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 an impermeable 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.

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.

Why is this happening?

Stucco finish detaching from conc. block structure due to moisture infiltration. Eastern Ontario.

Somehow, water vapour carried in the air is finding its way from the inside of the building and into the built assembly. This may be happening because of poor air sealing and poor management of interior humidity. This can be made worse by not having enough insulation and unchecked thermal bridges.

In wood construction, moisture infiltration may cause mould, rot or deterioration of materials.

In masonry construction, condensation inside an assembly may show up as spalling brick or stone, or delaminating layers like stucco and parging. You may also see white stains where moisture passes through masonry - this is called efflorescence.