March 1, 2001
by John Straube
Awareness of mould in buildings has increased dramatically over the last decade. The indoor air quality and health impacts of rotting condos in British Columbia and mouldy portable classrooms in Ontario have been splashed across the news. While it should not be blown out of proportion into the next asbestos scare, all those involved in the building process–architects, engineers, builders, and building code officials–must understand mould and must work to minimize its incidence.
Moulds in buildings are undesirable for three primary reasons: they cause decay of materials, including structural members; they cause objectionable staining; and they can have negative health impacts. Health problems can arise because moulds produce spores, mycotoxins and volatile organic compounds (VOCs). Spores are small particles that can cause allergic reactions which irritate the respiratory tract because of their small size–as small as 5 millionths of a metre for Penicillium spores, so small they float in still air for hours. Mycotoxins can cause neurological damage and immune system responses, while VOCs are irritants.
Although the health impacts are not completely understood, there is no doubt that some moulds, especially some of those that occur within decaying buildings (for example, Stahybotras Chartoroum and Aspergillus) will cause health problems in most people exposed to a sufficient dosage for a sufficient time. Many moulds have no health impact; in fact we eat mouldy cheese and use the mycotoxin from Penicillium as an antibiotic.
Four factors affect the health impact of mould in buildings: the type of mould, the size of the mould colony, the sensitivity of occupants to mould, and the nature of the building’s air distribution. Response to mould exposure is quite variable. Some people become hypersensitive through excessive or repeated exposure, while others appear unaffected in the midst of a building full of sick co-workers. In general, the greater the size of the mould colony, the greater the severity and frequency of health effects. The type of mould is another critical factor.
Mould Growth Requirements
Fungal growth requires an infestation, nutrients appealing to the mould, temperature and moisture conditions conducive to growth, and air. Except for submerged components of buildings (wood piles, for instance) air is always available.
Mould spores exist outdoors in significant concentrations, especially in summer and in areas with vegetation, as well as in building materials delivered to construction sites. If conditions for fungal growth are favourable, there are likely mould spores present. The type of moulds in the indoor environment that cause health and comfort problems are usually very different from exterior species, so species identification and counts may be useful in a mould investigation.
Many building products are organic–for example, wood and paper, many glues and paints, etc.–and act as a ready source of nutrients. However, mould can even grow on ceramic tile since small amounts of captured airborne dust, dirt and even soap provide nutrients. The larger the internal surface area of the material, and the more processed it is, the more quickly and easily mould will bloom. For instance, the paper facing on gypsum board and the cellulose in ceiling tiles, which can be broken down into sugar, provide ideal sources of food, while the original source for these products–solid wood–is not nearly as attractive to mould.
Ideal temperature conditions for mould growth tend to be in the range of 20C to 35C. Most moulds will grow much more slowly outside this range, and little growth occurs below 5C or above 50C. Mould spores can survive outside this range–from well below freezing to over about 60C–and growth will resume when the temperature becomes favourable again.
Fungi typically require a surface relative humidity of 80%. Some moulds can grow with less moisture, but these species are less important to building problems, and growth is very slow, even on nutrient-rich surfaces.
Alkalinity and radiation also play a role. Most fungi require the pH of the growth substrate to remain within the bounds of about five to eight. This sensitivity to alkalinity is the reason that lime washes–with a fatal pH of 10 to 13–were used in the past to control fungal growth. Exposure to ultraviolet radiation may slow or kill fungi, but this depends on the intensity. For exterior cladding, the UV intensity is often high enough to stop growth. Exposure to direct sunlight typically reduces fungal growth since the light warms (and hence dries) the surface, and the UV intensity can be high.
Since buildings are infested with mould spores, building materials provide nutrients, and interiors are maintained at temperatures conducive to mould growth, the obvious and most practical means of controlling mould is to restrict the last variable: moisture. Hence, the industry has reached the important consensus that mould control is merely an extension of moisture control. The building enclosure must be designed to control moisture–rain penetration, interstitial and surface condensation, built-in moisture, and air leakage condensation.
HVAC systems are a great location for mould growth, and air distribution ducts broadcast the results throughout the building. Fibrous sound-absorbing duct linings collect dust from the air, and condensation from summer air conditioning allows mould growth to form on this ideal substrate. Condensate pans or clogged condensate drains are often sources of moisture infestation. Porous internal duct linings should be avoided, and the HVAC system (especially filters and pans) must be properly maintained.
Cool potable water and chilled water lines can allow summertime condensation to form and then drip onto ceiling tiles or within drywall partitions. Such lines should be insulated with closed-cell insulation or at least routed through building areas constructed of materials able to withstand occasional wetting.
For parts of a building or building enclosure that can be expected to be warm and moist, materials with low or no food value should be used. There are no truly mould-proof materials. Some materials do, however, provide a ready source of food for fungal growth, and hence have been found to be prone to infestation (these include gypsum board, carpet, and ceiling tiles). It is best to avoid getting the materials wet in the first place.
Finally, beware of wetting from cleaning activities; excessive wash water can wet drywall, subfloors, and even the ceiling below.
If control strategies fail and mould is suspected, either because of building occupant symptoms or complaints of musty odour, it should be found by tracking the smell and/or the symptoms. Occupant surveys and a detailed and expert visual investigation of building spaces and HVAC systems are very useful strategies.
Interior environments can often be very mouldy and yet have no visible mould. Since surfaces exposed to the interior tend to dry quickly, it is often concealed surfaces that support mould growth: inside wall cavities, above ceiling tiles, in crawlspaces, behind baseboards, in condensation pans below air conditioning units, and within porous insulation inside air ducts.
While it is easier and less expensive to avoid mould than to take remedial action, remediation may be necessary if wetting occurs because of a design oversight, a catastrophic flood, or construction error. The first stage in remediation is identifying the source of the moisture and solving this problem. The next step is the removal of heavily contaminated materials and the cleaning of slightly stained ones. Materials like drywall, carpet, and insulation will often need to be discarded, since their inner pores cannot easily be cleaned. Hard and non-porous surfaces like steel and concrete can be cleaned using mechanical action and then treated with anti-microbials. During cleaning the area must be contained to prevent the spread of the millions of mould spores that will be released when disturbed–this is normally achieved
by depressurizing the work area. Another concern is that too much water is used during cleaning, leaving the building moist enough for mould to begin again. Any space cleaned for a mould infestation should be clean enough–in concealed spaces as well as exposed–that a white glove will not show dirt or dust.
It should be stressed that although remediation is certainly possible, the best strategy is to avoid the build-up of moisture in the building enclosure in the first place, and this is a major topic that requires detailed attention.
John Straube teaches in the Department of Civil Engineering and the School of Architecture at the University of Waterloo.