Too Much Humidity?

At times a house can have too much humidity

When outdoor temperatures are cold you want to lower the indoor humidity

For instance at -20 celcius the maximum  relative humidity level with Dual Glaze windows is about 25% before condensation. Dropping to -30 means a maximum humidity of 15%

To keep track, it is a good idea to have a hygrometer  which tells you the humidity and temperature

To reduce the humidity you can increase the air temperature or you can dilute it by adding dryer outdoor air.

Use the ventilation fan switch under the thermostat

This fan is designed to remove air, it turn on the furnace which brings dryer air in, and that air is mixed with the house air to reduce humidity.

Watch the humidity and run the fan when it gets too high. Buy a hygrometer ( left) to track relative humidity levels. 

If  your humidity does not reduce you may need to check the air volume of the ventilation system. At times these are undersized and are the cause of high humidity.


You can record how the home is operated by placing a data logger in the house for several weeks. You can place one outside to see the exterior temperatures as well. 

It can record temperature, relative humidity and dew point.

This can be useful in helping a homeowner see where they can reduce humidity levels through ventilation.

Below is a sample graph showing the operational choices for this home. You can see where the levels spike each day due to showers and when they drop when everyone leaves the house. 

The dew point is at almost 10 degrees celcius - in other words any surface colder than this temperature will condense.

Humidity is the measurement of moisture in the air in a gas  vapour form

There is Relative Humidity and Absolute Humidity


Warmer air holds more humidity (vapour) than cool air. This does not change the amount of vapour in the air (absolute humidity), warm air can hold more vapour so it is relative to the air temperature. If your Relative Humidity is too high, increase the temperature, which will allow the air to hold more humidity before it deposits it in the form of condensation


The amount of vapour in the air does not change as the temperature changes. It is absolute, until you add more humidity by evaporation or other sources or we reduce the humidity by removing or diluting with dryer air.


The maximum amount of vapour in the air at a given temperature before it cannot hold any more humidity and it turns to liquid.

If your home is at 20 degrees Celcius and 30% Relative humidity, the dew point is around 3-4 degrees celcius. In other words surfaces that are colder than 3 - 4 degrees will condense.

As you increase the temperature the relative humidity is reduced.

However here is the WARNING - if you reduce the temperature overnight to a cooler temperature, i.e. 15 degrees celcius, you will have more moisture on the windows in the morning because the air cannot hold that moisture and because the air is not keeping the glass warm.


What about Hardwood Flooring?

measures humidity
Since 1986

Houses built to Building Code are designed for a maximum interior Relative Humidity at 21 Celcius of 35% during winter. 
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