Preventing Mold and Rot in a Crawl Space
Tom Silva’s tips for preventing condensation in areas that need proper ventilation
Our bungalow-style house was built in 1918, mainly of cypress, over a crawl space with a dirt floor. In the ’50s, tar paper and solid-oak flooring were laid over the cypress floor. While the oak floors are still beautiful, the original cypress has mold and rot in some areas. Is there a way to keep the floor from deteriorating further? By the way, we added central AC in the late ’90s and vented the crawl-space walls in 2013.
—Robin Bagala, Cut Off, LA
Cypress is a very rot-resistant wood, but it will decay if kept long enough in warm, moist conditions. And that’s what your crawl space was like after the tar paper and oak flooring were added; they didn’t allow the cypress to dry toward the inside of the house, as it could before.
But I suspect the real culprit is your AC system. It probably lowered the floor temperature enough in the summer to cause condensation to form on the cypress the same way water collects on a glass of ice tea. That moisture, trapped under the tar paper in the sauna-like crawl space, gave mold and rot fungi the perfect environment to take hold. The decay had probably already started by the time you added the vents, which just gave warm, humid air easier access to the crawl space.
The first step in fixing this issue is to get rid of the mold and stop the decay. To do that, spray all the exposed wood in the crawl space with a penetrating, borate-based liquid such as Bora-Care with Mold-Care. It offers long-term protection against fungi and termites.
Next, block up the foundation vents, remove any debris or rocks from the crawl-space floor, and cover the dirt with a vapor barrier—12-mil plastic sheeting—overlapped at the edges by 6 inches. This barrier will stop moisture from coming through the dirt floor. Run the sheets up the piers and up the inside of the foundation wall to within a few inches of the mudsill. Seal all overlaps and edges with a nonhardening acoustic sealant, such as OSI SC-175, and an acrylic adhesive tape, such as 3M 8087CW seaming tape. Finally, insulate the foundation walls with 2-inch-thick, fire-rated foam panels like Kooltherm. Taken together, those measures will sharply reduce the moisture buildup under your floors.
The plastic needs to remain intact after it’s installed; carpet runners will help protect it from rips when someone crawls over it.
Code requires that a sealed-up crawl space receive conditioned air at a rate of 1 cubic foot per minute for every 50 square feet of underfloor area. So figure on adding registers and a return duct down there, and consult with a local HVAC pro to make sure your system can handle the extra airflow.
When this work is done, you should find your house to be more comfortable, more energy efficient, and less prone to musty odors.