Ask a builder: Venting away mildew on floor joists
Q. What will remove mildew on the floor joists under my house? D. E. Stanberry Oakland, Calif.
A. A lack of adequate ventilation in the crawl space under your house has likely set up the mildew. To get rid of it, increase the ventilation by installing foundation vents between the joists, or otherwise adding screen access.
If standing water occurs in the crawl space, divert it by sloping the ground away outside the foundation. Compel downspout rainwater to flow away from the building. Also, reduce or eliminate plant irrigation next to the foundation.
Should groundwater incursion be the source of the moisture, you may need to install footing drains.
A 6-mil layer of polyethylene sheet laid over the dirt under the crawl space is a good moisture deterrent. Tape the joints and run the sheets up the inside of the foundation wall.
Once the moisture problem is under control, kill the fungus on the joists by applying a mixture of three quarts of water to one quart household bleach; or use a chemical mildew inhibitor sold by hardware and paint stores.
You may have to repeat the process, because mildew is tenacious.