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Conensation on sheathing

CanadianExpy | Posted in General Questions on


I have built my house based on the great information on this site, so now that I am insulating my walls, I have a question. 
My walls are as follows, exterior vinyl siding, 3/4 furring strips, Tyvek, 2 layers of 3″ poly iso Insulation (6′ total), Advantech OSB, 2×4 studs with R14 Rockwool, CertainTeed Membrain (smart vapor barrier), Drywall, Latex paint.  I noticed that a couple of spots under the Rockwool had condensation on the OSB, the membrane had been partly installed in this section, but not full attached, none of the other stud bays without Memebrain had moisture.  The house is currently heat at 67F and outside temp is roughly 30F.

Should I be worried about this?

I know that there may be extra moisture in the house being a new build, and no ERV working yet.    

The building inspector is requiring the vapor barrier.  


Climate Zone 6 (Ontario)

Thanks Dave

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  1. maine_tyler | | #1

    Dave, this sounds pretty much like what's being discussed in this other recent Q&A:

    But to feel confident in the why, you could measure the interior RH and take a temperature reading of the sheathing. It's a little surprising to me given the 3 inches of polyiso (edit: 6" not 3") and only R14 rockwool, leading me to believe your RH is quite high (you can do the actual calc), or your wall is colder than it should be due to an air leak or something.

    That would be my only concern: if your sheathing is colder due to some imperfection in your air/thermal layer. Otherwise I suspect you just need to get the membrain and drywall tightened up.

    1. CanadianExpy | | #2

      Thanks for the Reply , the polyiso is 6" thick. As for the air layer, I had a blower door test recently and it came back at 0.81 ACH50 which I think is pretty good. I do suspect the RH is high, but as I said the visual of seeing moisture on the intreior sheathing was a little shocking, worrysome ( alought only a small amount) . I did a temp measurement on the sheathing with a gun( not sure how acurate) but it showed 55F with surrounding wall with rockwool at 63F

      1. Expert Member
        Michael Maines | | #3

        Dave, for 65°F air to condense on a 55° surface, the relative humidity would be 72%, or at 70°F it would be 60% RH--not unheard of but higher than normal.

        With 70% of your R-value on the exterior, the sheathing should be about 70% of the difference between the indoor and outdoor temperatures, or about 56°F--so your gun is likely accurate.

        Have you ruled out bulk water entry, from bad flashing or other sources?

        I'd want to measure the indoor RH and also set up a dehumidifier to dry things out.

        1. CanadianExpy | | #4

          Thanks for the reply, I think I will try and measure RH and bring out a dehumidifier

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