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Unique cathedral roof has condensation issues.

idahocottage | Posted in General Questions on


I am building a small bunkhouse. I built the walls double stud, 8″ thick total. Sheathed the walls with osb, except for one gable end, which was sheathed with plywood. The roof is a 20/12 pitch. The roof has 2×8 rafters, 24″ OC   with a layer of tyvek commercial wrap( taped seams) , which goes from the peak of the roof to the bottom of the walls, followed by a 2×3 cross hatch ventilation gap. Double lock standing seam on top of that. No sheathing. 

I initially was going to blow cellulose in the cavity, but decided that it might not be worth it to get a insulation contractor out for such a small building. I purchased r30 faced fiberglass batts. I installed them in the roof, and walls, and I turned the heat up. It got down to 15 degrees overnight. Climate zone 5.  I noticed the next morning, that where the bats touched the walls, it was wet, except where there was plywood sheathing.  I also noticed significant condensation on the underside of the Tyvek, and damp fiberglass insulation in the roof portion. 

Some of the fiberglass was damp before I installed it, so that contributed to some of the excess condensation. I am running a dehumidifier in the loft currently, and it is showing 75 degrees 40% humidity on the intake side. Are my problems due the excess humidity from the insulation? 

I thought this assembly was going to protect the insulation from wind washing, provide a really thick ventilation layer, and be a really good roof ladder as well. Unfortunately the moisture condensed on the tyvek. since it is really cold. 

What is the way out of this? This seems like a bad assembly.  I have thought of installing a smart vapor barrier on the inside of the fiberglass, but some articles I have read indicate that a vapor barrier only slows down the vapor.

Also thought of installing 2″ of eps in the cavities under the tyvek. 

It seems like that dense pack cellulose would work better in this scenario then fiberglass bats. The bats seem so light and airy against the tyvek. 

What about my walls? Seems like a pretty typical installation, but the osb is getting damp behind the insulation. I could still add exterior insulation, but its going to cut into my already small overhang. Smart vapor barrier here as well?

Any advice is appreciated. 


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  1. Expert Member
    Michael Maines | | #1

    You have discovered why it's important to have an interior vapor retarder. When using air-permeable insulation, the indoor air can circulate through the insulation and reach the sheathing, or in your case the Tyvek, which is close to the outdoor temperature and will condense water out of the air. 75°F air at 40% RH will condense on a 49°F surface. The Tyvek is vapor-permeable enough to allow the wall to dry to the exterior but it takes time; an interior vapor retarder limits how much moisture can get into the wall and allows the Tyvek to pass the small amount of moisture that will accumulate.

  2. idahocottage | | #2

    Ok, so would a smart vapor retarder be the way to go? I was thinking MemBrain . My plans were to install wood on the walls and the ceiling. I also was intrigued by the fact that the plywood is apparently quite a bit better at moving the moisture then the osb. If you are not very good at thinking things through like me, plywood looks like the safer option.

    1. Expert Member
      Michael Maines | | #3

      Membrain is ok, it's the flimsiest and most vapor-open of that class of product but also the least expensive. I spec Siga Majrex or Pro Clima Intello. In some situations, painted drywall can work just fine.

      1. BuildingFun | | #7

        I thought the new claim is that vapor barriers/retarders aren't necessary if the surface layer of drywall and all penetrations are fully air sealed? Wont drywall/primer/paint do the same as a vapor barrier anyways?

        1. Jason_K | | #9

          This is straying into nitpicking, but primed + painted drywall would be a class 2 vapor retarder (0.8 perm), not a vapor barrier (class 1 vapor retarder, < 0.1 perm).

        2. Expert Member
          Michael Maines | | #10

          Drywall painted with standard latex paint is a class 3 vapor retarder, which is compliant with IRC code requirements in certain situations but not in others. Some building science experts are ok with an airtight assembly and minimal vapor control. I'd say that most of us on the GBA forum are in favor of the safer approach that includes a variable permeance membrane, unless there is enough exterior insulation that it's not required.

  3. Jason_K | | #4

    Sorry if I missed this, but is there drywall on the ceiling and walls? Wondering if it's a question of needing a vapor barrier vs needing an air barrier...

    1. idahocottage | | #5

      No drywall. I was hoping my tyvek on the outside would be a sufficient air barrier,

      1. Jason_K | | #8

        I'm not in a place to go spelunking right now for links, but I suspect the issue is moisture from inside getting through the insulation to the tyvek, not air from the outside (sounds like the condensation is on the inside surface of the Tyvek?).

        My understanding is drywall would normally/often serve as that air barrier to prevent warm, moisture-laden air from migrating to the cooler surface of the tyvek. A smart vapor retarder could also serve as an air barrier on the warm side of the insulation

        1. Expert Member
          MALCOLM TAYLOR | | #12


          With more primary air-barriers migrating to the exterior I think a misapprehension has developed that it isn't important to stop both air and moisture from moving into walls from the interior.

          As Michael said, the air-barrier stops exfiltration, but doesn't do much to prevent condensation on the interior face of the sheathing or house-wrap.

          1. Jason_K | | #13

            Agreed, Malcolm. I should have been more clear and distinguished between the exterior air barrier to stop exfiltration and an interior one to keep warm, mosit air from exterior surfaces in the colder months.

          2. Expert Member
            MALCOLM TAYLOR | | #14


            Not at all. Your post seemed spot on to me.

      2. Expert Member
        Michael Maines | | #11

        The air barrier is important, to keep indoor air from rushing outside and vice versa. But you also need to keep indoor air, which carries more moisture than cold outdoor air, from reaching a cold surface.

  4. Deleted | | #6


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