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Jun 18, 2021

Moisture, Moisture Everywhere

Jeffrey C. May shares insight about how to make sure that homes and buildings are mold-free.

By: Jeffrey C. May

Finished basement living area with a couch, chairs and rug

Fungi require moisture. I want to start with a brief review of two types of fungi, because it’s important for building professionals to know the difference.

Macrofungi produce fruiting bodies that we call mushrooms or toadstools. These organisms have hyphae that can fan out a few feet to many yards (or in some cases across acres of soil) from the location of the mushroom. The hyphae can be visible or more often, can be present within the substrate. Macrofungi are wood-decaying organisms, and most do not grow in temperatures below 55oF.

Microfungi, commonly called “mold,” grow in colonies and fan out along a surface. These organisms subsist on biodegradable surface materials that include dust, oils, fats, starches, wood sugars, pet-dander particles, and skin scales. While most microfungi do not degrade wood, they produce large numbers of spores that can impact human health.

Both kinds of fungi require moisture, but macrofungi need more moisture than most species of microfungi need. Some molds (microfungi) such as Aspergillus and Penicillium can 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. Stachybotrys and Chaetomium molds commonly grow on the paper of wet drywall and not the plaster. Both of these fungi are black, so some people mistake one for the other. Cladosporium mold is also black but can grow under drier conditions. This type of mold is commonly found on attic sheathing and on basement foundations.

I am more concerned about the presence of Aspergillus and Penicillium molds than I am about the presence of Stachybotrys mold, because Aspergillus and Penicillium spores are more readily aerosolized than Stachybotrys spores are. Thus exposures by inhalation are more likely; in addition, some species of Aspergillus and Penicillium molds can produce mycotoxins. Some species Aspergillus can even grow inside the lungs of immune-compromised individuals, resulting in an illness called aspergillosis.

Home Conditions to Look Out For

Water intrusion: Roof water should not flow down the outside of a building; and chimney flashings are an important deterrent to water intrusion in attics.

There are two other conditions that a building professional should mention to a client: window-cap and door-cap flashings bent toward rather than away from cladding, and wires and cables leading with a downward slope into a house.

Certain conditions that can lead to water-intrusion below grade include: a building without a gutter system and with inadequate overhang; reverse grading (even from an adjacent property); and a “garage under” with a driveway that slopes toward the house.

I’d like to point out three other conditions that concern me: a deck on a house without a gutter overhead; downspouts that empty into drywells; and concrete patios or walkways next to foundation walls.

I’ve inspected many decks that suffered structural decay due to roof water. And a deck doesn’t prevent roof water from ponding next to a foundation wall. Drywells can silt up over time. I recommend that clients stand outside during a heavy rain to see if a drywell is taking downspout water. A great deal of rainwater can flow through cracks in concrete; this water may end up in a basement or crawl space.

Internal sources of moisture: Dryers should not vent into crawl spaces. Bathroom exhaust fans should vent to the exterior and not into an attic or soffit. Drop-down stairs or a hatch leading to an attic should be airtight to prevent the upward flow of moist house air; otherwise the moisture can condense on cool sheathing in the fall, winter or spring, leading to mold growth.

The wrong way to vent a dryer – May Indoor Air Investigations LLC

Some older houses in New England have hot-water or steam heat with a separate, ducted air-conditioning system. Returns (especially those located in hallway ceilings near bathrooms) as well as ceiling supplies in such houses should be closed during the winter to prevent the flow of moisture into the ducts, where mold could then flourish.

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

Unfortunately, mold spores remain potentially allergenic even when dead.

The RH in unfinished basements 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 60oF.

Mold growth can also be found above-grade in properties located near water, properties with intermittent occupancy, and properties that have gone through foreclosure and have been empty for a while.

It’s common to find mold growth on the bottom of piano benches – May Indoor Air Investigations LLC

Members of my own family own a vacation home in northern New England. They used to go to the house during winter vacations, when they operated the wood stove and cooked lots of food for their children and their friends, filling the house with warmth and moisture. They put a pot of water on the wood stove to moisten the indoor air. When they left the house, they turned the heat down below 50 oF.

Conditions of elevated RH developed. I found mold growth on the bottoms, sides and backs of many pieces of furniture, as well as on the bottom foot or so of exterior walls and curtains covering sliding glass doors.


Here are some tips to help you with your inspections as well as to pass along to your clients:

  • A raised floor in a finished basement can hide all sorts of ills, including moisture intrusion, mold growth and wood rot.
  • Window-cap and door-cap flashing should be sloped away from rather than toward the cladding. People can add a bead of caulk at the inner edge of such a flashing to redirect the slope. They can also create “caulk dams” at the ends of the flashing to prevent water from entering the trim/siding interface.

Water flowed to the end of the flashing into the miter joint and rotted the trim and framing – May Indoor Air Investigations LLC

  • An alternative to a drywell would be installation of solid, 4-inch PVC piping a few inches below the soil. The piping can extend to daylight downhill from a building or at the edge of 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 subject to water flow from a downspout.
  • A mushroom growing out of siding may be an indication of extensive fungal growth and rot beneath the surface.

There was a roof leak behind the hardboard siding – May Indoor Air Investigations LLC

  • A thermo-hygrometer should be used to measure the relative humidity separately from the set point on a dehumidifier. The thermo-hygrometer should be placed near the floor in an exterior corner.
  • 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.
  • Access to an attic should be covered with an airtight, insulated box to prevent the upward migration of moist house air.

This article originally appeared in the December 2019 Digital Edition of Healthy Indoors Magazine and is reprinted with permission.

Jeffrey C. May
Founder and Principal Scientist, May Indoor Air Investigations LLC

Jeffrey C. May is the Founder and Principal Scientist of May Indoor Air Investigations LLC in Tyngsborough, MA. He combines his education as an organic chemist and his over 25 years of experience investigating building problems to specialize in indoor air quality (IAQ). He is a former Adjunct Faculty Member in the Department of Work Environment at University of Massachusetts Lowell, and is author or co-author of four books on indoor air quality (published by The Johns Hopkins University Press), including My House is Killing Me: The Home Guide for Families with Allergies and Asthma.

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