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Cathedral ceiling with 2×6 rafters question

Noah_questions | Posted in General Questions on

Hello everyone,

I have a question about my situation. I live in zone 6 and I am redoing the ceiling in my home because the drywall was failing. The roof is in very good shape, but there was only r11 in the ceiling. I want to redo the insulation right now, and I have two options. The windows go too high up the walls to fur out the ceiling, so I am left with r15 and vent channels, or spray foaming the cavity and getting to r-30, but no ventilation and possibly a vapor barrier sandwich. 

Eventually, I would like a metal roof and I want to future proof the home so I can add more insulation above. Financially, I would really love to just do the r15 for now and try to find a way to seal up the ventilation space in the future (next year, we just bought the house and the project got out of hand quick) so that I can add foam above. However, I am not sure since I would have to use a vapor barrier in the ceiling and that might cause a problem. Any help would be appreciated. 


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  1. Expert Member
    Michael Maines | | #1

    I avoid spray foam when I can, for environmental and health reasons, but there are situations where it's clearly the best product for the job. Your situation is one of those.

    It's ok to have a vapor sandwich as long as the sheathing is dry when the exterior insulation is installed and there are no thermal bypasses. If you want a safer assembly, though, you can use rigid wood fiber insulation on the exterior, or over-frame the existing roof and install a permeable insulation with venting so the sheathing can dry readily to the exterior.

    1. Noah_questions | | #2

      Hello, Michael. Thanks for the reply. I think I understand the second option you gave, and that’s what I want to do. But to clarify, if I just hang faced batts now, I need an air gap which will be achieved with baffles. When the time comes that I use mineral wool or some other permeable insulation above the existing sheathing, would the air gap be a problem? How could I safely (safely for the roof, that is) close that gap if I needed to? I plan on using durovent foam baffles/vents. I know I could open the ridge vent and close that off from above, and I am not worried about closing the soffit vents since they would be replaced with the roof.

      And just to clarify, when you say over-frame the roof, you mean just do a permeable insulation on top, new sheathing, vents on top new sheathing?

    2. Noah_questions | | #4

      I think I misunderstood your comment. You are saying spray foam and then a permeable insulation on top later down the line. Correct?

      1. Expert Member
        Michael Maines | | #17

        Yes, the opposite of more typical situations--spray foam the cavities, then when you're ready, insulate above the roof sheathing with fluffy insulation.

  2. freyr_design | | #3

    you could fur down in a scissor truss like fashion, so that you would only have the limited insulation toward the eaves

    1. Noah_questions | | #5

      That is an option, I’m just not sure the wife will go for that. If I had a way to safely insulate it now with batts and leave the ventilation, then somehow close it up and do 6 inches of foam on the roof deck later on I would. I’m just not sure I could simply fill the void in the vent chutes with dense pack cellulose when I do the roof and not have moisture problems.

      1. freyr_design | | #8

        An airspace in that location shouldn’t cause condensation issues under 6” of foam. If you can seal top and bottom very well and consider a smart air barrier on interior I don’t see how issues would manifest. If you seal up later you could add spray foam into eave air gap to limit cooling at that location and possible condensation, or maybe consider exterior rigid in that location

        Edit: also at ridge when you do seal it consider making it a high perm membrane

  3. Expert Member
    Akos | | #6

    Post some pictures of the setup. If this is your typical sloped ceiling with mini attic on top, you can dense pack the rafter space completely long as the attic is vented. The idea is the cavity can dry through diffusion into the attic space and cellulose works as a moisture buffer and also help redistribute any moisture that makes into the space. This would let you easily add exterior insulation down the road as there is no vent gap.

    Even if you rafters go to the peak, you can always frame in a mini attic area near the ridge to do this. That bit of flat ceiling always comes in handy for mounting light fixtures without effecting the feel of the space.

    1. Noah_questions | | #7

      I will get some pictures, but no mini attic. We have this stupid oversized beam that we are trying to get rid of, but not sure if we can yet, and it would cut the attic in half. There is also a center wall directly in the middle through most or the house. Hence, the drywall would be attached directly to bottom of rafters.

      If I added an attic, how could I vent it? Would I need to add gable vents?

    2. Noah_questions | | #9

      Here is the photo. I am planning to take more insulation out this weekend, but I am afraid to in case I need to get spray foam done since it would be over a week out, but I really want to avoid it.

      I see now the header is a little larger than I thought, but I ran it by the wife and she does not want to give up ceiling height.

      It just a soffit vent and ridge vent setup. 5.5 inches deep, 16 O.C. Spacing. It is a low pitch roof (2.5/12)

      1. Expert Member
        Akos | | #10

        Looking at the pictures, Freyer_design has the right idea. Fur it down to get some reasonable R value (unless code requires more, R30 batts gets you most of your energy savings) and keep it vented. Simple effective and cheap to do. Much easier than an exterior rigid project. To save height, near the outside wall use a a thick layer of rigid as vent baffle (say 2") and R14 batts. This means only a 1.5" fur down by the window so not much space lost at all.

        You can also install narrower vent baffles (make sure these are permeable) with some squished 2x6 batts with a layer rigid under the rafters for more R value. The rigid with taped seams gives you a solid warm side air barrier and if you go with say 2" polyiso, a pretty decent assembly R value. Does mean hanging drywall with long screws (standard stuff at commercial drywall places) or strapping out the rigid first.

        As for the beam, looks to be a big span, that might actually be needed. I would leave it exposed and clad it in stained veneer.

        The beam will be cutting through your ceiling air barrier, so make sure to figure out how to get continuity there. What I've done for similar setup is glued 2x nailers onto the beam and sealed the ceiling air barrier to these.

        1. Noah_questions | | #13

          R49 is code. That’s why I sort of wanted to future proof it so we can fix it with exterior foam later on. I’m not sure if spray foam would let us do that even. I suppose I could always get the sheathing off at some point and fill it with fiberglass again, then do exterior foam. I think with MemBrain it would be able to dry to the interior yet.

  4. Expert Member
    DCcontrarian | | #11

    "We have this stupid oversized beam that we are trying to get rid of, but not sure if we can yet, and it would cut the attic in half. "

    You need to either have that beam or horizontal pieces called rafter ties from wall to wall perpendicular to it. That beam is supporting the middle of the roof. Without it the weight of the roof is supported entirely by the outside walls, and the pitch of the roof will tend to push the walls away from each other. The rafter ties keep the walls from spreading.

    Another option is to keep the beam there, but see if you can replace it with something smaller, like a steel beam. That would be a question for an engineer.

    But it's serving a purpose.

    1. Noah_questions | | #12

      I’m not sure the bottom half of the beam is doing anything. The bottom half is 1.5x2.75” pieces of wood vertical every 42 inches with 3/8 plywood on the sides. The bottom is 3-8’ long 1.5x2.75” sticks spanning horizontally. The top half is solid wood under the plywood.

      1. gusfhb | | #14

        It is possible you can replace that beam with a physically smaller glulam etc, but it looks to me like you are at the end of the span for 2x6 rafters so there needs to be a beam there. If those are 8 foot studs on the far wall then you have 8 foot at the wall, you will never notice the difference. My inside wall height is 7'4" but you only notice the 10'+ at the peak

        1. Noah_questions | | #15

          7’ 6” is current height on short walls.

          I understand the need for a beam. I just think they put something over the actual structural part of the beam to make it look bigger. I do have someone coming to look at it as I’m not a structural engineer.

          1. gusfhb | | #16

            Well, as suggested you could find a clever way to fir out without changing that dimension. I personally have no qualms about spray foam, and have used it in my house. If you are later going to want to add insulation on top it may be the way to go since a ventilation channel would seem counterproductive in that scenario.

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