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Mold on Insulation in Crawl Spaces

Mold growth on crawl space insulation is fairly rare.  Fiberglass isn’t the best food source for microbial growth and typically condensation will occur on other surfaces first.  However, insulation in the crawl space can become easily damaged by rodents and other unpleasant circumstances.

Most common crawl space insulation problems

Rodent activity.  Unfortunately, the most disgusting source of insulation problems is also the most common.  Rats, mice and occasionally larger foes such as raccoons and cats.  Rats cause the most destruction, as they commonly inhabit the warm space between the insulation and the subfloor.  Eventually the weight of their body causes the insulation to droop or fall completely.

Missing or inadequate underfloor insulation.  Although a fairly uncommon sight, some older homes still lack underfloor insulation.  Occasionally a homeowner will remove the insulation due to a rodent problem and fail to install new insulation.  The energy loss of missing underfloor insulation is not nearly as duct insulation or attic insulation.  However, many homeowners will complain of comfort issues related to a cold floor during the winter months.

Missing or inadequate duct insulation.  Although far more important than underfloor insulation, duct wrapping is often neglected.  Newer homes typically utilize flexduct which comes pre-insulated.  In these homes, the only areas require traditional insulation are the large trunk lines.  Much like the underfloor insulation, duct insulation is a prime target for rodents and critters.  Typically the insulation won’t fall completely, but instead will become saturated with droppings and urine.

Project Report > Inspection for Mold on Crawlspace Insulation


  • RH: 35.5%  Temp: 72 deg F.  CO2 = 560 ppm   CO = 0 ppm
  • Sections of hardwood flooring had been removed from main walking area of kitchen floor.
  • Moisture content readings of all subfloor was below 15%.
  • All hardwood measured below 15% moisture content.

Mold in Insulation in Crawlspace


  • Some sections of insulation were pulled down from around the location where the water intrusion occurred.
  • Subfloor under the cabinets measured below 15% moisture content


  • Recommend replacing all fallen and/or damaged insulation batts into floor cavities.

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