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Old house wall assembly with no insulation

alpinesummits | Posted in General Questions on

Hello, I am an owner-builder in the Columbia Valley in BC. I am doing a deep retro fit of a 1950’s, 1 and half story house. The wall assembly is considered R12 but for all practical purposes they have no insulation.

The walls consist of 2×4 construction with 3/4 inch planks on both the interior and exterior walls with some cardboard and t-shirts mixed in (opening up the walls has been challenging). I want to put a blanket of xps foam on the exterior instead of ripping the 2 layers of drywall and plank out on the interior to insulate my 2×4 walls (plus my engineer doesn’t want me ripping the plank out as it is holding the house together).

I want to go with xps foam as it is lighter then the ‘safe and sound’, which my engineer also recommends.

Question: If I have R12 walls with no air barrier on the inside but a weather barrier and a single layer  of 4″(or 2 layers of 2″)  xps on the exterior, will I have enough permeability to keep the dew point on the exterior of the assembly? 

Thanks in advance

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