Figure B: 13 Common Breeding Grounds for Mold and Mildew
Leaky air-conditioning duct
joints, especially those running
through a hot attic,
create a moist environment
duct joints with the special
flexible mastic available at
heating and cooling supply
In warm environments, impermeable vinyl
wallcoverings can trap moisture-laden air as
it moves from the warm exterior to the
cooler interior. Mold degrades the drywall
and adhesive behind the vinyl wallcovering.
Use paint or apply wallcoverings
with permeable paper backings that
don’t trap moisture on exterior walls.
When washing machines in a room without a floor drain
overflow or hose connections burst, water with no point
of exit will soak into adjacent carpet, drywall and
Always provide a floor drain near the
washing machine. Install an overflow pan directly under
the machine or install a 1-in. lip at the doorway to contain
overflows in main-level or second-story laundry
Water-resistant drywall used as a tile backer quickly
degrades once subjected to moisture.
Install cement backer board, which will
remain structurally sound even if repeatedly subjected to
Poorly ventilated bathrooms allow surface mold to grow.
Install a bathroom fan (or at least, open a
window) to exhaust moisture. Remove surface mildew by
scrubbing the area with a 1/2 percent bleach solution. When
the area is dry, prime it with an alcohol-based, white pigmented
shellac, such as Zinsser Bullseye, and use a paint
Poorly constructed crawlspaces promote mildew
growth. Bare earth floors transmit huge amounts of
There are many regional differences
and solutions. Cover bare earth with 6-mil poly sheeting.
Heat, cool and humidify the area the same as the
rest of the house.
Freshly cut firewood stored
indoors emits huge
amounts of moisture.
Humidifiers (especially reservoir-type central units
and portable units) provide both a growth medium
and a distribution system for mold and mildew.
Clean and treat the reservoir
often with an antimicrobial solution, available at
most hardware stores.
The condensation pan directly under the coil of your
central air conditioner can harbor mold.
Before each cooling season, clean
the pan with a 1/2 percent bleach solution and make
sure the continuous drain is working.
Finished concrete basements that haven’t been thoroughly
waterproofed from the outside are problematic.
When moisture migrates through the earth and non-waterproofed
concrete walls, it can get trapped behind
vapor barriers, carpet, layers of insulation and drywall.
Thoroughly waterproof the exterior of
concrete walls before backfilling. Install 6 in. of gravel
under concrete floors during construction to prevent
moisture from wicking up through concrete floors and
into floor coverings.
Yards that slope toward foundations invite water to
enter basements and crawlspaces.
Regrade yard surrounding house
so it slopes away at a rate of 1 in. per foot.
Improperly flashed or caulked windows (and those with
large amounts of surface condensation) let moisture seep
into the surrounding wood, drywall and insulation.
Properly flash and caulk windows during
installation; minimize condensation with good ventilation
Leaky flashings and shingles allow rain to infiltrate attics,
insulation, eaves and other areas that can trap moisture and
be difficult to inspect.
Perform yearly roof inspections—even if
you do it from the ground with binoculars.
Note: You can download and enlarge Figure B in “Additional Information” below.