Why We Should Care About Mouldy Houses

report prepared for CMHC by Morrison Hershfield Consulting Engineers

There has been much coverage recently about mould because of the link between mould in a home and the health of its occupants. Studies have shown that this link exists even after taking into account the effects of smoking, the level of volatile organic compounds, CO2 in the indoor air, or possibly-related biologically household dust contamination including dust mite and pet antigen levels.

To ensure a healthy indoor environment, contaminants must be eliminated at the source. We can prevent the construction of problem houses, and we can also make strong recommendations for the renovation of houses with existing problems.

Studies have shown that there can be large variations between houses and even within an individual house, when considering levels of exposure to some types of bio-active materials.

Moulds generally require four conditions for growth: a suitable surface to grow on, appropriate temperature, food and moisture. The first three conditions are present in all environments in which people live. The only way mould can be controlled is to control the level of moisture. Condensation on the building envelope is one of many sources of water, enabling moulds to grow. This is why well-insulated building envelopes are desirable.

The presence of local moisture sources is a much more important factor in the level of biological contamination than the amount of general ventilation in the house. The number of people in the home will affect the humidity level but so will the amount of air change.

Moisture problems and mould exposure do lead to some types of disease and the rates are significant. The problems may be the result of envelope performance failures, occupant behaviour or a combination of both. In homes with reported health problems, there may be a large range of bacterial endotoxins, dust mite antigens, cat allergens and mould mass. Tests on visible mould have shown an unusually large fraction of mycotoxin-producing moulds. If we could reduce the incidence of moisture and mould problems, we could improve the health of occupants significantly.

What are the conditions in a house that could point to mould problems?
In CMHC's Wallaceburg studies, a higher proportion of problem houses did not have a forced warm air distribution system and were heated with fuel sources other than natural gas. The lack of forced air distribution could be a factor in the growth of biologically active contaminants. This may be because, without an effective ventilation system, the forced warm air system circulates the air in the house. Whatever air change may be present (caused by natural air change and stack action) will at least be redistributed. Homes with wood burning equipment (wood stoves or wood burning fireplaces) had more problems than those without.

Visible evidence of water damage and areas of mould growth by itself may not be a good predictor of whether houses have high levels of biologically-active contaminants. Much more important is the relationship between the airtightness of the house, the air change, and the relative humidity.

Conventional logic would suggest that a tighter building, which has a lower air change, will result in higher relative humidity, higher levels of condensation and biologically active contamination. These were not the findings. Biological contamination problems were not strongly related to low levels of air change and resultant high levels of general humidity.

The "bad" houses, on average, had higher tested air leakage, higher air change rates and lower average humidities. The bad houses also had more smokers, waterbeds, humidifiers in the bedrooms and residents had a much lower income level.

The Wallaceburg studies showed that the relationship between house construction and operation, biological contamination levels and health measurements is complex and very specific to an individual house.

Moisture sources are not just related to occupancy and the control of moisture sources is not just a ventilation issue. A major moisture source was "wicking" of ground water through the concrete. Control of moisture is also important.

Local sources of moisture, rather than high general humidity levels, may be the dominant factor in visible mould growth. Interior mould growth is connected with condensation on the building envelope. The most commonly reported trouble area is mould associated with windows, because windows have the lowest insulation value.

Common locations of mould growth not related to envelope condensation include:

    bathroom surfaces

    leaking basement walls

    pools of stagnant water (such as slow drying refrigerator drain pans)

    moisture in areas with direct soil contact (such as crawl spaces, below grade basement walls and floor slabs)

General ventilation, important as it may be for other reasons, cannot be relied on to control mould growth and biological contamination.

Control of moisture sources in the house, particularly in soil contact areas, may be an important requirement in avoiding mould growth and biological contamination.

The Condominium Owner's Guide to Mould gives a simple to follow procedure a homeowner can use to assess mould problems and how to do a clean up of small mould areas. This is one in CMHC's "About Your House" series of fact sheets. Available from your local CMHC office or


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