Actually rather benign and even ubiquitous in our watery world, mold is becoming the scourge of many building owners, health practitioners and among the general populace. Mold is nothing new; it appears quickly when we have moisture and a growth medium, combined with approximate temperature and humidity conditions. But mold problems appear to be on the increase.
For one thing, more people have asthma, which can be triggered by mold growth. Studies have shown a linear progression in asthma over the past 40 years. There are several theories as to why.
?Mold?s untold damage? was the title of an article in USA Weekend October 12-14. The article, read by millions, told the story of a Texas family that was driven from their home by runaway mold growth. Leaks from copper plumbing were blamed for the growth of a lethal mold that can cause breathing difficulty, memory loss and bleeding in the lungs. The family sued, and was subsequently awarded $32 million in damages against their insurance company, which the family says mishandled their claim for mold damage. The case is still under review.
Building practices since the 1970s are also contributing to increased mold growth. ?Tighter, warmer, with more furnishings? is how one researcher described this trend. There is reduced air infiltration into our buildings because of added insulation and tighter fitting doors and windows, which saves energy, but also which can lead to a build-up of humidity and other pollutants.
There are other factors which also contribute, but some of these are on a more individualized per-building basis. For instance, oversized cooling systems continue to be a sporadic problem. Many building owners, including homeowners, demand a high capacity air conditioner in the theory that this will make them more comfortable in the summer. Of course, hvac contractors know that this is not the case, that a too-large cooling system will run less, reducing the amount of time there is to remove humidity from the interior space. The mistake nevertheless continues to be made.
Factory-built homesManufactured housing contributes problems of its own, and while still a small sector of the overall housing stock it is one that is rising faster than other sectors. Of the two million homes built in the U.S. in 1999, 18.5% were built in factories. In 2000, one in six new single-family homes were built in a factory.
Tears and rips in the belly board, which is intended to exclude ground moisture from the interior of the building, are often found, according to one expert. Frequent use of vinyl, plastic and other nonporous wall coverings allows moisture to condense.
Manufactured homes are built to HUD Code, Title 24 CFR 3280. This code includes elements of several national building codes. According to researcher Neil Moyer, Florida Energy Center, a study of manufactured homes revealed that “All had a forced air distribution system that would generally considered to be oversized. Most had duct leakage that was significant and caused the building to operate in a negative pressure.”
There are even more specific challenges that lie in special use buildings, such as museums. Mike Eissenberg, with the National Park Service, gave a presentation on humidity control in Independence Hall, Philadelphia. It was part of a multi-million dollar construction project that began in 1993 to replace the building’s aging utility system. There were three needs that had to be met: the building itself; its many fragile, historical collections and artifacts; and of course the comfort needs of the many visitors who visit this national treasure annually.
If the interior of such a building is maintained at around 68∞F, which is the lower range of the ASHRAE comfort scale, it tends to have an indoor humidity in the 5-20% RH range. This is low enough to induce cracking in some of the artifacts. Raising the indoor dew point with humidification, however, can lead to water condensing from the warm, humid indoor air on building envelope elements such as windows, roof sheathing, metal tie rods, etc.
A humidity monitoring system was installed in this particular building: Hanwell and Vaisala brands. Indoor relative humidity was controlled to between 20 and 70% RH “with minor excursions.” As a result, “little physical damage has been recorded… as a result of the relative humidity swings” and ultimately “this control strategy has been successful in providing protection for the building.”
Poor maintenance blamedLack of building maintenance, poor or reduced air filtration, and improper operating procedures also all contribute individually.
Mold problems are frequently seen in many school buildings, particularly but not limited to the Southeastern portion of the United States. Schools regularly encounter mold problems because of operating practices that call for shutting off the building cooling and/or ventilation systems during the summer when school is out. A regular practice in many areas is removing mold from books and furnishings just before students return in the fall. Another common practice is to turn the ventilation and/or dehumidification systems off at night, or at the end of the school day, to save energy. Humidity quickly builds up inside the building, with the hvac system forced to “catch up” just before students return to classes the next day.
“Ventilation air dilutes and removes stale, contaminated air from occupied building zones replacing it with cleaner, conditioned air. Cycling of hvac systems results in periods of time when contaminants are not being diluted and removed but rising in concentrations,” she said.
Charlene Bayer, Georgia Tech Research Institute, said schools are “far more susceptible” to developing IAQ problems than most other types of facilities, and the children who occupy them are also far more significantly affected than adults. She quoted a U.S. General Accounting Office rep[ort that one in five schools in the U.S. has IAQ problems and 36% of the schools studies had “less than adequate” hvac systems.
Use of desiccant systems to remove moisture is recommended in many areas. Bayer said, “These systems are able to supply continuous ventilation to a space while continuing to control humidity.” A study she helped to conduct found that schools had problems with moisture, especially among those that met the recommended ASHRAE ventilation standard of 15 cfm per building occupant – unless they had an active desiccant system.
With a desiccant system operating, the maximum CO2 concentration was about 800 ppm. It rose to about 1,150 when the desiccant system was turned off. “The CO2 levels throughout the day with the (desiccant system) not operating were much more variable and these variations were dependent on the movement of the students in and out of the room, such as at lunchtime, and the CO2 concentrations decreased very slowly after the end of the school day. It can be assumed that VOCs and other indoor pollutants would behave in a similar manner to the CO2, since CO2 can be used as a surrogate to indicate gaseous pollutant behavior indoors.”
Maintenance also continues to be an issue in schools. Bayer told of a school custodian who told her he was unable to change the filters in ceiling-located heat pumps because his school district repeatedly turned down his requests for a taller ladder!
In one research project she was involved in, “The CO2 levels resulting from reduced ventilation air clearly demonstrated the need for optimum maintenance practices in the schools.”
A frequent source of moisture intrusion in buildings comes from hvac equipment located on the roof, such as chillers or ventilators. Many of these are protected by sheet metal enclosures which are placed directly onto the roof surface or a roof curb, but the tops of the cabinets lack a roof-like protective coating or surface. Sometimes an improvised temporary vinyl sheet or similar material is placed over them. Leaks that develop can allow moisture to infiltrate into the cabinets, with mold growth taking place right where the air is circulated throughout the building.
Roofs are more likely to fail at equipment curbs, Schoen pointed out, versus flat expanses where the roofing material is continuous. “The weight of rooftop equipment can further depress the roof structure, resulting in ponding in a location where leaks are common.”
There are other sources of building moisture. “In some cases,” according to Larry Schoen, PE, “ducts on the exterior of a building, often the roof, supply air to the interior. If the ducts carry heated or cooled air, there is insulation system present and it is often sealed with a brushed mastic and sometimes a reflective coating. Our experience is that such coatings remain waterproof for a maximum of five years. The fact that personnel commonly walk on ducts on roofs further compounds this problem. This compromise of the insulation can have several results. In hot and humid climates, condensation can occur on the outside of the duct when it contains cooler air. In cold climates, condensation can occur on the inside of the duct. In a climate with significant precipitation, liquid water, ice and snow can collect on the metal surface of the duct. All these effects can eventually lead to leaks in the duct itself and the resultant entry of moisture into the building.”
To prevent this, Schoen advises treating the top and side surfaces of the duct with “an appropriate roofing material and not a brush on product.” He said a rigid insulation board covered by a welded seam roof membrane works well. “Although this costs about three times as much as the alternatives, it performs leak free four to 10 times as long.” Ducts that do not require insulation for thermal purposes, he added, should nevertheless have an insulation layer thick enough to cover sharp edges of the metal and an outer roof membrane. Alternatively, the seams in the metal duct may be welded and no insulation applied. In all cases, the top of the duct and insulation system (when present) should be crowned to reduce ponding.