Abcott Construction

Engineers of

Markham Waxers

Abatement Council
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Moisture levels in the building envelope can cause biological, chemical, or structural damage to construction materials in houses. The type of damage depends on the temperature, moisture levels, time of exposure and mechanical loading and also the types of building materials. In wood-frame construction, wood products may be damaged by biological activity.

The Institute for Research in Construction (IRC) at the National Research Council of Canada (NRC) is developing a method to assess durability by linking structural and hygrothermal analysis computer models.

Determination of Rate of Damage
The development of a good moisture damage model requires 

experimental data. Damage due to biological action is one mechanism that affects wood durability. A wide range of organisms, including fungi, termites, carpenter ants, wood-boring beetles and marine borers, can live in and consume wood depending on the materials, the location and the micro-environment.

Fungi damaging building envelopes are mainly mould, mildew, and wood rotting fungi. Mould fungi also cause odours and health problems in some individuals. Under the right conditions fungi cause significant loss of strength in wood materials. A fungal attack depends on six critical requirements:

  1. wood type

  2. free or unbound water,

  3. favourable temperature,

  4. a characteristic time of exposure,

  5. oxygen and,

  6. available, digestible carbon compounds.

The results of experiments conducted in Finland were used to develop a formula for the rate of mould growth. The experiments identified how much time it took for initial growth and percentage of area covered with moulds.

To predict the mould growth, it is necessary to know the type of building materials and relative humidity and temperatures of the surface over the simulated period. The temperature and relative humidity are obtained either from field measurements or from predictions.

Mould Growth Prediction
For the case of exfiltration, the bottom and topof the sheathing membrane is most susceptible to mould growth. Mould growth increases as the air leakage rate increases and decreases as the relative humidity of the surface decreases.

An indoor relative humidity of 20% to 35% gives the same amount of mould growth. Only the bottom 40mm of the wall was covered by mould for indoor relative humidities higher or equal to 35%. These results show that hygrothermal analysis alone cannot be used to assess performance and durability. The results show that exfiltration rates are much more important for mould growth than once thought.








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