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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.

enter image description here Source: http://blog.rockwelltools.com/how-to-build-a-better-backyard-storage-shed/

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  • City lot or backwoods? The intensity of rodent effort (and the appropriate solution) varies with location.
    – isherwood
    Mar 10, 2020 at 19:55
  • It's in my back garden. Small town very near the countryside. I think there is a possibility of there being mice or rats nearby, but not a lot else.
    – Matthew Dresser
    Mar 10, 2020 at 23:45

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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.

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  • I would not do plywood although metal lath is good (but like chicken wire won't keep mice out). Plywood in situations like this will drive you nuts. Animals are going to get in - the issue with plywood is you are basically building them a house and the worst part is you will hear them really well.
    – DMoore
    Mar 10, 2020 at 20:04
  • Pressure treated plywood seems to be quite hard to come by in the UK. Is there anything I could paint onto ordinary plywood which would do the job?
    – Matthew Dresser
    Mar 11, 2020 at 1:14
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Don't leave it open, like Alaska Man said. My son has a sunroom that was built atop existing framing for a deck. The underneath of the floor was insulated, just like you're describing. He had all the problems Alaska Man mentioned, until he finally got around to covering the underside of the joists and closing off any openings.

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Any sort of outdoor space we put insulation in we put a couple layers of chicken wire under the joists to not only keep creatures out but to keep insulation in place. It really is that easy.

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  • The mice appreciate your support.
    – isherwood
    Mar 10, 2020 at 19:54
  • @isherwood - it won't fix mice but it will deter almost everything else. Mice are easy to get rid of by a shed and really have no reason to build a nest so far away from a food source. Mice can be supported by almost anything. The insulation itself is probably more than enough.
    – DMoore
    Mar 10, 2020 at 20:02

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