Hugh Cairns: Proper bathroom ventilation


It’s unfortunate, but I’ve seen plenty of examples of damage in attics resulting from bath fans that are vented to the attic space only and not directly outside. When bathroom fans exhaust into attic spaces, mold, delaminated plywood and saturated insulation can often be found. (Photo: Hugh Cairns)

Hugh Cairns: Proper bathroom ventilation

The mechanical ventilation of bathrooms is an important part of your home’s ventilation system. Your bathroom fan improves your indoor air quality by extracting humidity and odors. Unless your bathroom is adequately power ventilated, the moisture generated from your shower and bathing activities have no place to go and can lead to mildew or mould growth. If your bathroom mirror is steamed up from your shower, or if you see a build-up of condensation on your bathroom walls or window, it’s time to use your fan, or it may be time to service, upgrade or install a fan.

Indoor air quality professionals point out that the average family produces 25 litres of moisture a day from indoor activities like cooking, bathing, washing dishes. Interestingly, adults can introduce 1 litre into the air as moisture in a day just from breathing. Short showers and bathing activities can release 1 to 2 litres alone. High humidity can damage building materials and may affect your family’s health. 

While a small amount of intermittent condensation appearing on a wall or window surface may not necessarily be a problem, how long the condensation lingers and if it accumulates, can lead to problems. Sometimes we see short-term condensation on bathroom walls or windows during a cold snap.  In many instances, condensation moisture simply evaporates back into the air once the surfaces warm up or the moisture source is reduced, but the general rule is to ventilate adequately in the first place to avoid moisture build-up.

If your bathroom fan isn’t working properly, several causes may be contributing. Bathroom fans should be replaced or serviced for inadequate airflow, inability to overcome static pressure, high leakage rates or because they are in generally poor condition.

 A simple test to see if your bathroom fan is working is to turn the fan on and hold tissue paper or a plastic bag up to the grill.  The fan should draw the tissue paper or plastic tightly against the grill, if it doesn’t you may have a problem:

•           Check the flapper in the exhaust vent hood or the exhaust fan itself; it could be jammed

•           Check to see if the fan is connected to exhaust ducting; usually, no duct means better air flow, but in some cases the fan may be blowing directly into compressed insulation.

•           Duct terminates at obstruction; several times I have seen exhaust ducting terminate before it has a chance to exit the structure. In some cases siding or roofing materials have been installed over the exhaust penetration before the vent boot has been installed.

•           Check the grill for build-up and clean if needed or check for obstructions in the ductwork such as birds’ nests or other debris.

•           Incorrect position of fan in room

•           Incorrect size of duct piping used to exhaust room

•           Check if your fan is a recirculating model. If this is the case it is designed to filter out odors instead of exhausting air. If you find filters when you remove the fan grill, you have a recirculating fan

Loud fan operation can be caused by a loose motor. Is so, turn off power to the unit, remove the grill and check that all mounting screws are secured. Sometimes the fan blade hits the housing of the fan unit. If so, turn off power to the unit, remove the grill and check the fan blade is fully inserted on the motor shaft.

Sometimes the problem isn’t with the exhaust fan itself, but rather other factors such as an undersized fan, improper fan selection or lack of make-up air coming into the bathroom.