Dew Point and Wet Insulation

  • slide42.jpgWhen insulation is added, the building exterior becomes much colder
  • Cold air is able to hold much less moisture than warm air
  • As warm, moist air cools in the cold outer layers of the building, the water vapour it holds may condense as liquid
  • The temperature at which water condenses is called the DEW POINT

Condensation in the building envelope can reduce the effectiveness of insulation and even cause rot, peeling paint, buckled siding, spalling and efflorescence on brick and concrete walls, mould growth and other problems.

As absorbent insulation becomes wet, its thermal value drops, thereby intensifying the problem and moving the dew point toward the source of the flow of moisture (i.e. toward the interior surface of the wall).

The frost and condensation end up on the backside of the sheathing, not at the original dew point location (Building Science Insights; BSI-049: Confusion About Diffusion; by Joseph Lstiburek; 2011)