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Building Science by Norman Aceron Garcia

Washroom ventilation

by Norman Aceron Garcia

nachi.16012Ventilation systems must be installed in all washrooms as they are designed to exhaust moist air and odours to the outside. This includes washrooms with windows because they are normally closed during the winter in cold climates like Manitoba. Usual washroom ventilation systems comprise of a ceiling fan unit attached to a duct that terminates at the roof. The exhaust fan may be controlled by a conventional wall switch, a timer wall switch, or a humidistat that can be automatically set to turn the fan on and off depending on the relative humidity inside. Newer exhaust fans manufactured these days are very quiet, in contrast to older fans that are very noisy when operated. A quiet older fan is an indication that it is not working properly.

Signs of insufficient washroom ventilation include high humidity levels, frost on windows, flaking paint, visible mould on ceilings or walls, corrosion of metal, and moisture stains. Ducts that terminate or leak in attics can cause warm moist air to condense on attic insulation, framing, and other materials. This circumstance increases health risks from mould and decay damage to building materials. Moreover, high moisture content also decreases the effectiveness of thermal insulation installed. Even though the growth of mould may occur in the attic, mould spores can be drawn into the home’s living areas by low air pressure. Low air pressure is normally generated by the discharge of household air from exhaust fans in furnaces, water heaters, dryers and washrooms.

The most common unacceptable washroom vent terminations are located at mid-level in the attic, underneath the insulation, and just below attic vents. Poorly terminated ventilation systems may seem to operate very well from inside the washroom. However, improperly installed ducts will eventually become disconnected or loosen at connections or joints. Washroom ventilation fans must be inspected for dirt and dust accumulation that can hamper air current.

Washroom ventilation ducts must:

  • terminate outdoors and not within the building envelope;
  • be made from rigid and inflexible material such as metal or PVC. Unlike clothes dryer exhaust vents, washroom ventilation ducts should not dangle;
  • be installed as per manufacturer’s instructions;
  • be fitted with a roof termination cap to shield the duct from the elements;
  • stick out at least six inches (150 mm) from the roof;
  • be insulated to prevent condensation;
  • be as straight and short as possible with minimal elbows. Longer ducts compel the exhaust fan to work harder and let more time for vapour to condense;
  • contain louvered (angled) slats or screen at its termination to prevent bird, rodent and insect entry;
  • have smooth interiors because internal ridges can encourage vapour to condense, which would permit water to flow back into the exhaust fan or leak through joints;

Most important, a washroom ventilation fan must be connected to a duct capable of venting odours and water vapour into the outdoors. Mould growth within the washroom or attic is an obvious sign of improper ventilation that should be repaired in order to prevent respiratory health issues to occupants and structural degradation of the home.

Norman Aceron Garcia is a registered Professional Engineer and a Certified Professional Inspector of Mr. Peg Property Inspections Inc. Please visit for more information on building science and home maintenance.

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