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Community and Q&A

Signs of Moisture in New Cathedral Ceiling

green654 | Posted in General Questions on


A few weeks ago, we finally moved into a new construction home we built. Everything has been fine so far. The front bedroom of the house is directly above a insulated and unconditioned garage. The ceiling of this bedroom is cathedral. Early this morning when I was out walking the dogs, I noticed moisture on the roof of the cathedral portion of this room on the steep slopes. The cathedral roof has a soffit vent along the top and I’m 99.99% certain a baffle vent was installed. The insulation is rock wool. We used CertainTeed Membraine on top of the floor sheathing in the attic. I’m incredibly allergic to mold and we do NOT want any issues. I’m so glad I noticed the moisture but now what??

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  1. kbentley57 | | #1

    Are you saying that you noticed moisture on the exterior section of the roof, in the morning?

    There's a few things that might help us solve this puzzle together. A picture is worth a thousand words if you can upload one that is representative of what you're talking about. More information we need is

    (1) What climate zone / location you live in
    (2) Construction details of the roof
    (3) Insulation thickness
    (4) Conditions (Time, Temperature, RH if known) when you noticed the problem

    1. green654 | | #3

      I attached a photo. If you zoom in, you can see the moisture. We live in Chicago which is Zone 5. Insulation is a r49 rockwool. Hi noticed the problem at 7am. No rain for the previous three days. Relative humidity was about 75% and temp was roughly 60 degrees. The previous two days the humidity outdoors was very low. That bedroom humidity level is usually between 40-45%. The room below is the garage. The walls are insulated and the garage is not conditioned.

      The wall construction is as follows: painted Hardy board, Advantech zip with Tyvec drain wrap, r49 rockwool, CertainTeed membrane, drywall, latex paint.

      Any ideas of what I can do?

  2. Expert Member


    Can I ask a few questions which may help you get better advice:

    - You say it's a cathedral ceiling, but refer to the attic floor. Is it a roof with a sloped lower portion and a flat ceiling at a certain height?
    - How is this roof framed? Trusses? Dimension lumber?
    - You saw the moisture on the roof while outside. What did you see - Condensation on the shingles? Was it uniform, or only over the framing members?

    1. green654 | | #4

      To answer your questions:
      1. There is no flat ceiling at all in the bedroom. It’s angled.
      2. No trusses. Only framing lumber.
      3. I did see condensation on the shingles. It was not completely uniform. I’m attaching the photo I took. Zoom in and you can see the moisture.

      I’m trying not to panic. I want to deal with this immediately. I just don’t know what to do and who to call. I’m not too keen on calling the builder since it appears he messed it up in the first place.

      1. Expert Member
        MALCOLM TAYLOR | | #5


        I'm not sure I see any problem there. The condensation on the surface of the shingles appears to occur on the overhangs as well as the insulated roof. It looks a lot like the result of night time radiant cooling to me. That is the shingles falling below the dew point in the early morning. I'll be interested to hear what others think.

  3. PBP1 | | #6

    I have a flash-n-batt shed roof (2018 build) with standing seam metal, on the north side, especially where not under the tree canopy (less radiant cooling under canopy) often have dew running off the roof in the morning. Usually gone by midday, I have my moss/lichen garden below the drip line, doing fine.

    Your photo probably shows where the effective R-value may be higher, or greater radiant loss (cooler shingles). What direction is that part of the roof facing?

    Or that fire hydrant is spraying at night lol

    1. green654 | | #7

      That part of the roof is facing south. Do you think it’s wet on the inside if I can see wetness on the shingles outside?

  4. Expert Member
    PETER Engle | | #8

    This is not a problem. It is condensation from night-time radiant cooling. The wet areas got a little bit cooler than the dry areas, probably signifying somewhat better insulation performance in those areas. The shingles are designed to get rained on and they don't care about a little bit of dew. This does not in any way suggest that there will be interior moisture problems.

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