DamnYankee wrote:
arw01 wrote:
Radiant heat shop floor:

Top soil has been removed
very hard base ground (use a breaking bar to be able to dig in it)
3-7" of 3/4" crush gravel

moisture barrier HERE?

2" polystyryene extruded foam insulation

moisture barrier HERE?

concrete reinforcement mesh
1/2" pex heat tubing
4" concrete with plasticizers and fiber reinforcement



we put the moisture barrier under the foam for two reasons. first, we staple the heat tubing to the foam, and you'd have to punch the poly full of holes. more importantly, you would tear the hell out of it during the pour.

the moisture barrier under a slab does not function the same as a
moisture barrier on a wall. the under slab barrier's role is to block
moisture movement by capillary action from the ground into the
concrete. a vapor barrier on a wall is blocking moisture from moving
from inside the heated space into the insulated wall cavity.

i believe the IRC was recently changed to allow NO vapor barrier if
foam is used under the slab (since the foam blocks capillary action
anyway).