We Should Care About Mouldy Houses
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
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
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.
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
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
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
of stagnant water (such as slow drying refrigerator drain pans)
in areas with direct soil contact (such as crawl spaces, below grade basement walls and floor slabs)
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
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 www.cmhc-schl.gc.ca