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.