Mold & Rot: Look Before You Leap When Weatherizing
January 06, 2010
A version of this article appears in the January/February 2010 issue of Home Energy Magazine.
a blanket of spores. Wood rots make moresophisticated spore-producing structures, which we refer to as mushrooms. Fungal moisture requirements vary. Wood rots grow only on wet surfaces, while some molds can grow on a dry surface that is surrounded with high humidity.
Mold and IAQ
It is the fungal spores that can cause problems with indoor air quality (IAQ). Fungi can be remarkably fertile. A mold colony can produce millions of spores per square inch. A single portabella mushroom cap can contain billions of spores. When a moldy surface is disturbed, the spores can become airborne. Because they are so tiny, the density of the air behaves like a thick liquid, and the spores float about, seemingly in defiance of the law of gravity. In dead still air, common mold spores can have falling velocities of less than a yard an hour. Because air moves readily inside a
structure, a few square yards of moldy surface can fill a house with spores. When the spores eventually land, they become part of the settled dust. Once mixed into house dust, mold spores can be difficult and expensive to capture and remove. Houses with moldy dust can be irritating to sensitive occupants. Indoor mold is a common cause of IAQ complaints; reported symptoms include headaches, skin and eye irritation, and respiratory problems ranging from sneezing to full-blown asthma attacks. Instead of being green by design, an energyefficient structure can turn green by accident, if it is covered with mold. Making buildings more energy efficient involves reducing heat transfer across the envelope. This is done by using materials that resist heat transfer and prevent air leaks. But using these materials can encourage the growth of mold. As is often the case, a superb engineering solution to one problem creates another. When we conserve energy, we make it harder for the building to dry. When water wets a surface, it takes some sort of energy to move it away; heat energy is needed to evaporate liquid water into vapor, and some motive force is needed to move the humid air out of the envelope. When we prevent energy loss through walls, we lose evaporative potential, and when we seal air leaks, we lose ventilation. Whether it is new or retrofitted, an energy-efficient home must be properly designed to shed water from the outside and ventilated to remove humidity build up on the inside. Surfaces that are persistently damp or surrounded by still, saturated air are fertile substrates for mold growth, no matter what they are made out of. Even a fine film of dust on window glass can support mold growth if there is sufficient moisture.
Look Before Leaping
I was an expert witness for the losing side in a court case that involved mold. A nonprofit weatherization group was being sued for causing a mold problem when it worked on a preexisting home. The house was a rickety-looking older two-story home with a dirt crawl space. The windows, siding and roof were in poor shape prior to weatherizing. An aggressive representative persuaded the homeowners to pay to get some insulation work done. Within a year the homeowners were bombarding the agency with complaints about mold problems that were aggravating their allergies. Eventually they hired a lawyer, who moved them out of the house and sued for damages.
The long and short of the case was that the house had a soaking wet dirt crawl space. Prior to insulating and air sealing, heat loss and air leakage had dried the interior out, but the crawlspace remained wet. Due to the stack effect, we tend to exhaust air from the upper part of a heated structure and take it through the lower part. So the damp crawl space wound up humidifying the upstairs as soon as the heat came on. The insulating and air sealing that created an airtight space proved to be a disaster, because the house was sick to start with. When winter arrived, water came dripping out of the attic, ceilings buckled, and mold growth on the drywall developed in a pattern that looked like a faux finish paint job. The customers complained, and then sued when no one returned their calls.
In court I presented evidence that the house was probably damp and moldy before any work was done. But it was easy for the plaintiff’s attorney to suggest that professionals should have known better than to foist their services on the unsuspecting homeowners. Eventually the jury awarded them a new home. Oops. The house was a wreck to start with, but trying to fix it without addressing the preexisting problems made things worse. Just because the defendant (the weatherization group) had good intentions didn’t excuse it from taking responsibility for the problems it had created.
From a building science perspective, the critical lesson is to always consider the house as a system, and to try to anticipate the longterm effects of specific modifications we make on the total performance of the home. From a legal standpoint, the worst mistake was ignoring the client’s repeated complaints. Before any structure is weatherized, a thorough moisture survey is a must. Both penetrating and nonpenetrating moisture meters should be part of the estimator’s tool kit. Infrared camera technology is getting better and cheaper. In trained hands, an IR camera can be helpful in locating hidden wet spots. Stains and streaks should be documented with digital photography. Pictures can be kept in the job file. Contractors should take lots of pictures to avoid being blamed for preexisting conditions.
If a home has a bare dirt floor in a crawlspace or basement, that floor must be sealed before any other work is done. If the concrete foundation shows the white staining called efflorescence, the source of below-grade wetting must be identified and corrected. From top to bottom, all seams and joints and penetrations in the exterior shell should be examined for head and kick-out flashings that divert surface water away.
If a house has a preexisting mold problem, no work should begin until the moisture source is shut off and the moldy surfaces are properly treated. Mold problems can be expensive to address. Tiny mold spores are easily made airborne. Careless handling of moldy surfaces can spread what was a local problem throughout the indoor space. Wherever possible, moldy areas should be isolated with barriers of plastic sheeting and tape. Exhaust ventilation can then be used to create clean-to-dirty air flow.
Even a leak-free house can have interior moisture problems. This is especially true when ventilation is inadequate to handle such moisture-rich activities as showering and cooking. In addition, human metabolic activity generates large amounts of water vapor that can condense and create mold-friendly habitats, such as bedroom window frames. Examine windows for peeling paint and decay that may be signs of condensation. Make sure that all exhaust fans are vented all the way outdoors. Leaking ducts can create a variety of condensation problems, especially when air conditioning is used. Remember: Water vapor moves from hot to cold and from wet to dry. Make sure that this path doesn’t terminate inside the house.
As building scientist Joe Lstiburek says, “durability trumps energy efficiency.” Both are important, but the cost-saving goal of energy efficiency is defeated when mold and rot result from failing to treat the structure as a system. Just as an ounce of prevention can save a pound of cure, a moment of carelessness, ignorance, or stupidity can create a ton of aggravation. When decay shows up, it is at the end of a train wreck of circumstances. Arresting problems before they occur is the best preventive strategy.
Mac Pierce is Master of Public Health, Environmental and Occupational Health, School of Public Health, University of Minnesota. He is also a private consultant on indoor air quality, moisture and microbiological problems. He specializes in water damage cases, and designs mold clean-up protocols.
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