lara ramsey
1/11/2012
We have a new standing seam roof on a new construction. We used englert's "nailable underlayment" on top of zip board or green board which it taped of course. We have cathedral ceilings, if that matters. No insulation is installed yet. We are paying for this house as we go so the roof has been on the house for several months as we get ready for insulation. We just noticed large damp horizontal strips all over the underside of the zip board (looking up at the ceiling from inside the house) and the damp strips are even outside on the zip board on the covered porch. Is it possible that we have 2 vapor barriers and that is a problem? Or is excess moisture from condensation trapped under the vapor barrier and the roof cannot breath causing wetness to seep through the zip board? I am afraid that insulating will only hide the problem not dry it out and our roof will rot, not to mention mold. I am afraid to depend on "positive air flow" to dry it out. Is it not better for the moisture not to seep through in the first place? Thank you so much for your help.
Todd Miller
Classic Products, Inc.
1/11/2012
Lara, sounds like a great place you're building! Once, from the inside out, you get up paint, drywall, vapor barrier, and insulation, you should keep moisture from reaching and condensing on the bottom side of the roof deck. I'd love to see ventilation in there above the insulation but if that is not feasible, as long as you have a complete vapor barrier behind your drywall and a good amount of insulation, things should be okay. Make sure that all moisture sources inside the home are vented to the outside, such as bathroom and kitchen fans, dryer exhaust, etc. I am also not a fan of ventless gas stoves inside. They generate a lot of moisture that has to go someplace!
Todd Miller
Classic Products, Inc.
1/12/2012
Hi there. I will try to answer your numerated questions. 1) Ultimately, yes, that is the risk. Now, that said, until construction is complete, other odd things can occur but ultimately and really I think even at present, that is the risk. 2) This is not normally an issue. Condensation does not often occur there because there is no temperature differential in completed construction. As long as the underlayment on the decking is well sealed and doing its job, this is not an issue. 3) I would say this is because the joints are the coldest areas, so moisture from inside condenses there first.
Info @windowsonwashington.net
An informed customer is our best customer.
1/12/2012
The fact that this issue is just beginning to manifest itself is a clear indication of the condensation issue with us being in the middle of the heating season now. If the roof/ceiling will not be buttoned up until spring/summer, you are going to want to put up some sort of temporary vapor barrier in this application to prevent the furthering of this issue. I too would like to see a vent channel if possible, however, if you air barrier details are right on target and tight, you should not have any issues. Perfect scenario is to have insulation above the deck to thermally uncouple the roof structure as Todd highlights above but that is no longer an option here.

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