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 organism.

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 conditions.

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 in.

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 flourish.

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 buildings.

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 Houses

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 "Water-Managed Wall 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 leaks (see "Troubleshooting 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 fungus.

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 basement.

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 "Foundation Drainage," 3/99).

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 in."

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 air.