Indoor Mold Causes & Cures
Over the last few years, "mold" has turned into the ugliest
four-letter word on the job site. The mold monster has become
the bane of builders, insurers, realtors, and homeowners from
coast to coast.
The effects of mold on human health are not fully known, and
there's a lot of debate about it. But whatever the effect on
personal health, there is no doubt that mold can threaten the
health and even the survival of your business. To prevent
problems, all builders need to know what mold is, how to
prevent it from growing indoors, and how to get rid of it when
it gains a temporary foothold.
There's no real mystery to this. As a building science
consultant based in Texas, I've gotten pretty familiar with
mold, and I've come to see the issues in terms of elementary
building performance principles. The practical solutions are
found in simple good construction practice.
Many of today's mold problems are related to recent changes in
building materials and techniques. Every house is actually a
complex system made up of interacting components, materials,
and subsystems. Change one, and others may change in
unanticipated ways. In some of today's houses, this has
resulted in conditions that favor the growth of mold.
But while many types of construction problems can lead to
mold, mold prevention starts with one basic principle: Keep the
building dry. Build it dry to begin with, design it to stay
dry, and make sure it can dry out if it does get wet. And if
you tangle with a building that has an existing mold problem,
remember that every mold problem is a water problem first. To
fix the mold, you have to fix the water problem.
Some of the answers are as simple as drains that work and
pipes that don't leak. We also need to pay careful attention to
water-shedding exteriors, air and vapor barriers, insulation
details, and hvac system design, and consider how those systems
affect one another.
JLC deals with some aspect of dry building details in
almost every issue; in this article I'll just hit a few high
points, then discuss ways of tackling existing mold problems.
But first let's take a look at the beast itself: the mold
Why Mold Grows
Molds are fungi, a group that includes 100,000 known species.
Mildew in an old trunk, mushrooms in the woods or fields, brown
and white rot in a woodpile, and black mold growing in a
basement are all closely related organisms.
Fungi exist for a reason: They serve to recycle organic
material on earth. They help keep the planet in balance,
because for life to continue, everything that grows must decay
and break down again.
Mold and other fungal spores are everywhere: Every cubic foot
of air contains thousands of them, and every surface -- natural
or manmade, indoors or out -- is covered with thousands more.
When the environment is right, the spores sprout and grow. They
need only three things: food, water, and the right temperature
Food. Anything that was once
alive can serve as food for one fungus or another. In the wild,
molds invade dead trees and grasses and consume the sugars
stored in their cells. Mold colonies can live on the surface of
sawn lumber, but they really thrive on products in which the
wood has been prechewed and predigested, like OSB,
particleboard, and paper-faced drywall.
Water. To germinate and
grow, mold needs high levels of moisture -- meaning greater
than 60% relative humidity, or a surface that stays damp for
about three days. Without water, molds die or go dormant (but
only after releasing millions of spores into the air). From the
builder's perspective, this need for water is where mold is
most vulnerable: If we can keep the building dry, we can stop
mold in its tracks. But if we build any part of the building
wet, or let it get wet in service, we're inviting mold to move
Temperature. Molds can
handle temperatures from 40°F to 100°F; their
ideal zone is 50°F to 90°F. Generally speaking,
if the temperature is reasonably comfortable for you, mold will
be happy, too.
Mold in the Modern World
Builders at my training sessions often ask why we seem to hear
so many more reports of mold problems these days than in the
past. Several factors are involved. For one thing, houses are
tighter now than in the old days. Tighter homes also tend to
dry more slowly, so they may accumulate more moisture.
Increased levels of insulation also play a role. Heat flow
through walls and ceilings is expensive, but it does dry out
wet assemblies. The uninsulated or poorly insulated homes of a
generation ago could dry before mold had a chance to begin
growing. Better insulation cuts heat flow and slows the rate of
drying, so even a minor leak in a home insulated to modern
standards can lead to persistent wetness that can allow mold to
The other big consideration is that today's building materials
are better mold food. We're using less brick, stone, and
concrete (substances mold can't process); when we use wood,
we're using juvenile lumber that contains a higher proportion
of mold-prone sapwood than the lumber of years past. Sawn
lumber has been replaced by OSB and particleboard -- materials
that are easy for water to penetrate and that have lots of the
sugars and starches that molds can readily break down and
absorb. And instead of traditional plaster, which is a hostile
environment for mold, we're using paper-faced gypsum, which
amounts to mold candy (see Figure 1).
Figure 1.Paper-faced gypsum board is ideal mold
food. This photo shows a firewall between two multifamily units
that stood in the rain before the building was closed in. After
odor complaints from residents, every firewall in the project
had to be removed and rebuilt. Wet conditions during
construction are a major cause of mold in
To these factors you can add all the small things that
contribute to moisture in homes: air conditioners that achieve
greater energy efficiency at the price of reduced
dehumidification, exhaust-only ventilation systems that suck in
moist humid air in warm climates, leaky air ducts, vinyl
wallpaper that acts as a wrong-side vapor barrier -- the list
goes on and on.
All of these problems are avoidable, but they won't take care
of themselves. We need to actively address each one.
Preventing Mold in New
Stopping mold in homes is mainly a matter of attending to a
few key elements, starting with the building envelope. Remember
that there's no such thing as a waterproof wall. Windows,
doors, brick, and wood siding leak; every joint leaks, and all
caulks and sealants eventually leak, too. So water protection
in walls is not about waterproofing -- it's about drainage (see
Systems," 3/03). If you don't want mold, you can't let rain
into the walls.
Many leaks result from bad flashing details (Figure 2). For
roof leaks, poorly detailed chimneys and wall-roof
intersections are major culprits. Make sure your employees and
subs handle those details right, and you'll avoid most roof
Roof Leaks," 10/99).
Figure 2.Bad flashing at a deck ledger attachment
allowed water to soak this walkout basement wall. The OSB
sheathing shows both mold growth and white rot
Foundations. Basements and
crawlspaces are notorious for being damp and smelly, and they
are a common location for mold growth (Figure 3). Moisture in
the basement or crawlspace often moves directly into the home
above, and then into the attic. It's a common source of
condensation on air-conditioner supply grilles in the South and
of frost under roof sheathing in the North.
Figure 3.Damp basements are a friendly environment
for mold. Extensive mold growth is visible on a drywall panel
in a humid basement (left). The sawn lumber, which is less
moisture absorbent and has less available nutrition for mold,
has not been colonized. At right, high relative humidity has
allowed mold to grow on engineered I-joist floor framing in a
Building codes may say otherwise, but from the standpoint of
building science, the best way to keep crawlspaces dry is to
stop ventilating them with outside air. Seal them up, insulate
the perimeter, cover the floor with a continuous vapor barrier,
and introduce a small amount of conditioned air from the main
house. The ground cover is key: Left bare, the soil in a
crawlspace evaporates 12 gallons of water per thousand square
feet of exposed soil per day. Ventilating a crawlspace with
damp exterior air only adds more moisture. Whether your crawl
has vents or not, sealed crawlspace expert Jeff Tooley warns
that you must place an effective barrier to block ground
moisture before you close in the building. Otherwise,
you create humid conditions while the house is still under
construction, inviting fungi to attack.
Research also tells us that we should insulate a cold-climate
basement on the outside of the wall, because this keeps the
wall interior warm and dry. And in any climate that gets rain,
gutters, downspouts, and good foundation drainage are important
to protect the foundation against water intrusion (see
Warm-side vapor barriers. In
the northern U.S., the primary vapor flow in homes takes place
in the winter, as warm-humid interior air moves toward the dry
exterior. But in the South, the forces switch sides of the
wall: The warmer, more humid conditions are found on the
outside of the home during most of the year, and the vapor
drive is toward the dry, air-conditioned inside. Literally
speaking, northern homes dry out, but southern homes "dry
Vapor barriers should be positioned accordingly: on the
interior wall face in cold climates, and on the exterior face
in hot climates. In the South, vapor barriers on the inside of
the wall actually tend to cause condensation, mold, and rot
(Figure 4). Building codes in many areas are starting to catch
up with this reality.
Figure 4.In hot, humid climates, vinyl wallpaper
forms a vapor barrier on the cool, air-conditioned side of the
wall, causing exterior moisture to accumulate under the
wallpaper and support the growth of mold. Walls should be
vapor-permeable on the side facing dry air, and vapor barriers
should be placed on the side facing moist, warm