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Zip R12 with Class III Vapor Retarder in Portland, ME Zone 6

brad17 | Posted in General Questions on

Hey folks,
For our house in Portland ME, Zone 6, here is the wall assembly we are planning:
Cedar Clapboard Siding (unfinished)
Vertical Rainscreen
Zip R12
R23 Rockwool in 2×6 cavity
Sheetrock with latex paint

We are assuming we don’t need a vapor barrier on the inside of our wall given that we are above the R11.25 necessary to keep the sheathing dry. Is this sound?


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  1. maxwell_mcgee | | #1

    I don't have a response to share but will bump your question as I'm curious to hear the replies also!

  2. brad17 | | #2


  3. Expert Member
    KOHTA UENO | | #3

    Yep, this meets the requirements shown in Table 2, derived from Table R 702.1 2018 International Residential Code. Okay, technically you're at 34%/66% instead of 35%/65%... but that doesn't really matter.

    BSI-100: Hybrid Assemblies

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