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I'm planning to build a 10 x 12 living space based on a shed design. The shed will rest on an existing concrete slab therefore my plan is to lay 6 mil polyethethylene sheeting and foam board insulation in between the floor joists which I will finally cover with plywood. Does this sound acceptable?

I am located in Las Vegas so it is a hot and dry climate although it does get cold in the winter with average night-time lows of 38F/3C.

Also is it necessary for me to install the foam board between the joists so that there isn't a gap between the foam and the plywood (which means more work because I may need to cut the foam to size in between the joists) or can I simply lay the foam boards down and put the joists on top (there will be an air gap between the foam and the plywood due to the height of the joists)?

Or a third option would be to insulate above the floor joists and then put the ply on top which would make it unnecessary to cut the foam? This would have the added benefit of having large continuous foam board areas rather than smaller pieces in between the joists.

Many shed designs recommend building the floor with equally spaced joists. This serves two purposes namely making the floor stronger and, since most sheds are built on soil or gravel, provides additional spacing and support between the ground and the plywood floor.

In my case, however, the weight isn't an issue since there won't be any heavy loads inside the cabin (unlike a shed) and the ground level is a concrete slab which provides structural support and some insulation.

My question is whether I can do away with the joists all together and only keep the outer bands?

This would save me time, money and make it easier to install foam board insulation since I won't need to cut it up to fit between the joists. It would also provide better insualtion since each foam board would cover a large area without need to be cut to fit between the joists.

Finally, am I correct in saying that I would put a sheet of poly to create a vapor barrier between the concrete slab and the foam board insulation on top of which I would put the plywood? Would fixing cut and groove plywood around the edges of the 10 x 12 floor be enough or does anyone have other ideas to fix the ply?

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  • I fail to see the need for a joist system at all. I would put a vapor barrier, then 2" foam, then 3/4 plywood on the foam. Fasten with power loads with washers. Would be a lot cheaper. But, this suggestion is not ideal for certain flooring--like tile. Should be fine for carpet, linoleum, LVT. If you use a joist system, I think you will need a thermal break between the concrete and joists. Good luck. – peinal Nov 10 '19 at 22:39
  • That's a very good point. It seems like joists are necessary when building on soil or gravel in order to make the floor firm but since I am building on a concrete slab I may be able to do away with joists all together. – Alex B Nov 11 '19 at 2:40
  • What do you mean by "Fasten with power loads with washers" ? – Alex B Nov 11 '19 at 2:46
  • that should've been powder not power. Nailer: homedepot.com/p/… Nails:homedepot.com/p/… – peinal Nov 11 '19 at 14:19
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    "It does get very cold in the winter time, particularly at night" - a quick google suggests average night-time lows of 38F/3C in the winter. That is not "very cold"! – Martin Bonner supports Monica Nov 11 '19 at 18:01
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The foam insulation usually supplied for building tends to match the "normal" joist spacing.

Whatever you do, you need to make sure that there are no air gaps, that reduces the efficiency.

Have you considered a vapor barrier?

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  • The vapor barrier was what I was referring to with the 6 mil poly. I assume it is to be placed between the cement slab and the foam insulatoin? – Alex B Nov 10 '19 at 19:51
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I would not overcomplicate this, I would not put foam board directly under the plywood floor, I'd just put foam board between the joists.

If you want more insulation in the floor, wall to wall carpeting with a thick carpet padding would be your best bet, or if you want a hard floor, there are insulated underlayments available now.

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