Is Foil-Faced Polyisocyanurate Foam Insulating Board Good to Use Below a Concrete Floor Slab?
Question: The articles I read concerning the insulation of concrete slabs refer to extruded polystyrene foam insulating board for use underneath the slab and for perimeter treatment. (See photo at left). Except for Owens-Corning literature that recommends use of its Energy Shield sheathing, foil-faced polyisocyanurate foam insulation use below slabs seemed to be ignored in the 1980’s – why? — Steve Scheller, Falls Church VA Answer: In 1984 the only published research to date on underground applications of rigid foam insulation was a study by Dow Chemical, in which samples of insulation were buried for extended periods and later unearthed. In those tests, foil-faced polyisocyanurate foam insulating board did not fare so well, absorbing on average 5 percent water by volume and losing 40 percent of its R-value. Manufacturers of the foil-faced products argued that the tests were not valid because the 2-foot square samples used had proportionally far more edge area than full sheets (edges
