• Adding insulation to exterior walls increases the likelihood of damage to framing and cladding components as a result of moisture accumulation
• Older homes often did not have vapour barriers intentionally installed.
• Insulation reduces the temperature of the outer wall cavity which can lead to condensation
• If the wall leaks, added insulation can hold the water
• Exterior siding, flashing and caulking should be checked and repaired to prevent leaks

ABC - A-9.25.2.4.(3) The addition of insulation into exterior walls of existing wood-frame buildings increases the likelihood of damage to framing and cladding components as a result of moisture accumulation. Many older homes were constructed with little or no regard for protection from vapour transmission or air leakage from the interior. Adding thermal insulation will substantially reduce the temperature of the siding or sheathing in winter months, possibly leading to condensation of moisture at this location.

Defects in exterior cladding, flashing and caulking could result in rain entering the wall cavity. This moisture, if retained by the added insulation, could initiate the process of decay.

Steps should be taken therefore, to minimize these effects prior to the retrofit of any insulation. Any openings in walls that could permit leakage of interior heated air into the wall cavity should be sealed. The inside surface should be coated with a low-permeability paint to reduce moisture transfer by diffusion. Finally, the exterior siding, flashing and caulking should be checked and repaired if necessary to prevent rain penetration.