Dew Point

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Added insulation and a new airtight layer on the exterior of a building runs the risk of changing the DEW POINT in framed assemblies. Consult with an Energy Advisor to make sure this condition is handled properly!

DEW POINT is the temperature where moisture vapour condenses back into a liquid. Many times, the dew point happens on the cold surface of a material inside a built assembly, like the back side of exterior sheathing.

Here's a graphic example of condensation happening in a wall:

Inside temp / humidity =20 deg. C and 50%. Outside temp / humidity = -5 deg. C and 80%

The diagram above shows a typical wood frame construction section view with fairly basic layers:

  1. Gypsum drywall,
  2. Poly vapour barrier,
  3. Insulation in stud cavities @ 24" o.c.,
  4. OSB,
  5. House wrap,
  6. Back vented siding

Note that around layer #4, 5 and 6, our assembly is experiencing moisture as the dew point is on the cold backside of the sheathing. This is happening because the temperature at that point allows moisture vapour to change into condensation. The moisture can be carried by air leakage and vapour diffusion through the assembly.

The water droplets in the diagram represent condensation. The point in temperature where moisture laden air (vapour) changes to condensation (water) is called the Dew Point.


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