Replies

1. 
   maine_tyler | Feb 02, 2021 03:46pm | #1

   Dave, this sounds pretty much like what's being discussed in this other recent Q&A:
   https://www.greenbuildingadvisor.com/question/condensation-issues-with-flash-and-batt

   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 | Feb 02, 2021 04:16pm | #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 | Feb 02, 2021 04:34pm | #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 | Feb 02, 2021 04:51pm | #4

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