December 1, 2006


Today’s houses make it easier for mold to find the food and water it needs to thrive. The cure is a quick cleanup and smarter choices in materials. Reprinted with permission from Fine Homebuilding, December 2006/January 2007, pages 70-75.

Mold isn’t a bad thing. Without it, we wouldn’t have beer, blue cheese, or penicillin. When mold starts attacking the inside of your house, however, it is a bad thing. Even before the memorable onslaught of hurricanes in 2004 and 2005, we’d seen a lot more moldy buildings. Mold claims in Texas alone increased by five times a few years ago and cost homeowners’ insurers more than $1 billion in 2001. Why?

Mold is a water problem. Excluding the flooding in the Gulf states, though, there suddenly isn’t more water, so why is there more mold? The problem is that the water we’ve always had is hanging around in a new generation of building materials that can’t tolerate water as well as yesterday’s building materials could. Today’s building materials are also more palatable to mold because they’re more refined.

Many of the wood products that make our lives convenient during the construction process can make our lives inconvenient later on. These deficiencies don’t show up, though, until something bad happens. It can be a single event like a hurricane, or an ongoing event like a roof valley that lets water into the house every time it rains.

While “toxic mold” makes headlines, news stories tend to offer more hype than hope. It’s really not difficult to avoid mold problems if you understand how mold works. Getting rid of mold isn’t so difficult, either.

Mold likes sugar, and trees are made with sugar

To engineers like me, trees are just big batteries. Let me explain: A tree stores the energy it converts from the sun (through photosynthesis) in the form of glucose, which is a hunk of sugar. When we burn the tree, we convert the energy in glucose into heat. Mold prefers the sugar found in dead plants, which is in the form of cellulose. Unfortunately, we build houses out of dead plants and dead-plant by-products. That’s OK as long as we don’t make it easy for mold to get to the cellulose. Unfortunately, as wood is refined from a log to the paper facing on drywall, the cellulose is chopped, baked, ground, and seasoned to become easier for mold to eat.

Mold likes water, and houses leak

Living inside of mold food isn’t too much of a problem as long as we make it hard for mold to eat the food. We do this by avoiding moisture problems. When houses get wet at a rate that exceeds their ability to dry out, moisture accumulates. When the rate of accumulation exceeds the storage capacity, we have a moisture problem. The house can’t dry out. The amount of water coming in isn’t the problem; it’s the balance of how much water is coming in compared with how much is going out.

Unfortunately, both the storage capacity and the drying potential of houses have decreased over the past 100 years. You can store a lot of water in lath and plaster, but you can’t store much in drywall. The drying potential has shrunk because of thermal insulation and the lower permeability of roof and wall materials (plastic vapor barriers).

Ductwork: the mold superhighway

We’ve made building materials into better mold food as we move down the processing stream from tree to paper, we’ve reduced the drying capacity of houses by adding insulation and vapor barriers, and we’ve reduced the storage capacity of houses by using newer materials that can’t store much water (if any at all).

The last big change in our buildings is that they now are hollow. Walls, floors, and ceilings are hollow cavities. These hollow cavities with insulation in them can’t control airflow or moisture. We’re inadvertently building complex three-dimensional airflow networks.

The classic example is a commercial building where the dropped ceiling acts as the return plenum. The dropped-ceiling return plenum is connected to the exterior walls, and the interior walls are connected, too. We’ve got a three-dimensional airflow network where every interior partition wall is connected to the exterior walls by the floor and the ceiling assembly, so everything is connected to everything else. That network is the contaminant interstate. It transports mold from wet walls to the breathing zone of the space via the building’s mechanical system.

Residentially, we do the same thing with forced-air heating and cooling when HVAC contractors bang a few pieces of sheet metal on the underside of a floor joist or a stud cavity and call it an air return. Basically, they’re drawing air through wall and floor cavities. And the floor and the partition walls are connected to the cavities of the exterior envelope with holes cut for plumbing and electrical. We’re connecting the envelope to the breathing zone via the mechanical system. To me, that’s the greatest danger and the greatest tragedy of the buildings that we’re constructing today. The principle is the same for residential and commercial, whether we’re in Alaska or in Florida. This inadvertent linkage of the occupants to the envelope via the mechanical system is something that nobody expected or predicted. When we add this piece to the three others that I’ve described, it answers the question, “Mold, why now?”

View mold as the canary for all other contaminants. If they’re carried by air, decay fungi and other pollutants are likely to be transported along this three-dimensional airflow network into the breathing zone of building occupants.

The fix: smart design and remediation

We build houses of materials with little buffer capacity and with thousands of pounds of water in cast concrete. We fill them with insulation, we wrap them with vapor barriers on the inside and on the outside, we heat them and cool them without any understanding of the physics, and we look around and wonder why they’re turning into mush and falling down. But there’s no mystery here.

The solution starts with selecting materials systems differently. We can choose smart materials and simple systems or simple materials and elegant systems. When you have a water problem, fix the problem quickly and dry it out, or replace the damaged materials. Don’t wait for the lawsuits to establish who’s at fault. Fix the problem quickly, and it won’t be worth quibbling over who pays for it. The cost will be minimal.

And please, don’t blame mold and indoor-air quality problems on energy conservation. Yes, if houses were leaky, we wouldn’t have these problems, but the answer isn’t to take out the insulation and make the house leaky. The answer is to understand what went wrong, get past this issue, and build even higher-performing buildings. As a society, we can’t afford to build disposable structures. People aren’t disposable, and buildings shouldn’t be disposable, either. . .

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