skills to decide your best method of attaching the floor.
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Old 05-21-2017, 10:11 AM   #12
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I put down 2" of ship-lap XPS foam and 5/8" tongue-and-groove plywood several years back and it has held up amazingly. Not a squeak, not a shift. Rock solid.

Here's what I did:
First clean, prep and paint the floor. Let the paint completely cure. If you're using Rustoleum then give it several days beyond what they recommend on the tin. That stuff is slooooowwwww to dry.

Start at the rear of the bus. Cut your first piece of rigid XPS foam (not EPS. PolyIso is ok) to fit the floor, but leave a gap at the walls. Put a zig-zag bead of original (slow dry) PL Premium adhesive on the downward side of the foam and fit it in place.
There is also a PL foam adhesive, but it's a weird, thick consistency and doesn't hold nearly as well. I tested PL Premium on XPS foam and it cured flawlessly without burning through and held much better. I've used both Liquid Nails products and PL products and PL Premium has always come out on top.

Put a light bead on the ship-lap edge and fit another. That should give enough of a foam surface to put down a piece of plywood. Doing it this way gives you two firm surfaces to stand on. Just like the foam, cut the plywood so there's a wall gap. Note that you want the plywood edges to be centered on the foam board. You don't want a foam edge and a plywood edge meeting up.

Now, at this point I went ahead with driving screws through the plywood, the foam and into the metal floor. If I were to do the job again I would skip this step and use weight to sandwich the layers (bricks perhaps?) while the adhesive cures. Screws turn out to be unnecessary and moisture condenses on the screw heads in the cold. Not a big loss of heat, but I commonly have little wet spots on the floor in the winter. I determined that the screws were unnecessary when removing a chunk of the flooring+foam to put in a shower. I had to break and scrape the foam out even after removing the screws.

Continue the process until the floor is covered. Once everything is laid and dry go ahead and put some canned spray foam into the wall gap. Use the window and door foam so the floor can expand and contract without cracking the spray foam.

That should do it! It has worked gangbusters for me. Once the foam is sandwiched between steel and plywood it is incredibly solid. That is, as long as the plywood is thick enough. I would say that 3/8" is a bit flimsy. I wouldn't go with anything less than 5/8".

EDIT: Oh, one more note. PL Premium cures in the presence of humidity. The tube recommends spraying a light mist of water onto non-permeable surfaces after laying the bead. XPS foam isn't 100% air tight so the remaining humidity between the surfaces will wick out. Don't be concerned about water being trapped between layers. Again, it all worked out very well for me.
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Old 05-23-2017, 05:29 PM   #13
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I just did my subfloor, what I did was lay 2in insulation long ways, started with a 4ft wide piece on the left side then trimmed 3ft 6in piece beside it, the next piece I swapped starting on the right side did this full length swapping back & forth then I laid 1 &1/8 in T&G plywood cross ways & glued the T&G I'm also installing screws just at the very back edge & very fwd edge.
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Old 05-23-2017, 06:08 PM   #14
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No cold feet for you.
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Old 05-23-2017, 07:49 PM   #15
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I am thrilled with my rough floor and insulation.
It has been a um ******* ball freezing ******* cold **** summer so far.
We have averaged 55 degrees with rain for a solid month and it sucks.
My three inches of floor insulation on its own has helped wonderfully.
One little electric space heater from 1956 keeps my bus toasty.
I cant wait to get the windows in, run the conduit and prep everything and insulate the hell out of it.
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Old 05-23-2017, 08:12 PM   #16
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Cool setup.

I've also found that with good insulation one small electric heater pretty well does it. What a difference.
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