Where is black mold coming from?

[Alex W.]

I just found your response about rain gutter slope.

I just discovered black mold on the floor joists in our crawl space. I had a contractor go under to check. He said it spread throughout the space.

Our rain gutter does overflow, and there is rot along the base plate, about 3 to 4 feet.

Eight years ago when we moved into this home, radon mitigation was done so the entire floor of the crawl space is covered with thick black plastic.

Our furnace and water heater are in the space.

What's your recommendation?

[home improvement guy]

The wood floor joists and the boards that enclose the floor system (rim joists) have serious decay damage. Decay can start to form in wood when the moisture content in the wood is around 28 percent or more. This is a point when surface molds and fungus and decay may become visible. To avoid decay and molds, the lumber delivered to a building site is expected to be below 19 percent moisture content, but this can increase if the home is not under roof and is left exposed to the weather. The decay requires liquid water to grow. Water from the air in the crawl space can condense on the colder wood and can saturate the floor joists and rim joists. Once the wood is dried, decay and mold will no longer form.

So where is the moisture/humidity/water coming from? You have addressed the overflowing gutters and added a vapor barrier. Make sure the vapor barrier overlaps at the seams by at least 6 inches and the seams are taped and sealed tight. Cover, seal and tape the foundation piers with a vapor barrier. Make sure the entry doors' sill plates or bottom plates are caulked and sealed to the subfloor. Add a storm door if needed to keep blowing rains away from the door openings. If the furnace and water heater are heated by gas or oil, make sure the flues are vented to the exterior and that flue gases are escaping through the flues. A flue must extend above the roof unless the appliances are high efficiency units that vent through PVC pipes. Flue gases can produce gallons of water vapor daily.

On the exterior, measure to ensure the bottom of the floor joists are at least 6 inches above the finished grade of the yard. Any lower and the floor joists can draw water through the foundation walls. Make sure all plumbing supply and drain lines are free of leaks. The foundation walls should be insulated with a nonflammable material. If you have electric appliances, you can use foam board or spray foam to insulate the walls being careful to avoid any appliance, plug or light fixture that could generate a spark or direct heat source. You can lower the humidity levels in the crawl by adding an opening in the supply duct or by installing a dehumidifier. If you decide to condition the crawl space, the appliances, venting and insulation must be inspected and approved by a licensed (where required) and qualified HVAC contractor.