The moisture calculator in ALF gives a general guide to the possibility of mould growth in a room.
There is a large body of evidence that excess moisture is a serious problem in New Zealand homes. The BRANZ 2015 House Condition Survey found that 11% of owner-occupied and 31% of rental houses felt damp. Assessors found signs of mould in almost half the houses visited. See the 2017 BRANZ study report Warm, dry, healthy? Insights from the 2015 House Condition Survey on insulation, ventilation, heating and mould in New Zealand houses, SR372.
The moisture calculator in ALF allows designers to assess which rooms, if any, are more likely to have internal moisture problems leading to mould. If potential problem areas are identified early, the cause(s) can be addressed early. The moisture calculation is not part of the BPI calculation.
Mould can grow on walls and ceilings where:
- moisture condenses on a cold surface, such as in a bathroom;
- relative humidity is high even though condensation does not occur, such as in a bedroom.
ALF's moisture calculator computes the temperature difference between inside and outside wall surfaces and the moisture content in the room. It then calculates the time any condensation is likely to remain on surfaces and the duration of high humidities on a wall.
Mould is likely to form if:
- condensation remains for longer than 3 hours per day for rooms in which liquid water is produced, such as kitchens and bathrooms;
- high wall relative humidities last for 6 hours or more for rooms where liquid water is not produced, such as bedrooms.
Rooms with thermal mass have smaller temperature fluctuations and the lag in room temperature change in response to outdoor temperature changes is longer. A separate calculation is required to take these effects into consideration.