I want to insulate under a garage/workshop/office floor, since it will include radiant. The floor is actually a new pour over an existing floor (which is a mess) (so the entire floor will be quite a bit higher than it's starting: this is an advantage in this case).

I can get some used polyiso/perlite board, which would generally be used on a roof. Any ideas how that might perform under a floor? Parts of the floor might be driven on, the pour will be at least 4" thick with welded wire reinforcement.

The other issue: part of the reason to raise the floor is the challenge of avoiding floods the way things are set up now. This means that there is some chance that the insulation will get wet under the floor, though I hope to figure out how to include some drainage (I'm just worried that it might serve to introduce water under the slab, instead of draining it! There's no really easy daylight to drain to).

Thanks for any help.

Keith

I wouldn't use it underground. It probably has a black paper on it, that is meant to bond with the hat tar of a built-up roof. It soaks up water like a sponge, also the perlite is a fireproofing material used to stop the iso. from flamming up. The perlite also loves water.

You can use polyiso under a slab. The paper facer does not matter. The perlite is another issue. I would not use perlite faced material here. When using polyiso, first install the polyethylene moisture barrier and then the polyiso.
You should insulate the perimeter with styrofoam since the paper facer will be an issue if exposed to weather.

Most polyiso with paper facing is rated at 20 psi compressive strength. It is more absorbent than styrofoam, but it has to be submerged to be affected. If that happens you have other things to worry about. We have used paper faced polyiso for years in this application with no issues. The paper will not mold since it is in contact with the concrete on the slab side. If any does form on the bottom, it will be segregated from the concrete by the foam.

In the worst case scenario, polyiso can absorb 10x its original weight in water. When this happens (it has to be submerged for a long time!), it will lose half its R value.