Class I Vapor Retarder in Cold Climate
As part of a ‘down to stud’ remodel project on a 1.5 story brick Cape-cod in Zone 5, I am re-insulating. I am furring out existing study bays to accept R23 Rockwool, and and planned to apply continuous interior 1″ foil faced polyiso for additional R value and to reduce thermal bridging. The polyiso would be applied to all surfaces that would be drywalled, including the ceiling, prior to reframing interior walls, effectively creating a seam-taped polyiso ‘hat’ for the house.
Reviewing the data sheets for Dow Super TUFF-R, which is the product I planned to use, I see the vapor permeance is .03, a Class I vapor retarder.
I was under the impression that Class I vapor retarders, such as the dreaded polyethylene, should be avoided in Climate Zone 5 due to in wall condensation concerns.
Is this risk mitigated with polyiso because of the inherent R value?
Are there any other concerns with this approach? The 1st floor of the home has not been reinsulated, so I do wonder if the lack of balance between a lower R value leaky 1st floor, and a higher R value, seam taped air tight 2nd floor could cause any issues?
Thank you for your time.
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