Researcher Sam Glass of the USDA Forest Products Laboratory (FPL) has published a final report on field trials of insulation options for raised floors (“Moisture Performance of Insulated, Raised, Wood-Frame Floors: A Study of Twelve Houses in Southern Louisiana,” by Samuel V. Glass et al). Researchers placed moisture sensors within the insulated floor assemblies of a dozen homes in south Louisiana, including eight recently built Habitat for Humanity homes at “Musicians Village” in New Orleans and four older raised-floor houses in Baton Rouge, then logged the measured moisture levels of the homes’ subfloors for a full year, from October 2008 to October 2009. An analysis of the moisture readings concludes that foil-faced board insulation systems and closed-cell high-density spray foam applications protected floor assemblies from moisture in the air more effectively than did either fiberglass batts or low-density, open-cell foam insulation — even when the open-cell foam was covered with a coat of vapor-retarder paint. A Louisiana State University report posted at the website of the LSU Ag Center, which co-sponsored the study, is available here:“Insulating Raised Floors in Hot, Humid Climates”.

Other factors besides insulation type also affected the seasonal moisture content of the subflooring, however. Finished flooring had a significant influence: Floors with carpet or with wood strip flooring stayed at lower moisture content during the hot, humid summer than did floors covered with resilient vinyl flooring, because of the drying allowed by the higher permeability of the carpet or wood compared to the relatively impermeable vinyl. In general, “impermeable floor finishes, by inhibiting inward drying of the subfloor, raise summertime subfloor moisture content,” the LSU report comments.

And occupant behavior — in particular, indoor thermostat set-points — also had a big effect on the subfloor moisture. Says the LSU study, “For a given type of insulation and interior floor finish, subfloor moisture content generally increased with decreasing indoor temperature during summer. That is, the cooler the air conditioning was keeping the temperature indoors, the wetter the subfloor. Homeowners in hot, humid climates can reduce the risk of seasonal moisture accumulation if they set the thermostats controlling their air conditioners as high as they feel is practical. Houses in the study with summertime indoor temperatures of 78°F or higher did not show elevated subfloor moisture levels, regardless of the type of floor insulation.” Of course, setting the thermostat higher in summer also saves energy, the report notes.

But is there a practical way to retrofit an existing house that is already insulated with low-density foam or with fiberglass, so as to allow the homeowner to keep the house cooler? “The study did not address this issue directly,” says LSU. “The study did find, however, that properly sealed foil-faced rigid foam insulation installed below the floor joists (without any insulation in the joist spaces) prevented summertime subfloor moisture accumulation. This performance is attributed to the vapor-impermeable aluminum foil facing and the air-sealing details at all edges and penetrations. We therefore expect this type of insulation to be a suitable retrofit for a raised floor system already equipped with fiberglass insulation. As long as the existing insulation and subfloor have not been exposed to elevated moisture levels, it would not be necessary to remove the insulation.”