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Keeping It Dry Inside

How much humidity is too much?

Keeping It Dry Inside

In the same way water is an enemy of the structure outside, so moisture is the enemy of healthy living inside.

Moisture inside the home is measured as relative humidity. 
Humidity indoors higher than 40% in winter and 60% in summer is considered excessive. High humidity is the main villan in most humidity related problems in the homes we inspect.

While lower levels of humidity are desireable, humans require about 25% Relative Humidity to be comfortable. In addition certain building materials such as hardwood flooring require a somewhat higher RH to avoid cracking. 

In the house as system approach, the entire structure is assessed for humidity problems. Climate, high occupancy loads, unusual occupant habits, too many plants, or especially tight construction can all lead to humidity problems. 

Solutions can range from continuous fan operation creating positive pressure in the home to the use of dehumidifier units and increased ventilation where applicable.

Climate and time of year/season will dictate the response to different conditions.

In newer homes, building materials and concrete may not have dried out completely and may have been subject to weather during construction or for a period of time afterward. Alternative materials such as moisture resistant gypsum should be considered. 

Avoiding the close-in of wet assemblies will avoid mold or material degradation. Checking the moisture content of framing assemblies can help the home owner avoid excessive wood shrinkage later which causes damage to walls and doorways.

Why not just seal up the house?

In fact houses must breathe and water vapour must be able to  move into the structure, be conditioned (i.e. warmed, cooled, dried or moisture added to it) and it must be able to escape without damaging the structure. The occupants must have a healthy and comfortable environment. 

Humidity management always walks a fine line between too much and too litte. Modern materials and awareness of the issues means we can enjoy our homes and not be parched or dripping.

Check out this video of water wicking up a brick.


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Doug Bowman

Certified home inspector, condominium document analyst and Realtor. More than 10 years experience in general real estate.

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