I'm building a garden office (shed) and will build a floor similar to that shown below (minus the flat beams between the joists and the concrete supports). I will insulate the floor using PIR (e.g. celotex or kingspan). On the top will be two layers of plywood and above that insulation, electric underfloor heating and then tiles.
My question is whether the lower surface (which will be between 5cm and 15cm off the ground) should be covered with ply, some sort of building wrap (tyvek?) or even polythene? Or can I just leave the lower surface of the insulation foam open to the elements? The PIR boards have an aluminium foil covering which is what would be facing the ground.
Source: http://blog.rockwelltools.com/how-to-build-a-better-backyard-storage-shed/
Best Answer
Normally a shed does not have an insulated floor but since yours is going to be insulated i would not leave it open for critters to nest in or chew up and take away for a nests.
I would cover the bottom of your joists with treated plywood or Metal lath.
The plywood keeps out bugs and squirrels etc, the lath keep out only things larger than the opening of the lath you choose.
Building wrap (tyvek?) or polythene is no match for squirrels. Unless you put some lath around the structure as skirting, i would bury the bottom of it in a trench several inches deep to keep critters from burrowing under it.
Mice a voles are different, they can chew through plywood, so you may need both PW & lath with very small openings.