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How do I fix condensation between ceiling and closed cell spray foam insulation?

MartyBiz | Posted in Energy Efficiency and Durability on

I have cathedral ceilings on my second floor with close-cell spray foam insulation on the bottom of my roof. There is only about a 1-2 inch gap between my ceiling and my “hot roof.” I had the roof done 4 years ago and it is completely covered with ice and water shield. Over the past few months (covering the spring and summer), I have noticed water stains and mineral effervescence pushing through the paint on the very highest point of my ceiling. I have asked the contractor to remove the top 3 layers of shingles to see if there was anything wrong with the exterior. That is supposed to be done soon. If there is nothing found up there, he suggested that I will have to run the bathroom exhaust fan constantly to remove humidity. I have two questions about that: #1. How will that help during summer months when all the windows will be open? #2. How will that remove any condensation that is accumulating above the ceiling in the small space between the ceiling and spray foam?

Has anyone experienced this issue or have any suggestions? It’s so weird that it just started happening after 4 years. It’s difficult to tell if it’s rain or condensation as we’ll have some really humid days here in Massachusetts and then it’ll poor buckets right after.

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Replies

  1. Kevin Spellman | | #1

    Are there any leak points near the roof peak? Plumbing vent boots are notoriously crappy these days. They split and crack easily and leak(bring back that poisonous lead!). Is there a chimney? How much closed cell foam was sprayed? Trusses or stick built? What type of roof?

  2. User avater
    Michael Maines | | #2

    Marty, in addition to Kevin's questions, do you have a structural ridge beam that goes right up to the peak, as opposed to being dropped below the rafters?

  3. User avater GBA Editor
    Martin Holladay | | #3

    Marty,
    Your question falls into a new (and increasingly common) category of questions here at GBA -- namely, observations of condensation near the peak of a cathedral ceiling in hot weather.

    My honest reaction is that these reports don't lend themselves to an obvious explanation. These reports are therefore intriguing.

    We all know that condensation occurs when warm, humid air contacts a cold surface -- specifically, a surface below the dew point temperature of the air.

    So what's the mechanism? In previous threads, I have proposed a hypothesis (assuming, of course, that the possibility of a roof leak has been excluded): warm, humid interior air is contacting roof components (ridge flashing, roofing, or roof sheathing) cooled by night sky radiation. If this theory is correct:

    1. The phenomenon would only happen in homes with ridges that "see" the sky, and would never happen in homes that have a full tree canopy shading the roof.

    2. The phenomenon would be more likely during clear weather than cloudy weather.

    3. The phenomenon would be more likely in homes without air conditioning than in homes that are actively cooled by an air conditioner.

    4. The phenomenon would be more likely in homes that have air leaks at the ceiling peak and insulation defects at the ceiling peak than in homes with tight ceilings at the ridge and full insulation depth at the ridge.

    I've noted that my hypothesis is somewhat unsatisfying, and I'm not convinced that it's correct.

    Note that building scientists who have observed moisture problems in attics insulated with open-cell spray foam are still arguing about what's going on. These problems aren't necessarily related to what's happening in your house, unless "hygric buoyancy" is a relevant factor. More information -- not much information, frankly -- is available here: "High Humidity in Unvented Conditioned Attics."

  4. Rick Evans | | #4

    MartyBiz, congrats: you just stumped Martin Holladay. :-)

    Any chance you have light grey shingles? Just wondering if this is cooling off the sheathing during low dew points and is causing condensation.

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