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Why am getting so much condensation in my cathedral ceilings? And will it stop?

CPWilcox | Posted in General Questions on

I am building a home in northern Vermont and just finished the skim coat plaster on the walls and ceilings about 2 weeks ago. Since the plaster has been done I noticed water spots in arbitrary locations but mostly where the walls and ceilings meet and mostly on the south side of the house. I have 2×12 rafters, horizontal 2×4 strapping across the top and just under the plywood sheathing. Then I installed 1×3 strapping in between each rafter from top of wall to the ridge for ventilation. To this I installed 1-1/2″ poly ISO (Atlas energy shield), then a layer of 5-1/2″ and 3-1/2″ roxul for an R-value of 48.5, VT energy code is 49.
I pulled down some Sheetrock and the roxul at a wet spot and the bottom of the rigid was very wet. After they plastered the house was loaded with moisture.
Do you think it just reminants or do I have a more serious problem?
Thank you
CW in VT

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  1. ranson | | #1

    It sounds to me like you have "cut-and-cobble" R-8.4 poly-iso between your rafters, followed by R-38 of mineral wool. This is a recipe for condensation. Code would require R-25 of foam for condensation control in your climate.

    Take a look at this article:


  2. ranson | | #2

    Here's why you have condensation. By installing polyiso directly against the mineral wool, you have effectively blocked the moisture rising through the insulation from reaching the ventilation channel. Because the polyiso is so thin relative to the mineral wool, the surface of the polyiso is cold, and the moisture can condense into liquid water.


  3. GBA Editor
    Martin Holladay | | #3

    It's rare to get condensation on the interior surface of an impermeable ventilation baffle, but it can happen. It sounds like you have a perfect storm caused by a lot of construction moisture.

    Do you have any interior polyethylene? If you don't have any interior polyethylene, your roof assembly will eventually dry out, once (a) your construction moisture dissipates, and (b) warm weather arrives.

    (Yes, I know. It's going to be below zero again on Wednesday morning. But eventually, spring will come.)

    For this type of roof assembly to work, you need to have an airtight ceiling. Does your ceiling have any penetrations (for example, electrical boxes) that aren't air sealed?

    The type of roof assembly you describe also needs an interior vapor retarder (according to code). Something like MemBrain (a smart vapor retarder) or vapor retarder paint would work. Did you include an interior vapor retarder?

    -- Martin Holladay

  4. Expert Member
    Dana Dorsett | | #4

    Plaster on blue-board is probably on the order of 20-30 perms, unless the plaster was something special.

    Standard latex ceiling paint on plaster + gypsum board is nearly an order of magnitude more vapor tight, at ~5 perms, give or take a couple.

    "Vapor Barrier Latex" is an order of magnitude tighter still, at ~0.5 perms, give or take.

    So, do all the air sealing right away, but don't paint it until June, after the assembly has had some time to dry out. You'll probably do just fine with standard latex paint, but vapor barrier latex primer would be more like code, since it qualifies as a Class-II vapor retarder.

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