Renovation with interior poly vapor barrier
We are working on a renovation including an addition of an existing house in Zone 5.
Existing wall: 2×6 @ 16″ o.c. w/ fiberglass and an interior poly vapor barrier.
We would like to improve the R-value of the assembly, but will not be removing the plaster from the interior face of the majority of the walls. This means we do not have the opportunity to remove the vapor barrier.
As a result, do we lose the opportunity to add exterior polyiso, because of the resulting vapor barrier sandwich?
Any suggestion for adding exterior insulation that would have an appropriate permeability given the interior poly?
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