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Proper temperature and relative humidity levels are important in maintaining good air quality and occupant comfort. Improper relative humidity settings can create a variety of problems for any building such as bacteria, viruses, dust mites and mould growth.
Mould can grow inside perimeter walls due to condensation from high relative humidity. This is usually a hidden issue that may not show actual mould growth on the surface. However, airborne mould spores may begin to negatively affect occupant health and comfort.
Elevated humidity levels indoors can be an indicator of poor ventilation or exchange between the building and outdoors. Poorly vented bathrooms, laundry facilities, cooking facilities or improper use of humidifiers can cause the relative humidity to rise leading to a mould and bacteria problem.
Low humidity levels are usually the result of low outdoor levels experienced in Canadian winters when the outdoor humidity levels can reach the low teens.
Humidity levels should be managed throughout the year at appropriate levels for the season. In general ASHRAE Standard 62-2001 for Indoor Air Quality states the following:
Maintain RH levels in winter between 30 to 40% and temperatures between 19-23 degrees Celsius. If windows show excessive condensation then the RH% levels will need to be closer to 30%. Never exceed 45% in winter.
In warmer seasons keep the relative humidity between 50 to 55% with the temperature around 23-26 degrees Celsius. Outdoor humidity levels may temporarily increase these levels.

Health Effects

Relative humidity settings that are too low may cause respiratory infections, allergic rhinitis, and asthma. Relative humidity that is too high may cause health effects due to mould growth, dust mite infestation as well as certain bacteria and viruses.
One of the leading indicators is whether your windows are showing excessive condensation. Should this be the case you may want to have an indoor air quality assessment performed to determine the source of the condensation issue.