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Community and Q&A

Vapor barrier in unvented attic living space

user-7561825 | Posted in General Questions on

We have an old two story log farmhouse in Northern Michigan that has been in the family for 100 years.  It is a simple saltbox design with just an up and over roof.  It has a full second floor (with full stairway and door, etc).  The roof assembly is unvented and the underside of the decking has been sprayed with closed cell foam.  All sides of the space up to about 4 feet high have knee walls built that were insulated with r-23 rockwool and a vapor barrier under the drywall.  The areas behind the knee walls have not been sprayed – you are looking at the inside of the top 4 feet of the log construction.  Roof assembly is blocked off from the knee walls.

The existing attic living space has a flat ceiling which causes a 6 foot high (at peak) triangular void between the attic living space ceiling and the ventless, closed cell insulated roof assembly.  

The floor is uninsulated and there is an old time vent installed in the floor that acts as a secondary heating source from the rising heat on the first floor (there is a primary heater in the second floor space as well.  The living space has windows on either end of the house that are open from late spring through early fall and there is no air conditioning in the home.  

We are now renovating a few things.  

My question is this:  Should we install a vapor barrier under the new drywall ceiling in the upstairs living space?  Or will that cause issues with the closed cell foam above that that acts as the conditioned space’s own vapor barrier?

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    The closed cell foam is highly vapor retardent, and is sufficient to protect the roof deck. If the closed cell foam is the only roof insulation in that attic, there is no point to an interior side vapor retarder, though standard latex paint would be OK.

    If there is fiber insulation between the closed cell foam and the attic space, and that fiber insulation exceeds the R-value of the closed cell foam layer a vapor retarder is called for to keep moisture from accumulating at the foam/fiber boundary, but it should NOT be a true vapor barrier such as 6 mil polyethylene, but rather a "smart" vapor barrier such as Intello Plus or Certinteed Membrain, snugged up to the fiber insulation and detailed as an air barrier.

    1. user-7561825 | | #2

      Thank youI. The closed cell foam is the only insulation above the living space on the slope and peak. Probably going to put a new dehumidifier in the living space up there to monitor and run when needed in winter months when it’s closed up tight.

      New roof is going on after the winter and I plan to add a couple inches of rigid foam above the decking to get the r-value higher and to limit the inevitable thermal transfer.

      But to my original question regarding vapor, I’ll limit myself to primer and latex paint on the slopes and flat ceilings:

      Old, old house so there are always concessions to be made and workarounds to identify it seems

  2. Expert Member
    Dana Dorsett | | #3

    >"Probably going to put a new dehumidifier in the living space up there to monitor and run when needed in winter months when it’s closed up tight."

    Winter is the time when the indoor air is naturally driest, and easy to control with ventilation rates. As long as the indoor air below the attic is below 40% RH @ 70F there wouldn't be much risk even if there were a layer of fiber insulation to deal with. But since the highly vapor retardent foam is the only insulation there won't be any significant mold risk to the attic, or any significant moisture accumulation in the roof deck from interior side moisture drives.

    >"But to my original question regarding vapor, I’ll limit myself to primer and latex paint on the slopes and flat ceilings"

    That would be the right plan- standard latex primers and paints would run 3-5 perms, and the humidity in the attic space would track the with dew point averages of the room below, even without actively ventilating the space with indoor air.

    A humidity monitor in the attic could verify whether active ventilation of the attic with conditioned space air is need or not, if you're the type who would lie awake at night wondering...

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