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How to prevent a cathedral ceiling from condensating?

shdavis | Posted in General Questions on

I noticed icicles on the exterior of my siding, after further investigation I determined that it was the result of my roof condensating. I have R-30 insulation with air chutes and ridge venting, I realize that the design of my cathedral roof system was a recipe for failure.

Only a few bays are problematic, the others are dry. Will installing a product like ‘Ventilation Maximium’ help or compound the problem by drawing more warm inside air out? Any advice would be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    A common cause of "icicles on the exterior of siding" is ice damming. Make sure that you know the difference between an ice dam and condensation on the underside of your sheathing before reaching a final conclusion on the cause of your problem. For more information on ice dams, see Prevent Ice Dams With Air Sealing and Insulation.

    Assuming that you are correct, and that your roof does not have any ice dams, then it's possible that the ice came dripping out of your cathedral ceiling. If you haven't read it yet, I suggest that you read How to Build an Insulated Cathedral Ceiling.

    The usual cause of moisture accumulation in a cathedral ceiling is air leakage from the interior to the rafter bays. This happens when your ceiling is leaky. If you add more soffit vents or turbine vents or the type of vent sold by the Quebec company, Ventilation Maximum, it's very possible that your situation will become worse, because the increased venting will pull more interior air than ever before into your cathedral ceiling. For more information on these problems, see All About Attic Venting.

    In your case, I advise you to take the following steps:

    1. Make sure that you understand the cause of your problem.

    2. If you have moisture accumulation on the underside of your sheathing, take steps to seal the air leaks through your ceiling.

    3. A long-term solution may require you to disassemble your ceiling, remove the existing insulation, and install closed-cell spray polyurethane foam.

  2. shdavis | | #2

    I don't believe it is ice damns as both time there was no snow/ice on the eave and it followed very cold nights (single digits). The roof is very flat (2/12), not visible from the ground and consist of rubber roofing, could I build a false roof over the trouble areas and insulate with two layers of rigid foam? I would also seal the air chutes at the eaves and ridge at these bays?? My theory is that this would keep the existing sheating warmer and hopefully prevent the inside air from reaching the dew point until its above the rubber roofing.

    I also have a similar problem in my dormer roof which is above the master bath. Since this is a smaller area I was thinking of insulating the interior ceiling with rigid foam board (foil faced??) and covering with pine. Would this sufice??

  3. GBA Editor
    Martin Holladay | | #3

    If you have a low-slope roof, you should follow the recommendations in this article: Insulating Low-Slope Residential Roofs.

    Unless you have a high attic above your insulation, I don't recommend trying to ventilate a low-slope roof. You need to adopt an unvented solution.

    Any solution will be expensive, unfortunately. You need to seal your soffit vents and ridge vents, and then you need to choose whether to install rigid foam above the roof sheathing (a solution that will require new roofing), or spray foam insulation under the roof sheathing (a solution that will require you to open up your ceiling).

    Further advice is given in the article I just linked to.

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