Condensation forming on window surfaces or anywhere else in the home is very often a sign of excess humidity. If your residence is a victim of this condition, you’ll want to take note of the following.

Situation normal…
Occasional condensation is nothing to worry about. Large gatherings during the Holidays, for instance, can easily create conditions that will make your house “sweat:” a large number of guests means plenty of snow-covered boots and coats. It takes longer to cook all that food, and there are way more dishes to wash afterward. All of this creates an awful lot of humidity! Since condensation is caused by warm, moist air coming into contact with cold surfaces, it’s quite likely that your home will host another kind of get-together: condensation on the inside of your windows.

… or worrisome?
If, however, the phenomenon is recurrent or persistent despite the fact that your windows are in good condition and properly sealed, then it should be cause for concern. If condensation regularly forms on window surfaces, chances are it will also form on bathroom walls and ceiling, as well as on basement walls or those of closets that are adjacent to exterior walls, in the attic and — most insidiously of all — inside the exterior walls. All this repeated “watering” will quickly turn these areas into gardens of mould, in turn resulting in damage to your home’s structural elements, finishes and equipments — not to mention the impact on air quality

The solution is to monitor ambient humidity, install mechanical systems to get rid of moisture at the source and, in problem situations, rapidly identify the cause.

Measuring humidity
The rate of relative humidity in a home’s interior is measured using a hygrometer. This instrument, which can be mechanical or electronic, is inexpensive and easy to find wherever room thermometers are sold (in fact, both devices are often combined into one). To get a meaningful reading, place the hygrometer in a living area, or in a part of the house where humidity seems to be a problem.

Relative humidity should never go above 45% in winter (30% when it is extremely cold out, i.e., below -10°C). If your hygrometer consistently displays a higher humidity reading, you should consider this a warning sign — one that it is best not to ignore.

Excess humidity: out with the bad air!
The members of an average household produce between 10 and 50 litres of moisture per day in their home* (presence of humans as well as pets, baths, showers, laundry, cooking, cleaning, plants, firewood, etc.). To get rid of excess humidity that is likely to produce condensation, a home should be equipped with an air extraction system connected to an outside exhaust vent.

Kitchen and bathroom
In the kitchen and the bathroom, you should choose a device equipped with a centrifugal fan: a system that looks like a turbine or a hamster-cage-type blower. Extraction efficiency will be much higher than with fans equipped with a small propeller that looks like a boat propeller.

For optimum performance, the exhaust duct should be made of smooth, rigid metal and have the shortest, straightest run possible. It must be insulated if it passes through an unheated space such as an attic, and must have a slight slope toward the vent hood to allow any water from condensation to drain outside.

A bathroom fan should preferably be controlled by a timer (or humidistat) to ensure that it operates continuously until enough water vapour has been removed from the room. This usually does not happen if the fan is controlled by an independent switch or connected to the bathroom light switch: people tend to turn it off when they leave the room, when there is still far too much moisture in the air.

To be efficient, the fan should have a ventilation rate calibrated according to the dimensions of the bathroom (1 cfm, or cubic foot/minute, per square foot of floor space + 20% of the total) and the length of the exhaust duct (1 cfm per linear foot for a rigid sheet-metal duct, or double that for a flexible duct).** The type and number of plumbing facilities (e.g., bath/shower, shower stall, whirlpool bath) and frequency of use in peak periods must also be considered. The fan should operate as silently as possible (2 sones or less) to encourage regular use.

Cooking vapours, meanwhile, must be vented outside using a range hood. Its ventilation rate will depend on such factors as the length of its exhaust duct. The duct must be of rigid sheet metal, either rectangular (3¼ in x 10 in) or circular (6 or 7 in). 

Clothes dryer
You must also make sure that all the hot, moisture-laden air generated by your clothes dryer is properly vented outside, using the same type of ductwork, and periodically ensure that the duct is not blocked.

Air exchanger
When you consider that, in a house with insufficient air flow or ventilation, between 2,000 and 10,000 litres of moisture can be trapped during the heating season,* can there be any doubt as to the usefulness of this particular device? In especially airtight homes, an air exchanger, preferably of the heat recovery ventilator (HRV) type, can complete the job of ensuring healthy air for occupants.

Beware of hidden sources of humidity!
Excess humidity resulting from occupants’ living habits is not the only culprit, however, when it comes to a home’s vulnerability to the dangers of condensation or mould proliferation.

What are the other guilty parties? Well, a large number of them are members of a dangerous gang known as water infiltrations and leaks: leaky roofs, aging exterior cladding and window openings, leaky plumbing, poorly drained or cracked foundations, etc. Any one of these shady characters can work in secret for long periods of time, creating plenty of dangerous humidity zones.

As a “home detective,” if you are well armed with a hygrometer and use it to take regular readings, you can in some cases track down any of these villains before it causes extensive damage. Once it is identified, however, mechanical devices for extracting moisture won’t be enough: you’ll have to attack the problem at its source.

*Source: Canada Mortgage and Housing Corporation (CMHC)
**Source: (“Installing a bathroom fan”)