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Moisture problems with Vapor Barrier – Part 2 – Another question

H7RM87gwJK | Posted in GBA Pro Help on

A follow-up question:
Is there a problem having paper backed insulation and a plastic vapor barrier on walls and ceiling? When I observed moisture problems it was not condensed on the paper or plastic, but on the coldest surface i.e. the roof sheathing and Styrofoam air ducts. Some have said the plastic is the problem and the walls/ceiling need to breathe while my architect says install the plastic for air seal and thermal efficiency. Confused! What’s correct?

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Replies

  1. davidmeiland | | #1

    Tony, the answer is climate-dependent, plastic sheeting is only used in the coldest climates at this point, and possibly in places where code enforcement has not kept up with the industry. Since this is a new thread, and others may not read the first, what is your zip code?

  2. jklingel | | #2

    Is your house air sealed well?

  3. H7RM87gwJK | | #3

    For more details of the problem please read the first thread "Moisture problems with vapor barrier". This is zone 5A 50 mls north of NYC, 10516 zip, on top of a small mountain in the Hudson Highlands, so it's colder in winter. The house has part stucco, part T1-11, Tyvec with taped seams, plywood sheathing, 6" studs, R19 on walls, R38 on a cathedral ceiling with typical soffit vents, Styrofoam vent ducts and Cobra ridge vent. I was very careful in installing the paper backed fiberglass rolls and batting + taping all the interior plastic vapor barrier seams. It's not dry walled yet. Thank you, I appreciate your responses.

  4. GBA Editor
    Martin Holladay | | #4

    Tony,
    People interested in high-quality insulation jobs don't usually choose fiberglass batts. Fiberglass batts can work without problems, but only if the builder pays close attention to air sealing. That usually requires a blower door to confirm a low air leakage rate, and to plug any leaks.

    Polyethylene is not usually recommended in your climate any more, but it can work if you have verified that the polyethylene air barrier has no leaks. This requires airtight electrical boxes and lots of Tremco acoustical sealant or high quality tape.

    If you are seeing signs of problems, you need diagnostics with a blower door, as well as efforts to lower the indoor humidity level. It's possible that everything will be fine once the construction moisture dries out, but it's hard to tell without more information.

  5. stuccofirst | | #5

    Tony,
    Condensation is due to a temperature difference with warm air meeting cold air, and not enough ventilation where the condensation is occurring. I would assume you have heat leaking to the surface where the condensation is building. I'm in Newburgh, not far from you. you can call me if you like. 845.863.5125
    Shane BPI Building analyst/ Envelope

  6. GBA Editor
    Martin Holladay | | #6

    Shane,
    You wrote, "I would assume you have heat leaking to the surface where the condensation is building."

    You may be right, but the problem is not a heat leak. (Leaking heat would warm up potential condensing surfaces, which reduces rather than increases the chance of condensation.)

    The problem is probably an air leak, not a heat leak. Escaping indoor air contains a high moisture level, and the moisture in the air is finding a cold surface in the wall assembly or roof assembly.

  7. H7RM87gwJK | | #7

    Thank you for your responses.

    I must admit to being confused due to what would appear to be contradictory advise from different sources. I've spent a lot of time on GBA reading articles and threads. I've read that nowhere in America should poly be used on a wall or ceiling assembly, that it might work and be OK 90% of the time, that folks have used it for years without problems, that walls have to breathe or be entirely air tight -- or that it could work if entirely sealed. What about screw holes and picture holes etc. But then I don't have complete faith in a contractor making an air tight drywall envelope either. The drywaller's who did a 1,200 sq ft building on my property never used a caulking gun, but just slapped up the sheetrock. How would one do this on a cathedral ceiling with the drywall sheets spanning joist bays at the top and bottom of the roof pitch? On walls isn't paper backed fiberglass more permeable than plastic, or is the idea that this allows for moisture from inside air to eventually breathe inside instead of being trapped behind the plastic?

    The moisture problems appeared this winter with new construction and slab evaporation, which has now more or less dried, but I need to rectify any future problems before going forward with the drywall. Plastic on or plastic off? In either case I will do my best to seal it. What does MemBrain do? Does it breathe, similar to Tyvec on the exterior and should it be used with paper backed fiberglass? By the way, I have stucco sections and T1-11 sheathing sections on the exterior and the space will be cooled with Mitsubishi A/C wall mounted units in the summer. I've read that plastic shouldn't be used with A/C, or with stucco. Is this correct?? The whole cathedral ceiling assembly is another issue/headache, that needs to be addressed! Thanks for the input. Tony

  8. GBA Editor
    Martin Holladay | | #8

    Tony,
    Q. "I've read that folks have used poly for years without problems."

    A. Where did your read that? I disagree. The statement would only be true if it were qualified; for example, "In cold-climate houses without air conditioning, interior poly has been used for years, and most houses haven't developed problems attributable to the polyethylene."

    Q. "I've read that that walls have to breathe."

    A. I disagree -- see this link: ‘Walls Need to Breathe’ and 9 Other Green Building Myths.

    Q. "What about screw holes and picture holes etc.?"

    A. In most cases, drywall screws or picture-hanging nails do not create significant air leaks into walls that follow the Airtight Drywall Approach or walls using interior polyethylene air barriers.

    Q. "I don't have complete faith in a contractor making an air tight drywall envelope either."

    A. Air leakage specs should be called for in the specifications and verified with a blower door.

    Q. "The drywallers who did a 1,200 sq ft building on my property never used a caulking gun, but just slapped up the sheetrock."

    A. That's too bad. However, the question arises: did the plans call for an air barrier? Was the air barrier shown on the plans? After all, a builder can't include something that the designer forgot to indicate.

    Q. "How would one do this on a cathedral ceiling with the drywall sheets spanning joist bays at the top and bottom of the roof pitch?"

    A. It's possible to use ceiling drywall as your air barrier, but you have to think through the details. At all transitions and changes of plane, you need some detail or method to maintain the integrity of your air barrier.

    Q. "On walls isn't paper backed fiberglass more permeable than plastic?"

    A. Yes.

    Q. "Is the idea that this allows for moisture from inside air to eventually breathe inside instead of being trapped behind the plastic?"

    A. Yes, kraft facing allows more drying towards the interior than poly.

    Q. "The moisture problems appeared this winter with new construction and slab evaporation, which has now more or less dried, but I need to rectify any future problems before going forward with the drywall. Plastic on or plastic off?"

    A. If you leave the plastic on, you need to be sure that it is fairly airtight, which requires airtight electrical boxes. If the poly isn't an air barrier, I would take it off.

    Q. "What does MemBrain do?"

    A. MemBrain is a smart vapor retarder with variable permeance.

    Q. "Does it breathe, similar to Tyvek?"

    A. No. The permeance of Tyvek is fixed; the permeance of MemBrain varies with the relative humidity. As the RH rises, MemBrain becomes more permeable.

    Q. "Should MemBrain be used with paper backed fiberglass?"

    A. It can if you want, although most builders would use unfaced fiberglass (or better yet, cellulose).

    Q. "I've read that plastic shouldn't be used with A/C or with stucco. Is this correct?"

    A. I certainly wouldn't include interior poly on the walls of a house with air conditioning. Stucco is a very problematic siding material; for more information on stucco, see To Install Stucco Right, Include an Air Gap.

  9. user-723121 | | #9

    Tony,

    Are there no building codes for New York? It seems to be one place to look for some answers. Martin has given you a good rundown, the stucco and poly combination is how the moisture in walls made headlines. I have used the combination successfully in Minneapolis but extruded polystyrene sheathing was used along with the typical felt paper and metal lath. I think the safe bet for New York is ADA.

  10. BobHr | | #10

    I think he needs to understand the sources of moisture and the ability of a structure to dry.

    The biggest moisture problems are a leak of some sort or air leakage carrying moisture to a cold surface.

    Doing an A+ job on air sealing is your best defense against air carrying moisture in to a wall.

    Diffusion or moisture passing through a material is a very minor concern.

    The plastic vapor barrier or kraft facing is more likely to have air leaks and let moisture and then trap it there.

    The water dripping off the insulation was from a very high humidity load from the concrete drying. Hopefully you have a vapor barrier in contact with the bottom of the slab to prevent future moisture problems. The condensation may also be showing flaws in the insulation.

    As it is you did not have the drywall wall installed that would act as the air barrier. Hopefully there are no can lights or other penetrations in the drywall that could be sources of air leaks.

    If you have batts in the ceiling what was done to give them a 6 sided air tight enclosure? The bats are not going to perform well well with wind washing over the top.

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