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What is the recommended method for insulating a cathedral ceiling in a moisture-laden home?

designc | Posted in Green Building Techniques on

My split level home was built in the 60’s with insulating practices of the the era. No moisture barrier under concrete basement floor. R-3 fiberglass batt, fiberboard sheathing.
After structuring for an open floor plan and removing all the old 2×6 ceiling joist I have to re-insulate as well as re-roof. I mention this because I have the opportunity to add neopor foam board to top of roof sheathing. Really can adapt to any recommendation for ceiling , but will have to live with leaky house for a while.
Appreciate any help, thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    If you have the opportunity to place all of your insulation above the roof sheathing -- in your case, that would probably be a thick layer of rigid foam -- then that approach is by far the best of any option.

    For more information on these issues, see How to Build an Insulated Cathedral Ceiling.

  2. user-945061 | | #2

    Why is the house moisture-laden? Is there evidence of moisture problems in the home?

  3. GBA Editor
    Martin Holladay | | #3

    [Editor's note: Cameron just posted the comments shown below. They were accidently posted as a new thread; the comments belong here.]

    Due to aesthetics, R-20 (4 1/2 inches of rigid foam) above the roof is my limit. Would Roxul plus an airspace in the rafter bays be a good choice for the remainder of the needed R-value?

    Jesse, 80% of home is concrete slab with remaining 20% being crawl space I closed in - conditioned basement. In a high water table area with no vapor barrier under slab or crawl. De-humidifier runs constantly.

    I plan to include air space in rafter bays under sheathing along with batt insulation for remainder of needed R-value. Is an airspace necessary with R-20 foam above roof sheathing?
    -- Cameron Morris

  4. GBA Editor
    Martin Holladay | | #4

    I strongly suggest that you carefully read the article that I linked to in my first response:
    How to Build an Insulated Cathedral Ceiling

    As that article makes clear, once you install rigid foam above the roof sheathing, you don't want to include a ventilation space under the roof sheathing. These two details are incompatible.

    If you really want a vented roof assembly, and you intend to install rigid foam above your roof sheathing, then the ventilation gap has to go above the rigid foam, not underneath the roof sheathing.

  5. designc | | #5

    Thank you Martin, that was the answer I was looking for!

  6. user-945061 | | #6

    Whenever people claim 'high water table' I'm reminded of my very sweet, but severely obese uncle, who insists he's just got a large frame. Your house isn't fat, it's big-boned!

    I would evaluate the exterior drainage of the house. Ensure that the gutters eject water >10' from the house where the grade slopes away from the house. Make certain there are no bypasses from the roof - gutters - leaders - sub-grade drainage - grade. I can almost guarantee that if you look, you'll find something. You can learn a lot about houses by walking around them in a torrential rainstorm.

  7. designc | | #7

    I understand your skepticism Jesse. One of my first projects after purchasing home was mold remediation in attics, and basement. I installed interior weeping drainage and weeping system in the cinder block foundation. Exterior schedule 40 drainpipe for downspouts to dump out to daylight. etc..
    It can be difficult to describe in a few sentences the make up of your homes history. I'll try to be more detailed in the future. I appreciate having this site to reference. Thanks for taking the time.

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