Prevent Subfloor Moisture

“Don’t even think about insulating the floor of your raised Louisiana home.”

It’s common advice, based upon two dreaded words: moisture problems. Many types of insulation can actually contribute to moisture problems, but unless you want to freeze on the cold days and mop sweat on the sultry ones, all while paying through the nose for utilities, there has to be a solution. According to the results of a 2011 LAS Ag Center study called “Insulating Raised Floors in Hot, Humid Climates: Research Findings on Moisture Management,” closed cell spray foam insulation may well be the answer.

Moisture management is critical in Southern Louisiana

Moisture management, especially in a climate like that of Southern Louisiana, is indeed crucial, not only for preventing termites, mold and decay, but also for warding off expansion/contraction damage to wood flooring and corrosion of nails, screws and other elements that keep your structure intact. Too much moisture, and even too much variation in moisture content, puts your home at unnecessary risk for any/all of these problems. In addition, cracks between floorboards and joints allow the passage of air, which means cool air is forced out in the summer and sucked in during the winter.

Water vapor is absorbed by the subfloor from the outside

LSU Ag Center’s Claudette Reichel warns homeowners not to relax just because they have vinyl or tile floors. Such coverings, she says, likely rest on a bed of plywood, which is susceptible to mold and decay. Air conditioning, she adds, is a big part of the problem. “Picture your house as a giant glass sitting in the summer sun. When you crank up the central AC, it’s like filling the glass with ice. What happens to the outside of the glass? It gets wet.”

In other words, as the LSU study explains, “when a building is air-conditioned and the outdoor climate is hot and humid, water vapor migrates through the building shell from outdoors to indoors. This is referred to as inward vapor drive. Inward vapor drive means water vapor will be absorbed by the subfloor from the outside. This absorbed moisture will migrate through the subfloor and will dry to the inside. When the rate of wetting is higher than the rate of drying, moisture will accumulate in the subfloor…To protect the subfloor from moisture accumulation, the insulation under the subfloor should be selected to provide enough resistance to the inward vapor drive.”

An LSU experimental study of traditional insulation and water vapor

With traditional insulation near the ground, water vapor often migrates to the subfloor and gets trapped. The LSU study’s purpose was to explore alternative types of insulation to keep homes dry, safe, comfortable, long-lasting and energy-efficient. Their experimental study focused on a dozen houses, 8 in New Orleans and 4 in Baton Rouge, monitored from October 2008 to October 2009. Eleven of the 12 homes were located in flood hazard areas and were constructed with open-pier foundations, while one had a wall-vented crawl space with a vapor-retarding soil cover.

Data loggers measured air temperature and humidity indoors, outdoors and in crawl spaces. Moisture content and temperature of the wood or plywood subfloor were generally measured twice each month. Houses in New Orleans originally were insulated with fiberglass batt insulation (which, like open-cell spray foam, is quite permeable), but contractors removed that from half of each home’s floor, replacing it with rigid foam or spray foam insulation. Floors in the Baton Rouge houses were insulated entirely with rigid foam and/or spray foam — some open cell, and some closed cell.

Closed cell insulation prevents moisture accumulation in LSU study

Results showed that the permeable types of insulation, open cell foam and batt insulation, permitted elevated moisture levels of subfloors. Generally anything above 20% is considered too high, though mold, decay and corrosion can actually occur at levels as low as 15%.

On the other hand, the report indicates that closed cell spray foam insulation, which is impermeable, showed good performance and is suitable for preventing summertime moisture accumulation in subfloors of raised floor systems in Southern Louisiana. And while closed cell spray foam costs a bit more than other types of insulation, no other method has proven as effective in protecting your greatest investment: your home.


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