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Concern about vapor diffusion vents increasing summertime moisture in cathedral ceiling

Nola_Sweats | Posted in General Questions on

I’m considering a vapor diffusion port on an existing unvented cathedral ceiling.  I’m in climate zone 2A, hot and humid Gulf Coast.  Existing insulation is fiberglass batts, but it’s impractical to re-roof or re-insulate.

A vapor diffusion port will allow vapor diffusion to move in both directions, with moisture going out in winter (as intended), but also coming in during summertime.  If I use a/c for 7-8 months of the year and heat for 2-3 months, is there a risk that the outdoor air in summer (usually 90%+ humidity and around 80 degrees overnight) will diffuse into the drier cathedral ceiling space and condense on the 70-something-degree drywall ceiling?  Do I risk creating a 7-8-month problem as serious as the potential 2-3-month problem I’m trying to solve?

“They” say vapor diffusion works in zones 1-3 to avoid wintertime condensation, but I don’t understand why vapor intrusion wouldn’t be a problem in the hotter months (i.e., most of the year), and the vapor diffusion studies seem to acknowledge higher humidity in the roofs in the summer.

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Replies

  1. Expert Member
    Michael Maines | | #1

    Nola, 80°F air at 90%RH will condense on surfaces that are 76°F, so I share your concern. I'm very interested in vapor diffusion ports but not an expert on them, or hot/humid climates, so maybe someone more knowledgeable will weigh in.

  2. Jon_R | | #2

    No data, but since the vapor diffusion port is very small compared to the ceiling area, I expect that the small amount of moisture diffusing in from the exterior would diffuse harmlessly into the interior. "some condensation/sorption" does not imply "causes a moisture accumulation problem". Most walls in cold climates prove this.

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Nola.

    I asked Kohta Ueno of Building Science Corp. about this. He explained that in general, inward vapor drive is only going to cause issues in homes with an interior vapor barrier and reservoir claddings (materials that can hold a lot of water).

    BSC studied vapor diffusion ports in homes with insulated roof assemblies and concrete roof tiles, which are a reservoir material. The short answer to your question is that while there was a higher level of moisture in the insulated roof assemblies with vapor diffusion ports than the control assemblies at warmer times of the year, it was not concerning.

    From their conclusions: "Although springtime ridge RH levels in the diffusion vent roofs are slightly higher than in the unvented roof, levels are well within the safe range. Moisture accumulation within the assembly due to inward drive is unlikely, given the vapor-open assembly (loose-blown fiberglass and air-permeable insulation mesh)."

    You can read the report here: BS Field Test/Vapor Diffusion Ports

    1. Nola_Sweats | | #4

      Thanks for the detailed response! That's encouraging. I assume insulation with a kraft paper backing would not count as a vapor "barrier"? My house has some batts backed with kraft paper, and some with plastic that's perforated, which I understand to be equivalent to unfaced as far as vapor is concerned. I can't see which is inside the cathedral ceiling in question.

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