By Ann Robinson and Annie Vernon

If you didn’t have any sleepless nights after we talked about lead paint, let’s see if a discussion of mold will do the trick! We have all heard horror stories about mold taking over homes and wreaking havoc. But in our dry climate, we don’t typically fret too much about mold. But we get mold even here, and the more you know, the more likely you’ll be able to avoid it.

Mold is an organism that thrives on eating the cellulose or sugar in dead plants. Unfortunately, we build our homes out of dead plants—from wood to paper to various wood by-products. Mold has become an increasing problem because the materials we use to build homes have changed.

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The mold in this home was a result of Hurricane Katrina. But it’s not just wet climates or severe storms that can make your house vulnerable to mold.

For instance, mold and fungus cannot penetrate heartwood—the wood at the core of a tree. But they easily feed on sapwood—all the other wood around the core. As a tree ages, more of its trunk becomes heartwood, and in years past we harvested large, old trees for lumber. With the advent of tree farms, trees today are harvested much younger, so lumber is now almost all sapwood.

To compound things, when sapwood is ground up and used as engineered lumber or plywood, the heat and pressure in the manufacturing process caramelize cellulose, making it even more delicious for mold. Particle board and medium density fiberboard are made with even smaller pieces of wood, so they are almost as easy for mold to digest as paper.

While that may be startling, all of this material is able to resist a mold takeover if it’s kept dry. But combine it with water, and you have mold nirvana.

Our first goal should always be to keep water away from a home. However, as far as mold is concerned, the critical thing is not how much water gets in, but the balance of how much water goes in vs. how much goes out. When a house gets wet at a rate that exceeds its ability to dry, moisture accumulates. And that moisture accumulation becomes critical.

For instance, a wood-framed house averages 5 percent moisture content (MC). Mold does not become active until around 16 percent MC, so this type of construction has a storage capacity of around 10 percent. The wood in a typical 2,000-square-foot house weighs about 5,000 pounds. With a storage capacity of 10 percent, this means a wood-frame home can absorb 500 pounds of water (or about 50 gallons) before mold will start to grow.

By contrast, a home built with metal studs and sheet rock has a storage capacity of about 1 percent. The same size home will have a structural frame that weighs about 3,000 pounds with a storage capacity of only 30 pounds or 3 gallons of water. Obviously, using metal studs and sheet rock will require you to pay much closer attention to keeping the structure dry, while wood frame is more forgiving.

But the answer is not to abandon newer construction methods that make a home more energy efficient. The answer is to pay attention to the materials you use and keep your home dry.

The numbers in this column were published in Fine Homebuilding Magazine, January 2007. As always, we welcome your home architect design questions at

Keep house dry to avoid mold