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double ceiling (similar idea to double wall)

Clark Lewis | Posted in Green Building Techniques on

can’t find any info on this idea.
i’d like to insulate an awkward existing post and beam structure. it has a low-angle metal roof built over the original flat roof with its torch-on membrane still intact. the attic space tapers to nothing at one end, so not enough attic space for insulation above the torch-on layer. inside flat ceiling height is much taller than i need.

wondering if i could build a “double ceiling”, similar to a double stud wall, but oriented horizontal instead of vertical?

ie. fill existing 2×10 rafters with fluffy insulation, hang 2x rafters several inches below that (joist hangers on ends at beams, plus perhaps metal straps hung along the 2x’s to prevent sagging). fill the gap with a continuous layer of fluffy insulation, plus insulate between the dropped ceiling joists. would be easy to reach R50+, and no thermal bridging.

because of torch-on, has to dry to the interior, so only other option to mitigate thermal bridging would be a layer of roxul comfortboard hung below the current rafters, but attaching drywall to that would be tricky (strapping with long screws through the comfortboard, working against gravity).

thoughts?

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Clark,

    Does this roof have soffits and a ridge vent? (I'm guessing not.) Where are you located?

  2. Clark Lewis | | #2

    whistler, canada, so zone 6 or 7. cold at times, humid at times. attic part of roof will be vented, but plan to insulate the ceiling (below the torch-on membrane) right up to the sheathing, since a flat roof with a membrane on it can only dry to the inside. no interior vapor barrier obviously.

  3. Steve Knapp CZ 3A Georgia | | #3

    So the slope roof and low-slope roof are in a connected attic space, correct? Hopefully others will comment, but I don't think you can count on using the sloped area to safeguard your low-slope roof. If you don't want to re-roof the low-slope section, a flash and batt strategy might be the best option. (This is just from my research. Other GBA members have actual experience with these assemblies and will give you safer advice.)

    This article described the flash and batt approach for cathedral ceiling, but I believe it applies here as well: https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

  4. Clark Lewis | | #4

    My question is about trying a double ceiling. Any reason it wouldn’t work the same as a double wall?

  5. Steve Knapp CZ 3A Georgia | | #5

    Clark,

    What issue are you trying to resolve? Ice dams? High heating bills? Both?

  6. Jon R | | #6

    A double wall with fluffy fill and exterior sheathing that cannot dry to the exterior isn't an idea you should copy. You could drop the new ceiling far enough to allow a vented to the outside air gap above the fluffy insulation.

  7. Clark Lewis | | #7

    steve, not trying to resolve an issue, trying to insulate a weird post and beam structure properly from the start.
    jon, from what i've read, flat roofs don't ventilate well, they should be built unvented. the low-slope over-roof (above the flat torch-on layer) will be vented, for what its worth, but the ceiling below won't breathe through the membrane, so it can only dry to the interior, which is fine. i have lots of height, just looking for an alternative to rigid interior insulation since i can't insulate externally.

    i guess this sounds confusing, i'll post a sketch later.

  8. Clark Lewis | | #8

    Hopefully this bad drawing is viewable

  9. Clark Lewis | | #9

    Here

  10. Clark Lewis | | #10

    i could bore out a hundred 4" holes in the plywood/membrane between the ceiling joists and leave an air gap between the top of the fluffy insulation and the underside of the sheathing and hope it vents into the attic above, or i could call the membrane an air and vapor barrier and insulate right up to the sheathing and hope it dries to the interior, though with 2 "roofs" above the ceiling, i doubt it would see moisture.

    regardless, mostly interested in the idea of the double ceiling to avoid having to hang drywall off strapping that's screwed through 4" of rigid insulation (ie. roxul comfortboard). attic space above tapers to nothing at outside beam, so not enough space for adequate insulation in there.

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