Crawl Space Mold Removal

An unvented crawl space running at 78% relative humidity, with visible growth across the floor joists and subfloor.

About the project

A 1930s house in Two Rivers with a dirt-floor crawl space under the rear addition. The owners called because of a smell in the kitchen that got worse in July and nearly disappeared in January — a very characteristic pattern, and one that points at a crawl space almost every time.

The crawl space was reading 78% relative humidity on the day we inspected, with an outdoor reading of 61%. The joists above the exposed soil were furred with white and grey growth over most of their length.

The challenge

There was no leak. That is what made this one worth writing up. The soil floor was evaporating an estimated fifteen to twenty gallons of water a day into a sealed void, and the foundation vents that were supposed to solve this were actively making it worse — in a Lake Michigan summer, venting a cool crawl space with warm humid outdoor air just condenses that moisture on every cool surface inside.

The stack effect then pulled that air upward into the kitchen, which is why the family were smelling a space they had never been inside.

The result

We remediated the joists and subfloor by HEPA vacuum, abrasive cleaning and antimicrobial treatment, then encapsulated the space: 20-mil reinforced vapor barrier across the soil and up the walls, mechanically fastened and sealed at every seam and penetration, foundation vents sealed, and a small dedicated dehumidifier draining to the sump.

Relative humidity in the crawl space now sits between 45% and 50% year round. Follow-up sampling twelve months later showed no regrowth.