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Forest Products Laboratory
One Gifford Pinchot Drive
Madison, WI 53726
Phone: (608) 231-9200
Fax: (608) 231-9592
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Frequently Asked Questions (FAQs)

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Mold and moisture

 

Question: What is mold?

Mold and/or mildew are microscopic fungi that are present virtually everywhere, indoors and outdoors. Under the proper conditions, they can grow on a variety of organic materials, including wood products (1, 11). Mold growth requires ample moisture in the form of liquid or high humidity. While there is not universal agreement on the lower threshold of humidity required for mold growth, most literature indicates that the relative humidity needs to be at least 70% to 90% to support mold growth (15). Once growth occurs, molds can spread by production of spores.


 

Question: How do molds get in the indoor environment, and how do they grow?

Mold and/or mildew are microscopic fungi that are present virtually everywhere, indoors and outdoors. Under the proper conditions, they can grow on a variety of organic materials, including wood products (1, 11). Mold growth requires ample moisture in the form of liquid or high humidity. While there is not universal agreement on the lower threshold of humidity required for mold growth, most literature indicates that the relative humidity needs to be at least 70% to 90% to support mold growth (15). Once growth occurs, molds can spread by production of spores.


 

Question: Isn't mold happening now because buildings are too tight?

This rationale for moisture/mold problems is often presented by the media as the main driving force for the current rash of moisture/mold claims. However, mold needs moisture to grow and moisture can come from multiple sources in a house, many of which have nothing to do with the amount of fresh air exchange a building experiences. A chronic plumbing leak releasing water onto drywall or a leaky foundation wall are two examples. There can indeed be cases where the humidity level in a house would be lower with more fresh air, potentially alleviating some moisture/mold problems, but to characterize this as the root cause of all mold problems is incorrect (6).


 

Question: The homeowners had a water leak. What is the potential for growing mold in this area?

Materials that are exposed to a constant leak or have been soaked and not dried thoroughly can support mold growth. Some molds can form a new colony in one or two days on damp materials. Additionally, molds do not require light, and growth can continue indefinitely without light (2).


 

Question: What about moisture in the crawl space?

A definitive answer about moisture problems in the crawl space does not exist. A symposium on the subject held by the American Society of Heating, Refrigeration and Air-Conditioning Engineers (14) produced the following recommendations:
  • There should be proper drainage, clearance and access.
  • Crawl spaces should have ground covers for moisture control. These should be installed carefully to limit evaporation from the soil.
  • Heated crawl spaces should not be vented with outdoor air.
  • Unheated crawl spaces may be vented, but there is no overriding need to do so for reasons of moisture control if an effective ground cover is present. (Note: Check with local building codes to verify whether these practices are permitted in your jurisdiction.)
The Building Science Corporation web site also contains crawl space information and recommendations. This information includes discussion of the differences between crawlspace design recommendations for cold climates versus warm (moist) climates in accordance with the International Energy Conservation Code (20).


 

Question: At what moisture content should someone be worried about mold?

A useful rule of thumb is that wood with a moisture content greater than 19% is sufficient to support mold growth (19). More specifically, it is the moisture content at the wood surface and the relative humidity in the air above the wood surface that is important. This detail sometimes causes confusion, since a 19% moisture content (average for the entire cross-section) for kiln-dried lumber results in about a 16% moisture content at the surface - dry enough to minimize even the slow growth of most molds. Much higher moisture levels are required for substantial mold growth. And very high moisture levels are required to support decay producing organisms (8), which, unlike mold, can cause permanent loss of structural strength (3). Wood can achieve excessive moisture content when exposed to direct wetting, or from extended exposure to high humidity.


 

Question: Do structural adhesives cause mold to grow more readily or faster?

A recent study (9) on fungal susceptibility of pine and aspen oriented strand board (OSB) found that the amount of mold growth on both OSB types was approximately equal to the mold growth on solid aspen. These results indicate that the adhesive has little or no effect on mold growth. Molds grow best on freely available sources of nutrients (4), and because the cured resins used in structural adhesives are a poor source of freely available nutrients (and may even block mold access to the cellulose fibers), these adhesives are generally assumed not to encourage mold growth.


 

Question: What is wrong with mold?

Mold and/or mildew fungi do not cause wood decay. However high moisture environments that foster mold growth also have the potential to support decay-producing organisms (3). Molds produce spores, which often become airborne, and may trigger an allergic reaction for some people (13). In addition some molds produce mycotoxins and microbial VOCs (volatile organic compounds). However these have not been linked conclusively to health problems in buildings.


 

Question: If homeowners think there may be mold in their houses, should they test for it?

According to the U.S. Environmental Protection Agency (EPA), in most cases, if visible mold growth is present, sampling is unnecessary. Because no EPA or other federal limits have been set for mold or mold spores, sampling cannot be used to check a building's compliance with federal mold standards (1). Additionally, testing for mold is difficult because mold is everywhere; therefore, testing will not prove that a house is free of mold (6). The U.S. Centers for Disease Control and Prevention (CDC) advises that it is not practical to test for mold growth in a house, because large mold infestations can usually be seen or smelled (13). When testing is done, it is usually to compare levels of mold spores inside the house with levels outside the house (2). However, results from air sampling are prone to large errors, and different sampling methods produce different results. A thorough inspection of a house, looking for signs of moisture problems or active mold growth, is likely to be more effective than testing as a way to size up potential problems (6). According to the Centers for Disease Control, it is not necessary to identify the species of mold growing in a residence, and CDC does not recommend routine sampling for molds (13). Current evidence indicates that allergies are the type of disease most often associated with molds (13). Since the susceptibility of individuals can vary greatly either because of the amount or type of mold, sampling and culturing would not be reliable in determining the health risk. CDC recommends that if the occupants are susceptible to mold allergy and mold is seen or smelled indoors, there is a potential health risk, and the homeowner should arrange for its removal (13).


 

Question: A qualified environmental lab took samples of the mold inside a home and Returned the results. Can CDC or anyone else interpret the results?

Standards for judging what is an acceptable, tolerable or normal quantity of mold have not been established. If the homeowner decides to pay for environmental sampling for molds, several questions must be addressed before the work starts. For example, decisions must be made regarding how sampling will be accomplished (i.e., surface wiping by tape or swab, air sampling for spores or bioaerosols, etc). Additionally, the CDC recommends addressing several other issues before the work starts (13):
  • Who will establish the criteria for interpreting the test results?
  • What are their qualifications?
  • What will be done, or what recommendations will be made based on the sampling results?
It is important to keep in mind that the results of samples taken in a unique situation cannot be interpreted without physical inspection of the contaminated area, or without considering the building's characteristics and the factors that led to the present condition.