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Location of Vapor Barrier

user-1020504 | Posted in General Questions on

I am insulating under a 1930’s house on brick piers open around the perimeter located on the Eastern Shore of Maryland. The floor is constructed with 3/4″ T&G flooring nailed to joist.  For insulation, there is currently R25 Rockwool insulation which is exposed to the elements below. There currently is no vapor barrier installed. I am planing on adding another layer of rigid foam insulation to protect the rockwool and to add another layer of R-value. 

I am concerned about moisture being trapped between the layers of insulation. Any recommendations would be appreciated.

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  1. Expert Member


    The rigid insulation is much more effective placed continuously on the underside of the joists where it can reduce thermal bridging. It should be taped and sealed as an air-barrier, and covered with something like plywood to protect it. The open area underneath would benefit from a layer of poly to stop moisture rising from the soil below.

    1. user-1020504 | | #2

      Thanks, unfortunately the rockwool insulation is already in place. I would prefer not to have to remove it to install the rigid installation adjacent to the underside of the T&G flooring. Just wondering if there is another option.

      1. Expert Member
        MALCOLM TAYLOR | | #4


        You can leave the rock wool where it is, or pull it down to the bottom of the joist cavities. The rigid foam isn't in the cavities at all, it's on the bottom of the joists as a continuous layer.

  2. user-2310254 | | #3


    Malcolm is suggesting this sort of solution:

    The foam is nailed to the bottom of the floor joists and then covered with a plywood protective layer.

  3. Expert Member
    NICK KEENAN | | #5

    This is a good article by Joe Lstiburek:

    You are geographically close to the house with the "icky crawlspace" that he profiles.

    He also recommends continuous foam below the joists.

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