On the proper ventilation of condos to avoid condensation problems

In a cold climate like Canada the important variations of temperature and relative humidity can cause the appearance of condensation inside of condos. When the air is saturated with humidity, the excess amount of humidity will start to condensate (i.e. transforms itself back into water). This can create numerous problems to the building, such as the rotting of materials and most importantly the appearance of mold. Mold spores can degrade the air inside a condo to the point where they can become a health hazard. This memo outlines how to prevent condensation on a daily basis.

What is a “normal” relative humidity level (RH)?

To have both a healthy building and comfortable and healthy people, the relative humidity level should be between 30% and 60%.
In the summer, a normal humidity level inside a condo can be between 50% and 60% RH.
In the winter, a normal humidity level inside a condo can be between 20% and 30% RH.
Below 20% RH, the air is generally too dry to feel comfortable, but the risk of condensation is nonexistent. Above 60% RH, however, the risk of condensation is high and it is not recommended.

Water run-off on the windows is the first visible sign that there is excess humidity in the air.

How to tell there is too much humidity in the air?

A high level of humidity in the air will tend to form water on cold surfaces such as windows. This is what we call condensation.

A severe condensation problem can lead to the growth of black mold inside of exterior walls. In such a case, the walls must be rebuilt.
Have I scared you enough?

Living “tips” to prevent condensation

Bathing and Showering: it’s an important source of humidity production. Turn the ceiling extract fan on every time you take a bath or shower to exhaust the hot and humid air outside. Leave the door closed for a few minutes after the bath or shower, with the fan running. If there is no fan, open the window.

Drying Clothes: always use an electric dryer vented to the outside. Verify that your clothes dryer conduit is clear and functioning properly. If the clothing takes too much time to dry, have your dryer duct cleaned. It may seem more economical to dry clothes on a clothes dryer inside the apartment, but from a condensation problem point-of-view, it is really a bad idea.

Cooking: when boiling water or slow cooking any food that evaporates water, turn on the stove hood at low speed to evacuate the humidity to the outside. Verify that your kitchen hood conduit is clear and functioning properly. If not, remove kitchen hood and clean duct.

Fresh Air: humans need to have a constant supply of oxygen to live and feel comfortable. Most condo units built before 2005 do not have a proper mechanical supply of fresh air and the air inside them can become stuffy. It’s always a good idea to regularly open a window to allow the inside air to be renewed by fresh air from the outside. By doing so, you will also allow excess humidity to escape to the outside.

Know your humidity level by purchasing an electronic Hygrometer (about 20 $)

An electronic Hygrometer looks like this. Most of them also show the temperature.

Jean-François Lavigne

Associate Architect M.Arch O.A.Q. PA LEED®