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Community and Q&A

Condensation on Cathedral Ceiling

GBA Editor | Posted in GBA Pro Help on

Nine months ago I had a new composition shingle roof installed. When the roofer removed the old roofing, he found there were two layers of comp shingles in place. The slope on the roof is 3/12. The drywall is attached directly to the 2×6 rafters, so no attic space. The underlayment is 30lb felt. The roof faces west. City permit was pulled.

About three months ago I noticed that the ceiling above the skylight was glistening in about a 2×2′ area. The moisture is evenly distributed — no water spots (no plumbing). The moisture appears overnight, every night, and for the most part, dries out during the day. The skylight was installed in 1991, and I have never had any problems with it. Still, I called out a local skylight installer to check the flashing, etc. No problems there. My roofer came out and installed an O’Haven roofing vent in the problem area. No improvement at all.

I live in Southern California less than a mile from the beach, and the humidity is usually around 65-70%. This summer has been hotter and more humid than normal, with humidity rising above 80% several times. The cathedral ceiling is in my master bedroom, and I sleep with the skylight and windows open every night. I never use the tiny shower in the master bath, and condensation never appears on any walls, windows, or mirrors. I use my air conditioner infrequently, and its use doesn’t seem to impact the problem one way or the other. The rest of the ceilings in the house have attic space and no problems.

Do you think that I never experienced this problem with the old roofing because its two layers provided a little extra insulation from the sun? Is this a vapor drive problem? Would you suggest a different underlayment? Other than the new O’Haven roofing vent, there are no other vents on the roof, or venting holes under the eaves. My roofer claims to have never run into this problem in his 20+ years of roofing. He admits to not knowing what to do to solve the problem. He wants to try drilling some ventilation holes under the eaves next. In your opinion, will this help? As for insulation in the rafter area, I assume it is there, but I haven’t checked yet. Please help. My roofer is clearly spinning like a top. All I know for sure is that the problem started with the new roof, and the only thing that really changed, is that I now have one layer of roofing where before I had two.

I recently lost my husband to cancer, and I’m feeling overwhelmed here and in need of some honest advice. Thank you listening. 🙂

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  1. GBA Editor
    Martin Holladay | | #1

    1. I'm not sure what you mean by "the ceiling above the skylight." Do you mean the sloping drywall ceiling between the skylight and the ridge?

    2. If I understand correctly, you open the skylight and go to bed. When you wake up, you notice that there are water droplets on the painted sloping drywall ceiling. Is this correct?

    3. If I understand correctly, the ceiling is cold enough to allow condensation to form. If this happens during the summer, it's certainly possible that the ceiling is cold because you keep your house air conditioned. Is that true? Do you have any air conditioning ducts or registers in your ceiling?

    4. Assuming that the ceiling is cold, the next question is, what's the source of the moisture? The most obvious source is the open skylight. When you open the skylight, you bring in warm, humid exterior air. The exterior air condenses on the nearby ceiling, which has been chilled by your air conditioner.

    5. The above scenario is only a guess. I don't know the weather conditions under which this phenomenon occurs.

  2. barbara | | #2

    The area of condensation is on the sloped ceiling between the skylight and the ridge. Every morning I find an approx. 2x2' area of evenly distributed condensation -- never any actual water droplets. I also notice a bit of condensation on the slope below the skylight, but it doesn't appear to get as wet there.

    I have tried sleeping with the skylight and windows closed, and it makes no difference at all in the amount of condensation I find the next morning. I very seldom use my air conditioner. I went four weeks this summer without turning it on at all, and the condensation problem persisted at the same level as before. There are no a/c ducts or registers in my ceilings. Although the new roof was installed last winter, I never noticed the ceiling "sweat" until this summer.

  3. John Brooks | | #3

    You say that the problem is not related to air conditioning use.....
    hmmm....could it be night sky radiation?
    does it occur on clear nights?
    did you change the color of the roof or the type of roof?

  4. barbara | | #4

    The type of roof (composition shingle) and color did not change. The problem occurs on clear nights, as well as heavy, marine-layer nights. Only the sloped ceiling above and below the skylight shows condensation. The rest of the ceiling remains dry.

  5. GBA Editor
    Martin Holladay | | #5

    This is the type of problem that is hard to diagnose over the Internet. A good home performance contractor should be able to help. There are many questions unanswered: is there any roof insulation? If so, how much and what type? Is the roof vented? How? What is the temperature of the drywall when this happens? (A good infra-red thermometer would be helpful to gather data.) What is the indoor humidity level when this happens?

  6. barbara | | #6

    Is a home performance contractor different from a building contractor? How could I go about finding one?

  7. barbara | | #7

    Martin, the thing that puzzles me the most is, why is this happening now? I have lived in my home for 23 years, and this is the first time I've ever had this problem. Neither the ventilation (there is none) nor the insulation has changed even a tiny bit. I'm wondering if an incorrect underlayment was used.

  8. Robert Riversong | | #8


    I think you hit the nail on the head. The only change is to a single layer of composition shingles.

    If you open up the ceiling above and below the skylight, I suspect you will find uninsulated areas that, with less roof-top insulation (the shingles) are allowing sufficient radiant cooling at night to bring those areas of ceiling to the dew point.

    By the way, shingle roofing is not recommended (and often not warranteed by the manufacturer) if installed on a roof of less than 4:12 slope.

  9. barbara | | #9

    Richard, I will call someone out to check the insulation around the skylight this week. What kind of insulation would you recommend? I really believe this is the answer. :)

  10. Robert Riversong | | #10

    For reasons I've never understood, many people have refered to me as "Richard" right after I introduced myself as Robert, but this is the first time that someone has made that mistake after putting my name in writing.

    Assuming your question was directed at me, if you're going to open up the ceiling, then batt insulation (fiberglass is the most common, although it's also the least effective and most problematic) would be simplest. If the ceiling is not going to be opened, but instead probed or otherwise checked for absence of insulation, then you might consider blown cellulose, which is both more effective at reducing conductive and convective heat loss but also resistant to fire, insects and rodents. While it would not be cost-effective to have an installer bring a blowing machine for two small cavities, you might have the rest of the ceiling "densified" with additional insulation at the same time (assuming there's low-density fiberglass in there now). This would, of course, require patching and refinishing the ceiling, but may be worth the effort and expense.

    I would also have your roofer remove the roof vent (and not install any others), as a vented roof with no continuous vent channel from bottom to top is worse than an unvented roof, as it can introduce moisture to an otherwise closed cavity.

  11. barbara | | #11

    Robert, I have no idea how I made that mistake. Don't know where Richard came from.

    Although it might be more expensive, I've been considering having the roofer pull up the sheathing and then install the new insulation himself. At the same time, he could remove the roof vent for me. The job would be quick, and I wouldn't have to hire an insulation installer, a drywall repairman, and a painter. If I decide to go this route, what kind of insulation would be best?

    This is my last question! I promise! Thanks so much for hanging in here with me. I really appreciate it!!

  12. Robert Riversong | | #12


    If you're going to have the roofer install the insulation, then it would probably have to be fiberglass batts, as he's not going to have the equipment or expertise for blown-in or sprayed-in material.

    If you're going to rip up the new roofing and sheathing, then I would recommend either adding batts to any empty areas or, if the existing insulation is in poor condition, removing it and replacing it with either R-19 or higher-density R-21 unfaced batts. I would suggest installing 1" of tongue & groove XPS rigid foam board insulation over the rafters and installing new sheathing (or reusing existing if OK), and re-roofing as before.

    The additional foam board will serve as a thermal break over the rafters, increase the thermal efficiency of the roof/ceiling assembly, and offer an additional weather barrier under shingles installed on a too-shallow roof.

    Adding the 1" of foam will, of course, increast the height of the roof by that amount and will require wider fascia and rake trim to compensate. Make sure there's metal drip edge installed at the eaves and the rakes and a sound underlayment (30# felt is fine) installed in shingle fashion and under the rake flashing and over the eaves flashing. The shingles should be installed with a 6-nail pattern rather than the standard 4-nail pattern, since it's such a low pitch and facing the windward side.

  13. barbara | | #13

    Actually, I was only thinking of having the roofer rip up the sheathing around the skylight area to install new insulation. By the way, he did just e-mail me to say that there was existing insulation where he put the new vent in last week. Just the same, I will have him install better insulation and remove the vent. If that fails to do the job, I will have him read and follow your last suggestion to a "T." I'm just hoping that the entire roof won't have to be ripped off. :(

  14. Jerry | | #14

    I think it could be radiant cooling to the night sky. Can you hang a ceiling fan below the skylight well? This would keep the wall surface temperature above the dew point.

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