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Retrofit insulation in cathedral ceiling with truss-joist.

Foxwedge_DB | Posted in General Questions on

I am looking for feedback and recommendations regarding retro-fitting insulation in the roof of an existing home with a cathedral ceiling.
Parameters of existing conditions:
– gable/pitched roof (9:12 on one side, 4:12 on other)
– roof framed with 18″ deep parallel chord truss, 24″ O/C
– Interior to exterior assembly: drywall, 1×3 strapping across truss, poly vapor barrier, 9-1/4″ fiberglass insulation, vent space, plywood roof sheathing, steel roofing.
– Climate zone 5, near Kingston, ON, Canada

At this point, my idea is to:
1)remove the steel and whatever underlayment
2) dense-pack the remaining space with cellulose on top of the fiberglass through holes cut from the exterior
3) apply a vapor-open air barrier on top of existing sheathing (Intello x??? or other recommendations, please!?!?)
4) apply 2×3 on the flat, on top of each truss, for vent space
5) apply new plywood roof sheathing
6) New underlayment (conventional vapor impermeable? or Solitex mento, if there is a compelling reason?) and steel roofing directly on top of new sheathing.

Can anyone point out the pitfalls of this approach or suggest improvements?

Thank you so much!!!
Ian

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Replies

  1. Tom Wheeler | | #1

    If you have any holes in your ceiling, lights /fans /ductwork, you need those completely sealed.

    Intello is great.

  2. Expert Member
    Akos | | #2

    Why? I'm all about saving energy but the work you propose will have an ROI measured in centuries.

    Your current roof with 9.5" batts is pretty decent, works out to around an R38 assembly. Using my design temperature of 2F, a 1000 sqft of R38 roof looses 1900BTU. Bumping the roof up to R92 (24" of dense pack) your looses go down to 800BTU. It is less but unless you have some energy target you are looking to make, it is noise for a house.

    Your effort and dollars are better spent on air sealing the house.

    1. Foxwedge_DB | | #3

      Thank you so much for your detailed response. The other aspect of proposed project is re doing the siding and windows. Would you say that adding 3" continuous rockwool to the walls is also "noise"?
      2x6, with fiberglass, wall.

      1. Expert Member
        Akos | | #4

        Hard to say. This is a good read on the topic:
        https://www.energyvanguard.com/blog/the-diminishing-returns-of-adding-more-insulation/

        A well built and sealed 2x6 wall is not too bad and gets you most of your energy savings. Few if any production builds are well built and have a class I insulation install, so there is a lot that can be improved. Taping the sheathing and exterior rigid would definitely improve the assembly R value and overall comfort.

        Is 3" of rigid mineral wool worth it? Probably not. That stuff is expensive and fussy to install. Installing 1.5" of polyiso or EPS would be much cheaper, some siding can even be nailed directly through the foam. There is probably long but a reasonable ROI on that.

        1. Foxwedge_DB | | #5

          Right. I said 3" because I was under the impression that in my climate zone it was the minimum, based on various articles posted here.
          That said, 1" is the new code minimum around these parts and I see lots of that going up...

        2. Foxwedge_DB | | #6

          And mineral wool because of the poly on the interior - possibility to dry outward.
          Am I missing something here?
          I know lots of less-than-perfect assemblies can do well in practice. I am going for the "right" thing here.

          1. Expert Member
            Akos | | #7

            In zone 5, with a 2x6 wall, for condensation control you want R7.5 exterior rigid insulation, much less than 3" of mineral wool. With interior poly, you can reduce this even further, you can read about it here:

            https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-092-doubling-down%E2%80%94how-come-double-vapor-barriers

            Going with exterior mineral wool does help with drying, but not a requirement. Unfaced EPS or permeable polyiso also work. Walls with foil faced insulation also work.

            The important item to get right with this type of work is your WRB and flashing details, those will make a much bigger difference for the reliability of your wall than the type of insulation.

            Get these details right, air seal the sheathing and you'll have a great wall no matter the insulation type or thickness.

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