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Community and Q&A

Ceiling Condensation and Dew Point

jdclassen | Posted in General Questions on

Clate Zone 4 non marine.
Client changed from wood roof to composite (dark).
Condensation now forming on the entry ceilings in the summertime and growing mold. 18-20 ft ceilings. Skylight in the middle (not having issues)
Solar roof vents installed with no improvement. Eave vents are lacking around the entry.
R-60 fiberglass, not well air sealed at the attic plane.

How is the ceiling reaching the dew point?

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  1. GBA Editor
    Kiley Jacques | | #1

    Hi Jeff,

    I’m giving your question a bump. While you wait for experts to weigh in, take a look at this article by Martin Holladay: Understanding Dew Point. There are some pertinent comments and links to multiple resources you might want to check out.

  2. Expert Member
    Michael Maines | | #2

    Jeff, this is just a guess but you could be seeing the result of solar vapor drive. Dark roofing absorbs the sun's energy and uses it to push moisture back toward the interior.

    Is the condensation only at the high ceilings? That suggests it could be the result of high relative humidity and/or air leaks near the ridge. What is the RH and indoor temperature?

    What were the wood shingles installed over? 30# felt or something else that was permeable, ice and water shield which is impermeable or something else? Were they installed directly on the underlayment, on Cedar Breather or on skip sheathing?

    1. jdclassen | | #4

      Was previously Cedar Breathers on the slopes no felt at all then a flat roof section where it was a rolled roof and is still. 20:12 pitch with a large flat section. Flat roof is about 12 feet above the top level ceiling. Problem started immediately after the change to asphalt shingles.
      Condensation really only over the entryway which has AC vents in the floor with 18-20 ft ceilings. The rest of the 2nd floor has no issues and another room with high ceilings has no issue.

      1. Expert Member
        Michael Maines | | #9

        Is that a round skylight? That, and the recessed lights, are your problem. They leak air and have framing that reduces insulation levels. Skylight shafts are notorious for being poorly constructed and insulated.

  3. Expert Member
    NICK KEENAN | | #3

    Do you have air conditioning?

    Moisture tends to move from warmer to cooler. During cooling season moisture gets driven toward the interior. In the right conditions it can condense on the interior and cause issues.

  4. maine_tyler | | #5

    I'm having a bit of difficulty picturing all this. It is a vented attic with R-60 fiberglass on the floor? Or this is happening directly beneath a flat roof?

    The area with condensation is a ceiling that caps two open stories? (view from 2nd story ceiling is to first story floor)?

    The key would be to figure out where moisture it coming from.
    If the AC vents are (either by convection or radiative coupling) bringing the ceiling to below dewpoint, is it simply the dewpoint of interior air, or is it the dewpoint of infiltrating air?

    Or is the ceiling temp actually lower now than before, perhaps due to exterior conductive/radiative cooling (night sky)?

    Did the venting characteristics of the roof (any part) change?

    1. jdclassen | | #6

      It is a vented attic (although not evenly distributed)
      Yes this is a 2 story area. Entryway and a large curved staircase.
      I not convinced it is the dew point of interior air or infiltrating air although I tend to think the latter.

      The only reason the ventilation changed is because of the change from wood shake roof to composite. Then the customer added solar vents to try and remedy the issue which did nothing.

  5. Expert Member
    Akos | | #7

    One problem I'm having with is summer time condensation. If it is happening now, that doesn't feel like summer time unless you are on the other side of the equator.

    A roof with wood shingles over something like cedar breather has a lot of drying capacity to the outside. Asphalt shingles are pretty much a vapor barrier, so any moisture that makes it into the attic can only dry through venting.

    Your picture pretty much shows what the real issue is though. You have a bunch of canned pot lights and skylight. Both are notorious air leaks in a ceiling. Adding in solar vents actually makes situation worst, now you have a fan that de-pressurizes your attic pulling more interior air out through those air leaks.

    The proper fix is to air seal the ceiling.

    This means covering the pot lights with air tight boxes and making sure the tunnel to the skylight is air sealed. Fluffy insulation does nothing for air sealing, for air sealing you need either spray foam, canned foam or caulking to fill in any gaps in the ceiling. . You have to seal all the gaps. Once that is done, the first job is to disconnect the solar vents and add in some intake vents. You want these about 60% larger than the exhaust vents to avoid de-pressurizing the attic.

    1. jdclassen | | #8

      I was just called in because they want to prevent this before summer hits. It is difficult to be called in when the problem is not happening right in front of you. The 5x Solar vents that were put on pull a total of 2600 CFM according to manufacturers specs.
      The can lights are definitely my top priority at this point, but the homeowner is still latched on to the roof and the ventilation (likely because of the money they spent on it.)
      There is definitely a lack of eave vents in that particular area, so that air is likely stagnant even with the powered fans.

  6. Expert Member
    NICK KEENAN | | #10

    Did we ever establish whether AC is being used? Could it be there is an uninsulated coolant line that is causing condensation? Or an uninsulated cold water line?

    What I have seen sometimes is you have a leak somewhere, and rainwater collects. In hot weather the rainwater evaporates and creates an area of high humidity. You don't see the direct effect of the leak but instead the secondary effect of the high humidity.

    1. jdclassen | | #11

      AC is being used and functioning within normal parameters.

      1. Expert Member
        NICK KEENAN | | #12

        My bet is that somewhere warm humid air is contacting either air conditioned air or a piece of the air conditioning equipment.

  7. maine_tyler | | #13

    My guess is winter time condensation (interior air) accumulates on the roof sheathing with the new asphalt and poor venting in that area. Then, come warmer weather, the drive reverses and is exacerbated by the AC vents cooling the ceiling. Basically what Akos said.

    Unless there is a roof leak of course.

    Getting into that space now, if possible, would reveal much (it is wet up in there now?)

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