Moisture Problems in Homes

Home Inspector with Electrospec Home Inspection Services

Why does condensation occur on interior surfaces?

Condensation can occur on any surface that is below the "dew point" of the air with which it is in contact. The "dew point" is the temperature at which condensation will occur for a given humidity level.


Inside many homes in winter, the inside of the windows are the coolest surfaces. Condensation will typically appear there before it appears on other surfaces. Condensation is less likely to occur on interior walls, because they are typically warmer than "dew point."

Occasionally however, condensation may occur on cold spots such as nail heads, in corners of outside walls where insulation is reduced, or in confined spaces where circulation of warm room air is restricted. In extreme cases, condensation may lead to mildew and the growth of mould.




What to do

To reduce or eliminate excessive condensation, the humidity level must be decreased or ventilation increased.


To reduce humidity levels:

  • Turn down or turn off humidifiers
  • Ensure clothes dryers and bath fans are vented to the exterior
  • Do not hang clothes to dry inside the house
  • If necessary operate a dehumidifier. These low-priced appliances are capable of removing 7.5 to 20 litres of water from the air per day!
  • Open blinds and curtains whenever possible
  • Avoid heavy draperies


To increase ventilation:

  • Operate bath fans when showering
  • Crack a window or operate exhaust fans as soon as condensation appears, until the source of the humidity is remedied.
  • Operate furnace with fan switched on for continuous flow.
  • Have range hoods vented to exterior.
  • In extreme cases, consider installation of a mechanical air exchanger or HRV (Heat Recovery Ventilator.)


If the obvious solutions don't eliminate the problem:

  • Before investing in a dehumidifier or an HRV, check the structure for other possible moisture sources.
  • Ensure the appropriate insulation and moisture barriers are installed in the attic, basement or crawlspace.
  • Check for seepage in basement or crawlspace.
  • If you have a damp crawlspace, especially with an earthen floor, cover the floor with plastic.
  • A sump pump can be used to remove moisture in extreme cases.
  • Ensure eaves troughs and lot drainage are effectively removing storm water and snow melt away from foundations.


Attic ventilation is normally provided by vent openings in the soffit or gables, and the roof. The total area of ventilation should equal one square foot for every 200 to 300 square of ceiling area. Not less than 50% of the attic ventilation should be through the soffit or gables. In turn, ot more than 50% should be through the roof.


Moisture Produced by Household Actvities

  • Family of 4 Cooking 3 Meals (per day) 1.0 litre
  • Running dishwasher 0.5 litre
  • One shower 2.5 litres
  • One bath 1.0 litre
  • Washing clothes (per week) 2.0 litres
  • Unvented clothes dryer (one week) 13.0 litres
  • Floor mopping (per 100 sq ft) 1.5 litre
  • Four occupants (per day) 6.0 litre


For more information:

If you have questions about this article, or about other maintenance and home improvement topics, call or send text to 613-391-8515 or visit us at


More about air exchangers:

Venmar Systems:

Lifebreath Systems:


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