Q: We just built a new home and moved into it in August. Two days after moving in, we found a considerable amount of mold growing on the wood joists and subflooring in the basement. It was spread throughout the entire basement. The contractor insisted this was a common problem and easily cleaned with bleach. We asked for an expert in mold remediation, but they refused. They bleached the wood and "guaranteed" us that the mold would not return. Within two months not only did the mold return, but it spread. It is growing on the joists where they meet the subflooring. We are not sure how to proceed. Our Board Certified Industrial Hygienist insists that all the contaminated wood be removed. Their "expert" says that if the wood is cleaned (soda) and encapsulated we should not have any more problems. There is no source of moisture...we have dehumidifiers running constantly (the air humidity is under 60%). This is a brand new home! We are afraid of the mold returning after remediation; and also know that if we need to resell the home it might be difficult after we disclose the mold problem.

A: Several moisture sources may be creating an environment conducive to fungal growth in your basement.

It may be that your house has no insulation below the first floor subfloor, and that the first floor is air-conditioned, but the basement is not. If this is the case, the moisture source is likely to be condensation. If the basement has multiple air leaks and/or the new building materials contained moisture and were never dried properly, it may not be possible for dehumidification to keep pace with the speed of air infiltration.

There are many other situations that may be factors, including:

• If the basement foundation is concrete, were the walls and floor sealed/insulated to minimize moisture penetrating through porous concrete?
• Do the rain gutters direct water away from the foundation?
• Does the landscaping direct surface water away from the house?
• If the water table is high, are there vapor barriers, drainage gravel, and/or French drains installed?

· The soil type -- clay or sandy -- can impact moisture migration into a basement. In some localities commercial or public building codes (not usually in residential construction) require insulation both under the foundation floor and along the exterior foundation walls. This would be in addition to drainage gravel and a vapor barrier under the concrete slab.

The first step in solving the fungal problem is to dry out the basement and prevent moisture from entering. Depending on your geographic location, the heating season may have already begun the drying process. If, or when, the appropriate vapor barriers, insulation and drainage are in place, effective mold remediation can begin.

I do not recommend bleach as a disinfectant because of its corrosive and toxic side effects (although it is effective). My recommended procedure is to clean with a HEPA filtered vacuum, followed by sanding of the area or use of an abrasive cleaner (soda and dry ice blasting are options). I am in favor of quaternary ammonium-type disinfectants and fungal encapsulates, as long as the residents of the building are not chemically sensitive. Some organizations prefer to avoid the use of chemicals and I do agree less is better. However, I find that in many situations, physical cleaning cannot be effective by itself because of the matrix of frame lumber and inacessible locations.

Thanks to Mark Goldman, who is a senior air quality scientist for Engineering and Fire Investigations (EFI), a firm that provides mold investigation and indoor air quality services nationwide. Mark can be reached at mark_goldman@efiglobal.com

QA archive >>

If you have a question for a mold expert, please send it to Brett Lyon, senior editor of construction.com at brett_lyon@mcgraw-hill.com.