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snow and condensation

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Guest User
12/12/2002
We are in the final stages of building a 20 x 52 addition onto our home. We installed an r- panel metal roof, 5/12 pitch , 14' rafters, directly on 2x4 purlins. We have full soffitt and ridge ventilation as well as 3 approx 32x42" windows in the attic space which is about 8" high to the ridge. Throughout the building process (over a year) I have kept a close eye on the underside of the roof and have not observed any leakage/moisture/condensation problems until last week when we had , for here in eastern VA , a fairly good snow storm that left a couple of inches of snow on the roof. The following day there was a significant amout of water accumulating and dripping from all the entire length of all the purlins. I suspect condensation; we took great care in not overtightening the screws(with neoprene washers) and am pretty confident its not leakage. The windows in the attic were all shut at the time. My question is : can I safely install a vapor barrier like tipar or tyvek directly to the undersides of the rafters and will that help? The attic floor (the living space ceiling) will be fully insulated with a vapor barrier. Thanks for your help.
Todd Miller
Isaiah Industries, Inc.
12/18/2002
Is there any reason that there is unusually high moisture in the building right now? Have you made sure that the ridge and soffit vents are not blocked by insulation or anything? If this is condensation, I would not feel too confident that a vapor barrier will correct it. The problem is that it is tough to make a vapor barrier be 100% effective. If you can, certainly, then that is a good option and it should work. I would certainly look also at flashings and things to make sure that there is no chance for roof leaks.
Allan Reid
Dura-Loc Roofing Systems, Inc.
12/18/2002
First it sounds like you have not completed the inside ceiling assembly and you have warm moist air escaping into the attic with the cold weather, condensating and dripping. You need an air barrier installed on the ceiling side such as a 6 mil visquene then the insulation. This will stop the condensation from this source. The condensation may have been agrevated with plugged ridge vents fron the snow and will clear itself out once it vents. As to the underside it is a poor detail to install anyl roof without underlayment and metal is amplifying your problems. You do not install a vapour barrier under roofing, you need a moisture barrier. Triflex 30 is a good material however Tyvec is not designed for this. Fix the other issues first and watch it. If need be you could tape some between the trusses or cutt some thin white styrofoam between the rafters.
Todd Miller
Isaiah Industries, Inc.
12/19/2002
Al's answer is excellent and I agree entirely.
Guest User
12/19/2002
Thanks for te advice...I think your analysis is right on. Wish we'd known about you guys sooner, but I'm sure we'll be ok.
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