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Q.A homeowner wants me to eliminate the moldy smell from a crawlspace. The crawlspace has a concrete floor, with fiberglass batt insulation between the floor joists. There are two louvered vents that are left open year-round. Would it help to insulate the walls with rigid foam and seal the vents shut?

A.Bill Rose, architect and building researcher at the University of Illinois Urbana-Champaign, responds: You want the crawlspace to be clean and dry when you’re finished. First, do whatever is necessary to make sure that rainwater cannot enter the crawlspace. Install gutters at the roof eaves, if necessary. Be sure that the discharge ends of any downspouts are as far as possible from the foundation. Keep the exterior grade at a minimum 5% slope away from the foundation, for a good 10 feet. If there is a persistent wet spot in the crawlspace, install a sump pump there. The existing concrete slab will be enough to keep evaporation from the surface down to a manageable level.

Any work in a smelly crawlspace should be approached as a mold remediation project. For any overhead work, goggles are a must. If the expected amount of visible mold colonization is minimal, then N-95 respirators and gloves should offer good protection. If the anticipated black area is large (more than 2 square feet), then P-100 respirators and disposable clothing are recommended. If the colonized area is very large — over 30 square feet — you should call for help.

If you are tackling the project, isolate the crawlspace from the basement and the rest of the house until the moldy smell is long gone. Install an exhaust fan at a vent opening. Remove any debris or stored material and anything else that is removable. Remove the fiberglass insulation from between the floor joists. See if there are black spots in the insulation, and keep track of any locations-that seem to have gotten moldy. If the band joist is moldy, it may be easier to remove it and replace it than to clean it from inside. (First, provide blocking so the joists don’t collapse.) Look for mold spots near toilet and shower locations. Check the concrete floor and the crawlspace walls for any sign of water entry.

Use a powerful vacuum cleaner outside the building, a long hose to the work area, and a wide assortment of scraping tools. Scrape any questionable surfaces, and vacuum away all the scrapings. Dispose of the vacuum bag or filter and clean the vacuum cleaner and hose thoroughly. Don’t use bleach around concrete — when bleach comes in contact with anything having a pH less than 8, chlorine gas is formed. TSP (trisodium phosphate) plus rinsing and drying may help to disinfect wood and concrete surfaces.

If you intend to insulate, I suggest that you insulate the foundation at the exterior. For now, and until termites evolve, my preferred insulation material is borate-treated expanded polystyrene or extruded polystyrene. Don’t re-insulate the floor framing — the heat from the house will help keep the crawlspace dry.

Keep the exhaust fan running as long as the smell persists. If you’re unsure by smell when the job is done, then have an industrial hygienist take some air samples. Once the air is back to fresh, the vents can and usually should be closed.