During the RCAC Housing & Community Conference I heard a lot of talk about mold showing up in crawlspaces during construction — particularly when unvented conditioned crawlspaces are being used. My organization builds in desert areas and I even heard this can occur in typically dry parts of the West where you might not expect it. Remediation can be costly – how can we avoid this?

Woes of mold

Dear Woes of mold,

We’ve been hearing more and more of this problem throughout the West and as you mention, even in dry areas. Here’s what you need to know:

Mold requires three things to grow 1) a food source such as wood 2) moisture (anything above 50 percent relative humidity with stagnant air) and 3) temperatures between 32 and 102 degrees with prime growing temperatures between 70 and 90 degrees.

Take away one of these three things and mold cannot grow. Most homes are made of wood and are full of things mold will grow on so removing the food source is not an option. Mold grows really well in the same temperature ranges that humans like so another dead end when it comes to eliminating temperatures in which mold grows. That leaves us with one viable option – remove the moisture and mold will not grow!

Some energy codes are requiring “conditioned crawlspaces” instead of the traditional vented crawlspace. In a conditioned crawlspace, the walls are unvented and insulated. The area is “conditioned” meaning a duct from the furnace/AC unit is left open to the crawlspace to heat and cool it. The heating and cooling will remove moisture from the crawlspace once the home is occupied. While the home is under construction however, conditions can be optimal for mold growth in sealed crawlspaces – very moist stagnant air, plenty of food (joists and subfloor) and prime mold growth temperatures.

Best practices to avoid mold during construction:

  • Keep fresh air circulating through the crawlspace once the subfloor is laid and sealed. Most homes have a crawlspace access opening somewhere on the plans – place a large box fan over the opening, with the air blowing down into the crawlspace. A heater in the crawlspace may not dry the air enough to do any good and could make the situation worse – especially if it’s a propane heater (propane heaters give off a lot of moisture).
  • Provide openings so the moist crawlspace air can escape. Openings should be placed as far from the fan as possible. If you know where your furnace vents will be placed, go ahead and cut those in or create a couple openings you can seal up later before the finished floor is installed. Without the exhaust openings, the fan will not circulate fresh dry air through the crawlspace.
  • A well-sealed vapor barrier is essential to keep moisture from entering conditioned crawlspaces.
  • Keep the air flowing until the furnace is operational and the home is ready for occupancy.

I hope this puts your concerns to rest, just keep in mind: “remove the moisture and the mold will not grow!”