Do you need a vapor barrier with cellulose insulation?


Do I need a standard 6 mil poly vapor barrier with dense cellulose exterior wall?

jeff Chormann May 1, 2020Last updated: Feb. 28, 2021

I'm renovating a 1915 bungalow in masachusetts with no  existing wall insulation. I've opened the wall cavity and plan to use a dense pack cellulose to achieve R-14.  The exterior wall is plank sheathing with paper wrap, followed by shingles, then 1/2 celotex foam board and vinyl siding installed in the 1980's.  No rot or leakage is visible from the inside.  My question is if I install a poly barrier over the cellulose on the inside under the drywall,  will I then cause vapor to be trapped between the outside vinyl /foam board and poly barrier.  The foil faced foam board is  not completly tight against the underlying shingles and I assume does allow for some air movement underneath it and  the vinyl siding. 


Responses (2)

jeff Chormann June 2, 2020, 8:44 a.m.

Gregg- Thanks, why do you favor rockwool?  Due to its being less prone to moisture issues?  I agree with the idea of a breathable  membrane on the inside where I will be putting up blueboard and plaster skim coat.