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Community and Q&A

Moisture problems with vapor barrier – Part 1

Tony Moore | Posted in GBA Pro Help on

I am a home owner building a 2,000 sq ft addition to our home 50 mls. north of NYC, on a small mountain in the Hudson Valley, so it’s colder up here in the winter and less humid than the river valley in the summer. This is a wood frame two story structure without central AC built on an exposed concrete slab floor. We are at finished rough-in stage. This winter, the radiant heat in the slab was first turned on, resulting in a lot of moisture being evaporated from the concrete into the building. Moisture started to drip from between the R38 paper backed fiberglass on the second floor cathedral ceiling and running down the interior plastic sheet vapor barrier. Upon investigation I discovered that the roof sheathing, Styrofoam air vents and top 2″ of insulation were soaking wet, with the bays with least venting around skylights and roof vent pipes being even wetter. The ceiling is vented with typical soffit vents and roof ridge vent. Skylights and pipes have Styrofoam bridging vents around them with 1″ holes drilled in the adjacent rafters. We have now drilled 6 holes for more venting.

The12″ roof sandwich is shingle, roof paper, 1/2″ plywood sheathing, Styrofoam air vents, R38 paper backed fiberglass insulation, plastic sheet vapor barrier and then it will be 1/2″ drywall. The 6″ exterior walls are 5/8″ T1-11, or Stucco on the outside, Tyvec, 1/2″ sheathing, R19 paper backed fiberglass, plastic vapor barrier then it will be 1/2″ drywall.

Now it is above freezing I have opened up the ceiling, drilled more holes in the rafters and provided better rigid foam sheet venting across the skylight headers, called back the roofer to re-do the ridge vent and dried it out. WILL I STILL HAVE MOISTURE PROBLEMS WHEN THIS IS DRY WALLED AND CLOSED UP? Tony

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Replies

  1. David Meiland | | #1

    You can't combat this problem with roof ventilation. You need to get an accurate hygrometer and a commercial dehumidifier. Yes it will improve after drywall if your drywall is airtight and not full of can lights and other holes... AND you monitor and control the indoor humidity.

  2. Bill Bradbury | | #2

    You shouldn't have a problem if you detail your air barrier better. Even a little bit of that hi humidity interior air contacting the cold sheathing will condense on the sheathing. You probably want to check for mold on the interior surface of the sheathing.
    I was reminded of this the other day when I fed wires into a plastic conduit that went from the living room to the vented attic, water poured out and not just a little. The air traveling up the 1/2" conduit condensed when it got to the cold attic and ran back down onto the clients hardwood floor. Doh!

  3. Tony Moore | | #3

    Is there a problem having paper backed insulation and a plastic vapor barrier on ceiling and walls? The insulation lets through draft because it's impossible to completely seal it up, no matter how careful one is, while the plastic is a very good air barrier. When I observed moisture it condensed on the cold roof sheathing and vent ducts and not on the plastic or paper. Tony

  4. GBA Editor
    Martin Holladay | | #4

    Tony,
    You are describing the type of cathedral ceiling assembly that has been causing condensation problems for decades. I describe the failures and the solutions here: How to Build an Insulated Cathedral Ceiling.

    You have chosen the worst possible insulation type -- fiberglass batts -- for this roof assembly. Fiberglass batts will only work if you have an airtight baffle above the fiberglass batts. Ordinary styrofoam baffles won't work in this application. If you insist on using fiberglass batts, you need to install a type of baffle that is completely airtight.

    Some people report success with Accuvents, installed with tape at the joints to make the product airtight. But if I were installing fiberglass batts, I'd install site-made baffles made out of plywood or fiberboard sheathing, with all seams carefully sealed with caulk or high quality tape.

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