Over the past year, the Consortium for Advanced Residential Buildings (CARB), part of the DOE's Building America Program, teamed up with Steven Winter Associates, a Norwalk, Conn.-based consulting firm, to look at moisture movement inside insulating wall systems, as well as water buildup on the inside of concrete walls in an insulated, dehumidified basement.
Researchers studied three different methods for insulating concrete foundation walls: 1) wood stud walls slightly offset from the concrete, with fiberglass batts; 2) polymer-faced fiberglass blankets between vertical 2x2s, attached to the foundation wall; and 3) foamboard insulation attached directly to the concrete wall. (Both polyisocyanurate foam and expanded polystyrene [EPS] foam were tested.)
The team found that the moisture level within the fiberglass batts was almost identical to the ambient moisture level in the basement. Apparently moisture follows air movement, moving through the batts installed atop the basement wall cavities. If the basement air was not excessively wet, this system did not tend to trap and hold water within the wall system or allow much buildup on the concrete.
The fiberglass blanket system also allowed air to flow through, but with less ease than the stud and batt system. The blanket walls did not dry out as quickly as the stud/batt walls, possibly because the system lacks an empty cavity that permits air to move freely and carry the moisture away.
Though less accessible to moisture in the air, the rigid-insulation-covered walls tended to hold moisture longer than the stud-framed walls. (The EPS dried more quickly than the polyisocyanurate, due to higher permeability.) What's not clear from the research is whether the slower drying of water trapped behind these products is significant with regard to mold growth or indoor air quality.