A crawlspace is a really short, unfinished basement with a dirt floor and is typically vented to outside air. Crawlspaces tend to be very humid, fostering the growth of many types of fungal organisms. Mold and mildew are types of fungus that can be found in most crawlspaces. Building science recognizes that traditional, vented crawlspaces are problematic and building codes in some states are starting to require crawlspaces be sealed, conditioned spaces.
Microbial Growth
Microbial growth is an umbrella term for any unidentified biological material, including: fungi/mold, algae, and bacterial mats or colonies. Any biological growth is reported as “microbial growth” because, unless it’s identified by a laboratory, we can’t legally declare what it is. Fungi and molds are of particular concern because of the damage they can cause to structural components and/or human health. Fungus is nature’s decomposer, reducing organic material to more basic compounds. Fungi is present everywhere but requires moisture to germinate and grow. Below a certain moisture level (16% moisture content) fungus dies but its reproductive parts (spores) remain in a dormant state, waiting for enough moisture to germinate.
All open-vented crawlspaces host a variety of fungal species, including molds and mildews. These are present mainly because of high humidity levels but also occur at areas of water leakage. There is no accepted amount of fungi/mold that is acceptable or safe, but I report unusually high amounts of growth or growth that appears to be of a structural concern.
Structural Concerns
All fungi, including molds, damage the surfaces they live on. However, some common molds, particularly surface-feeding mildews, cause little to no structural damage. Almost all traditionally-vented crawlspaces have at least some surface-feeding organisms visible. I have seen 60 year old houses with these mildews on floor joists and no structural damage or degradation. Other fungi will damage wooden floor structural components. Typically, these more destructive fungi are a sign of extremely high humidity levels or, more likely, water leakage. Any fungus-like growth (other than the common surface-feeding stuff) is a reason for concern. Note that because we’re not testing the growths we see during a home inspection, it’s not really possible to say with 100% certainty whether a common-looking growth is going to get worse and cause future damage–we’re just offering an opinion based on visual appearance and experience. However, generally wood with a moisture content below 20% will not foster the growth of structurally-damaging fungi.
Other Damage
Fiberglass insulation is a favorite growth media for many species of mold. Fiberglass insulation is used in floor batts and HVAC ductwork wrapping. Although mold can not eat or degrade fiberglass, it will contaminate it to the point it needs removal. This makes insulating a floor above a crawlspace particularly difficult. Without it, you get cold floors and higher heating bills. With it, you get a potential moldy mess that damages the subfloor and requires eventual removal. Pick your poison. Most homeowners wind up removing insulation because of fungal growth on the wooden areas covered by insulation.
Health Concerns
All fungi release spores and chemical gases that can enter the home through gaps and holes in the floor structure or HVAC ductwork. The most common effect of this is a musty smell in the home. Less commonly, these spores and gases can trigger allergic reactions or illnesses, especially in immunocompromised individuals. Some fungal species, such as the dreaded “black mold” (Stachybotrys chartarum), produce highly toxic gases called mycotoxins and are typically present in wet or constantly damp areas. The more common growths in a crawlspace do not typically produce dangerous mycotoxins or compounds toxic to most people.
Recommended Action
Whether common or severe, the presence of microbial growth in a crawlspace indicates high humidity and/or water leakage. The first steps in corrective action always include the following:
1. Ensure gutters are present, clean, and draining properly at least 5 feet away from the exterior walls.
2. Ensure drainage outside the home directs water away from the exterior walls. This may require the services of a landscaping professional.
3. Ensure the crawlspace dirt floor is completely covered with heavy plastic sheeting.
4. Any active water leakage into the floor structure needs to be corrected right away. Damaged or moldy materials should be treated or replaced.
These four steps reduce water seepage and intrusion into the crawlspace dirt floor. Coincidentally, standing water around the foundation wall can (but not always) lead to foundation issues such as settlement, so keeping the crawlspace dry is a good idea for more than one reason.
Next, humidity-related microbial growth can be addressed in the following ways. Ideally, a crawlspace is clean and dry with a relative humidity well below 60%. In reality, few crawlspaces achieve this except for encapsulated ones. It’s much easier to “do a crawlspace right” during construction rather than correcting it years later.
Encapsulation
This is the gold standard and the most expensive option. Encapsulation involves installing a heavy plastic sheet over the floor and sealing it to the walls, making a gas-tight covering. Vents are closed off and either a dehumidifier is added or HVAC ductwork is opened to air-condition the crawlspace. Typically, any visible microbial growth is first treated and removed. Drainage issues requiring a drainage system are installed first. A radon gas problem is fixed by installing collection pipes on the ground before the plastic liner is added. The resultant space is clean, dry, and much healthier. Encapsulation can cost upwards of $10,000 but is becoming a major feature / selling point.
Powered Ventilation
This involves installing fans at one or more crawlspace vents. The fans are controlled, ideally, with a combination thermostat and humidistat which runs the fans only when doing so would reduce (improve) humidity.
Structural Sealing
This involves coating all exposed wooden floor structure with a special sealant, such as Watsonseal LumberKote. Any visible growth is removed prior to the treatment. The coating prevents molds from growing but surface-feeding mildews can still be present in small amounts. This solution does not address fungal growth on floor and HVAC ductwork insulation.
Chemical/Mechanical Removal
Fungi on floor structural components can be removed with soda blasting (a kind of sandblasting) or with fungicides sprayed onto the wood. These treatments work but unless the underlying problem (humidity) is addressed, the growth will return.
Professional Resources
Crawlspace or foundation companies provide one or more of the above solutions. Be aware that some of them tend to oversell their services and sometimes make a bigger deal than is warranted out of common issues such as surface-feeding mildews and water intrusion into the crawlspace floor area. Check reviews, get multiple bids, and do your research. A pest control company can be a great resource. Unless you are physically sensitive to common molds some crawlspaces can be left as-is. However, if you have the financial means it’s always best to properly address crawlspace fungal growth by reducing the humidity and water intrusion. This will reduce the chances of problems down the road and will boost the value of the home.
(c) 2023 Todd Thuss, Integra Inspection Services, LLC