Fungi Require Moisture

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©2023, Jeffrey C. May

MACROFUNGI are wood-decaying organisms and produce fruiting bodies that we call mushrooms or toadstools. Outdoors, their hyphae can fan out in the soil a few feet to many yards (or in some cases across acres) from the location of the mushroom. In wood, their hyphae are present within the substrate.

MICROFUNGI, commonly called “mold,” grow in soil or fan out in colonies along a surface, and subsist on biodegradable materials that include pollen and tree sap on siding; and dust, oils, fats, starches, pet dander particles, and skin scales on interior surfaces. With few exceptions, microfungi do not degrade wood, but they produce large numbers of spores that indoors can impact human health.

Most species of microfungi require less moisture than macrofungi. Some microfungi, such as the
allergenic Aspergillus, Penicillium, and Cladosporium molds, can even flourish in conditions of elevated relative humidity (RH) without the presence of liquid water. Exceptions would be Stachybotrys mold, commonly known as “toxic black mold,” and Chaetomium mold; both need chronically damp conditions and commonly grow on the paper and not the plaster of wet drywall. Both of these microfungi are black, so some people mistake one for the other. Cladosporium mold is also black and is commonly found on attic sheathing and on basement foundations.

I am more concerned about the presence of Aspergillus and Penicillium mold in buildings than I am about the presence of Stachybotrys mold, because Aspergillus and Penicillium spores are more readily aerosolized than Stachybotrys spores are and thus pose a greater risk of exposure from inhalation.

Sources of Moisture

Water intrusion can occur from the exterior when –

  • Roof water flows down the siding or brick.
  • Chimney flashings are inadequate.
  • Wires and cables lead with a downward slope into a building.
  • A building lacks a gutter system, has an inadequate overhang, or has reverse grading (even from an adjacent property).
  • A driveway leading to a garage-under slopes toward the house.
  • Window-cap and door-cap flashing are sloped toward rather than away from the cladding.

A proper repair of the above flashing problem would entail removing a clapboard and installing a
flashing that is sloped to shed water away from the siding and that covers the mitered joint.

Some Internal Sources of Moisture

  • Dryers venting indoors or into crawlspaces.
  • A bathroom exhaust venting into an attic or soffit.
  • Pull-down stairs or a hatch leading to an attic that allows moist house air to flow up into the attic.
  • Open ceiling supplies and returns in an air conditioning-only system with ducts in an unconditioned attic, thus allowing moist house air to migrate into the system.

Relative humidity (RH) below-grade:

As air cools, its RH rises, so below-grade spaces are prone to developing high RH conditions. It’s therefore not surprising that some molds commonly grow below-grade when the RH has not been adequately controlled since construction.


The RH in unfinished basements and crawlspaces should be at or below 50% and in finished basement spaces, 60%. During the humid season (in New England generally between mid-April and mid-October), unfinished basements must be dehumidified. Finished basement must either be air conditioned or dehumidified (or both, if necessary). During the heating season, unfinished basements do not need to be dehumidified, but finished basements must be consistently heated, whether in use or not, with the thermostat set at a minimum of 60 o F.

Some Tips for IAQ Investigators and Building Inspectors:

  • A raised floor in a finished basement can hide all sorts of ills, including moisture intrusion, mold growth, and wood rot.
  • An alternative to a drywell would be installation of solid, 4” PVC piping a few inches below the soil. The piping can extend to daylight downhill from a building or into a deep landscape furrow, and a downspout can be inserted into such piping.
  • Be wary about cracks in concrete that are near a foundation wall and are subject to downspout water flow.
  • A cable or wire leading from the exterior into the siding or trim of a house should have a “drip loop” to prevent rainwater from entering the building.
  • Attic pull-down stairs should be covered with an airtight, insulated box to prevent the upward migration of moist house air.

The second edition of our book My House is Killing Me! A Complete Guide to a Healthier
Indoor Environment
was published in December 2020 and is available online.

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