Polyiso foam board under slab

[Possum]

I just scored a good deal on some polyisocyanurate sheets and was wondering what the consensus is on using this under the slab for my radiant setup. 4'x8' sheets 2" to 2-1/2" thick and has a fiberglass face on one side. Will it hold up the concrete ok? Will it deteriorate more than the extruded polystyrene?

Thanks!

 

[Possum]

Anybody? This is the material I am referring to -

http://en.wikipedia.org/wiki/Polyisocyanurate

 

[GearBeer]

Possum,

I would guess that it is not suited to under-slab installation, but you should really ask an expert. Polystyrene is extremely resilient, that's why we use it. The uses that wiki listed were not high compression.

 

[VHF]

The polyiso foam board is great for bulding solar collectors and storage tanks as it stands up to high temperatures.

 

[Possum]

Thanks for the responses. I think I may save it for the ceiling and go with extruded polystyrene. The solar collector idea is a good one too since I had planned on trying to build something to tie into my radiant setup.

 

[aleccolin]

EPS foam for perimeter insulation only, not under the whole slab. The slab is only as good as the subbase and compacted porous fill under it. Foam under concrete is only good for refrigerator slabs.

 

[devo]

Possum,
IMO, you're better off using the XPS under your slab because of the polyiso's potential to absorb water and become useless as an insulation. Polyiso works OK, and compressive strength is alright, not great (usually 17-25 psi), but it's not ideal because of the moisture absorption. It also warps pretty bad if it gets wet; won't lay flat.
Your XPS *should* have a higher compressive strength, but it's manufactured in a wide range of densities-check it out...

 

[Possum]

Been doing some checking today as a matter of fact! Found a place that can get Foamular 400 which is 40 psi, but it is special order and will cost an additional $350 more than the in stock 25 psi. Foamular 250.

So what is psi. XPS/EPS is everyone using under their slabs?