Controlling Winter Humidity: What Level of Humidity is Appropriate

If you are trying to maintain an indoor humidity level over 40% in your home during the cold winter months, there is a high probability that you are going to have indoor air quality and moisture issues. Here are some good guidelines to consider:

The Canadian Lung Association recommends the following:

• That a RH of 30 – 50% maximum for most of the year, because too much humidity can promote mould growth.
• If you notice condensation on your windows, consider replacing them with double pane windows, and getting an energy analysis of your home.
• Meanwhile, use the window condensation as an indicator that the humidifier needs to be turned down, or that your home needs more fresh air ventilation

While this information is reasonably good, it is directed more towards existing homes and does not fully reflect the trend towards tighter homes that have HRV’s and ERV’s and the move away from leaky homes that need humidifiers.

CMHC recommends that humidity be kept low enough that window condensation does not occur. In order to limit window condensation, you need to lower your relative indoor humidity. But what relative humidity level is needed to avoid condensation and frosting on cold surfaces in your home?

The Doug Tarry Homes Client Guidebook recommends maintaining winter humidity levels between 30-40% during the winter months and in a typical winter this range would work quite well. However this range was given for our area in Southwestern Ontario where we rarely go below -12°C for any extended period of time. This temperature range did not take into account -22°C (wind chill of -38°C) for days on end, such as our recent extreme cold spell in early January.

We’ve found that during extreme cold weather, clients trying to maintain their indoor humidity levels between 40-45% were having excessive moisture build up on their windows. Once we changed their humidity settings to reflect the outside air temperature, the excess moisture issue was resolved. On that early January -22°C day (with a wind chill of -38°C) we had to take the home’s relative humidity down to under 25%. Once we let the ERV run, the moisture problem went away. As the weather warmed up we were able to raise the relative humidity levels back up into the more normal range.