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Community and Q&A

Vapor barriers and Insulation

TheSkycrane | Posted in Energy Efficiency and Durability on

Hello again! I have a question for everyone. I have spent the last week researching this before asking and here I find myself very confused. I live in zone 2 central Florida, my home is cmu block 8” thick. I have r board 3/4” thick installed on the inside of the block wall. I have furring installed over that. All seams are taped and 1/2 drywall will go over this.  Now, I want to put the same r board 3/4” on the exterior of my block. I am putting hardie panels  up as board and batten siding. The hardie panels are being installed on 1/4 furring that is t nailed to the block. Do I need a vapor barrier too? The home has a unvented roof with open cell spray foam on the back side of the roof sheathing which is osb. Then tiger paw, purloins 3/4” and metal roof on that. I have fresh air supply’s for my hvac. Any issue with the foam board on both sides of the block?? Can’t seem to find an answer on that. 

thank you

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  1. climbing_carpenter | | #1

    More questions than answers- it seems to me, so far, you’re going to have moisture issues.

    What kind of R board? Different types of foam have different levels of vapor permeability.

    You don’t have much insulation, even for your climate zone.

    What will serve as your continuous air barrier?

    Any leakage from warmer, moist air will cause condensation against the block. Winter and summer months would be opposite.

    A hot roof with open cell foam sounds sketchy at best. This assembly requires carefully detailed Intello Plus on the conditioned side or roof side venting.

    1. TheSkycrane | | #2

      I know not much insulation that’s why I’m adding to the exterior. Wouldn’t the drywall be my air barrier? Here most homes are built this way with open cell on the roof. R board is sika pro brand.

  2. Expert Member
    Akos | | #3

    Vapor barriers are generally a bad idea in warm climate. You also already have a layer for foam which depending on the facing is a vapor barrier.

    I can't see there being issues with your wall provided there is no bulk water that can make it to your CMU, as long as the flashing details are good, you should not have an issue.

    If you are looking for a piece of mind, it doesn't take long to set the wall up here:

    They have examples, the default one is a block wall. It has a nice diagram that shows moisture profile through the assembly, you can run it for summer and winter conditions to see what happens. You'll have to do a bit of imperial to metric conversion though.

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