GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Insulated cathedral ceiling/roof layers

brentwilson | Posted in Energy Efficiency and Durability on

I am wanting to figure out the best build-up for an insulated cathedral ceiling/roof. Here are some of the basics:

1. Zone 6B – so should be R49 or higher.
2. Roof pitch 5:12.
3. The roof structure will be BCI rafters over the top of a ridge beam and two purlins (one on each side).
4. The layers I am thinking for the roof will be as follows (listed in order from the interior to the exterior):
          a. 1x tongue and groove pine ceiling attached to bottoms of rafters.
          b. Maybe drywall (is this needed?)
          c. 12″ or 14″ BCI rafters – with either of these sizes, there is not enough depth for R49 cellulose if 2 inches is needed for ventilation baffles.
          d. 2x4s attached horizontally to the top of the rafters (at 90 degrees to the rafters) to allow for some horizontal airflow along with the vertical airflow in the rafter bay baffles.
          e. 1/2″ or 5/8″ Zip System roof sheathing with Zip taped or Zip liquid flashed joints.
          f. Peel-and-stick ice and water shield for lower portions of roof and valleys (is this needed? How far up from the facia should it extend? Any advantage to adding it to the whole roof?)
          g. Standing seam metal roofing.
5. What am I missing in the above?
6. This roof won’t be super complex (no dormers), but will have several valleys (see attachment for something similiar).
7. How can I get this to R49+ affordably?
8. Do BCI rafters have much thermal bridging? It seems like it would be less with them vs. standard 2x rafters.
9. I appreciate the sample roof buildups presented in this document by Martin: http://conference2016.eeba.org/Data/Sites/2/media/presentations/Holladay-Getting-Cathedral-Ceilings-Right.pdf.
10. Of course, adding rigid foam above the sheathing has some benefits. But I’m concerned about cost as well as the added complexity of attaching everything. And adding strapping and a second layer of sheathing over the foam is something I would like to avoid. Adding foam between the rafters and interior T&G pine appears to be another option. But if BCI rafters don’t have much issue with thermal bridging, maybe the rigid foam is overkill?
11. One option to get the R factor up high enough in a 12″ to 14″ cavity appears to be using closed-cell spray foam with cellulose below it. In that case, does the spray foam get sprayed on the bottom of the vent baffle product? If going this approach, can the cellulose be dense-packed below the spray foam? Or does that risk breaking the air seal of the spray foam?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. insaneirish | | #1

    Thanks for providing lots of detail. The standard recommendations for these types of ceilings/roofs:

    - Rigid foam on roof, cellulose, fiberglass, or mineral wool in bays
    - Closed cell spray foam in bay (against roof deck), followed by cellulose, fiberglass, or mineral below (i.e. "flash and batt")
    - Closed cell spray foam filling bay

    In all cases, you need enough foam to prevent condensation issues. See this article: https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling

    And I think everyone would recommend installing drywall between the roof and T&G finished ceiling, both for air sealing and fire blocking reasons.

    1. brentwilson | | #3

      So if this were done as an unvented roof with flash and batt insulation method, would I be losing much through thermal bridging of the i-joist type rafters? In this scenario, would 5 inches of spray foam and 7 inches of dense-packed cellulose be appropriate in a 12" BCI rafter unvented ceiling? Is it correct that it would be something like R57? This could be sheathed with Zip sheathing and have taped and mudded drywall on the inside covered by T&G boards. Where do you think this would fall cost-wise and effectiveness-wise vs. either 1) vented cellulose with 1 inch of foam on the inside between the rafters and the T&G ceiling boards and 14" BCI rafters? Or 2) External foam, Zip sheathing, 12" BCI rafters with bays filled by dense-packed cellulose, drywall, and T&G boards?

  2. Expert Member
    AKOS TOTH | | #2

    If you are going with T&G ceiling, might be worth to put a layer of rigid insulation as the air barrier instead of drywall. You can go up to 1" and still be able to install the T&G with 2.5" nails.

    I joist have very little thermal bridging because the webs are very thin. Usually your roof assembly gets close to 90% of the R value of the fluffy insulation between the I joists.

    If you go with 1" interior plyiso bellow the TJIs, with 12" loose fill cellulose, you end up with an R50 roof.

    Make sure your combination of vent gap between TJIs and the cross strapping allows for a somewhat continuous vent space around the valleys in your roof.

    1. brentwilson | | #4

      So you are thinking a vented roof is the way to go on this? If so, why? Do you think the valleys would get enough airflow with the cross-strapping? Are there any fire-code/off-gassing issues with using the 1" of foam instead of drywall between the rafter bottoms and the T&G boards?

      1. Expert Member
        AKOS TOTH | | #5

        COST.

        When it comes to roofs my order is:
        -vented with blown in insulation
        -vented with batts
        -unvented with bats+exterior foam
        -SPF

        Generally, if you can figure out how to vent your entire roof, you'll have a much cheaper assembly that is also more forgiving to build.

        In terms of your valleys, you'll have to check if all bays can be vented. Take a look if you can get enough air flow. If you have an air path with the amount of cross section area required for ridge/soffit venting it should work.

        Most important thing. An air tight ceiling needs very little venting but you can't vent your way out of trouble with a leaky ceiling.

        Rigid insulation sheets are very stable, since it is made in a plant it doesn't have any of the issues like a bad spray foam install. You can always up-size your TIJ and go with drywall/osb.

        Check your local code if T&G is a good enough barrier, there is some good info here:

        https://www.greenbuildingadvisor.com/article/thermal-barriers-and-ignition-barriers-for-spray-foam

        1. Deleted | | #6

          Deleted

        2. brentwilson | | #7

          How can I determine whether or not the valleys will receive adequate ventilation? I will attach a BCI i-joist framing plan for the roof.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |