Building code dictates the mandatory venting of crawlspaces despite well recorded evidence proving the superior conditions in sealed crawlspaces. The idea was, long ago, that allowing airflow in and out of crawlspaces via vents would allow these unconditioned areas to dry naturally. This idea didn’t account for relative humidity. When warm air enters... a crawlspace, for every degree of temperature drop, the relative humidity is raised by ~2.2%. For example, if it’s 85 degrees outside and 70 degrees in the crawlspace, you’ve increased the humidity by 33%. And if the relative humidity of the crawlspace is 67% or higher—you’ve now reached the dew point—say hello to condensation and your new enemy, mold.

Keep in mind, you don’t need 100% humidity for mold to grow. In fact, many molds only require around 70% relative humidity to grow. If left untreated, mold can and will cause structural damage to joists, girders and subfloors. The only way to fix such problems is to enlist the help of a professional carpenter.

Mold can also cause a plethora of health problems—most notably being serious respiratory illnesses. Stack effect tells us that crawlspace air is cycled through a home before exiting through the attic. If you’ve got mold down below, assume that the spores will be airborne and will enter your home.

So if you’ve got a vented, dirt crawlspace and you’d like to avoid expensive joist replacement and trips to the doctor, listen up.

The solution is simple…seal your crawlspace.

Sealing your crawlspace allows the homeowner to dictate the humidity of the crawlspace with a dehumidifier. However, there are several steps needed in order to successfully seal a crawlspace.

Step 1. Install a vapor barrier on the crawlspace floor. Using 6mil to 12mil plastic (depending upon terrain), lay the barrier across the entire crawlspace floor, using tape on all seams. Use specialty spikes to secure the plastic along the outside edges of the crawlspace. Typically, you want to extend the vapor barrier at least 6 inches up the foundation wall.

Step 2. Seal vents with foamboard or vent covers. If using foamboard, cut it to exact dimensions of the crawlspace vent and foam around. If in a flood zone, make sure you keep your vents open in case water rises and needs to enter the crawlspace to prevent the foundation from collapsing.

Step 3. Seal crawlspace entrance. Whether it’s a piece of plywood or something else, it needs to be airtight. Adding weatherstripping is often a solution to warped wood.

Step 4. Install a dehumidifier in the crawlspace to keep relative humidity below 50%. You may need to install a condensate pump in order to drain the dehumidifier.

Having a sealed, mold-free crawlspace is an absolute must for anyone who is interested in keeping their home for decades to come. It’s an investment in your home as well as your personal health.

Fact to remember - mold needs moisture to survive. Control moisture and control the mold.

One more thing…
There is one alternative to sealed crawlspaces—closed cell spray foam insulation. This product is applied to crawlspace joists, girders and subfloors. This creates an air and thermal barrier and thereby eliminates the need for fiberglass insulation. This barrier fully segregates the living space from the unconditioned area and eliminates any need for sealing the crawlspace (unless your HVAC contractor has installed an air handler or ductwork in the crawlspace).

Thanks for reading.
Stephen Mogowski