Mold in Your Crawl Space – How Much is Too Much?

Summer is almost here, and with the warm temperatures comes more homeowner activity in the yard. A few will even open the door to their crawl space and peak inside. What happens if you find a bunch of stuff that seems to be growing on the wooden floor joists under your house?

First, understand that a few different kinds of things that look like mold can also stick to joists – namely insect feces. Second, understand that some mold on the wood holding up your house is normal and to be expected, especially if your crawl space is an ‘open’ design with foundation vents that allow outdoor air into the space. Some mold under the house may be defined as ‘normal fungal ecology’.

If you are reasonably sure that your floor system has mold on it and want to know if something needs to be addressed, you may want to consult with an expert, or be prepared to buy a few measurement tools and learn how to use them. An expert is best defined in this case as someone who can assess the crawl space without bias (i.e., they do not sell sealed crawl systems or perform mold remediation). 

The best time of year for an expert opinion of a crawl is during summer months. This allows the expert to view the crawl when mold is most active and when the house is using air-conditioning.
Here are the steps your expert should take:

 

  1. While wearing protective gear, crawl through the entire underside of the house and document conditions with photos of the vapor barrier, foundation walls, floor insulation, joists/sub-floor, and any mechanical equipment (especially air handler & ductwork;
  2. Collect wood moisture measurements of several joists and the sub-floor, especially near plumbing penetrations and near the foundation if moisture is observed;
  3. Check the vapor barrier for tears or gaps that could allow soil moisture to evaporate onto the floor system;
  4. Measure soil moisture content if it appears to be damp. Note if all the soil is damp or just certain areas;
  5. Note any strong musty odor. A crawl space should smell earthy, not musty;
  6. Note any debris such as old pipes, insulation or ductwork lying on the floor. These should be cleaned out;
  7. Warn the homeowner of the danger of storing gasoline-powered yard tools under the house should you see any of these devices, especially if they are near a gas-fired furnace (we have seen this more often than you might believe);
  8. Observe the level of air movement through the crawl space. We often find a dead zone near the center of larger crawls, which is usually home to more than the typical amount of fungal growth.

Your expert should be able to provide a report that includes a list of observations & recommendations. Here comes the tricky part – how do you know if the mold in the crawl space is at a normal level and is unlikely to harm the family living in the house?

This is where experience plays a large role. Someone who is new to crawl space inspections may miss something important, make a big deal out of something insignificant, or not know how to communicate their findings very well. Other variables that come into play are the tightness of the duct system under the house, the number of occupants who are sensitized to mold and are claim to be experiencing health effects, and the air quality inside the home. An expert should be able to address all of these variables or be able to direct you to someone who can.

A final note about the collection of samples from suspect mold in the crawl space. It is not always necessary to know the exact level or species of mold (information lab analysis will provide), however, sample collection is often useful. Ask your expert to explain their sampling method and what information they hope to gain by sending a sample into a lab for analysis. An experienced individual will be able to clearly explain why sampling is necessary and how it will guide the process of determining if the crawl space has a normal fungal ecology.

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