I discovered something by accident in regard to styrofoam placed under concrete slabs.

On a upper floor exterior deck that had a rubber membrane installed for water proofing, there were sheets of styrofoam laid on deck for temporary protection of the membrane.

These same type of 2" sheets were used for insulation under the whole house concrete slab for insulation and is a very good idea.

Always a problem with good ideas if not installed properly. I picked up a couple of the styrofoam sheets covering deck and thought, what the heck, these are heavy. The cerebral muscle began to flex and I thought, Hmmm, if these styrofoam sheets have absorbed moisture, how much moisture will the styrofoam sheets absorb under the concrete?

Styrofoam is comprised of voids and if filled with moisture, the R value will drop considerably.

I suggest if placing styrofoam under a slab, that minimum 6mill poly is laid under styrofoam and up side of slab to prevent moisture from contaminating styrofoam. Poly is cheap, overlap all joints and caulk or glue to prevent intrusion of water. I never install a concrete slab with out 2" of styrofoam under it. Styrofoam also takes up most expansion of soils under the styrofoam......acts as a pillow and decreases most pressures delivered by movement of soils onto concrete. Plus insulates your floor slab, which saves you money, forever and ever.

David Palmer is the owner of Big Block Home Inspections and is an expert in home inspections in the Barrie Area. He has built, renovated and inspected structures for the last 45 years.