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Control Layers for Vented Cathedral Ceiling

Ben Gowland | Posted in General Questions on

Hi,
I have a vented cathedral ceiling in Nova Scotia. Construction maybe mid 80’s as addition to an Existing older A-frame house. It has Ridge vent and soffits, more than the minimum recommendation for air vent area, so I think that’s ok.

Existing rafters 2×8 wood, around 16 OC. Existing vent is cheap baffles under the roof sheathing.
Existing insulation only 5.5” fibreglass batt.
Vapour barrier poly was installed between fibreglass Batt and strapping, then drywall.

I ripped off all existing material back to the roof sheathing and discarded old mouse damaged insulation and nasty old styrofoam baffles.
A light covering of black mold had grown on the inside of the roof sheathing. I treaded this with mold control spray.

In order to gain higher R value:
Nailed 2×2 to the insides of all rafters to make them 2×10.
installed 1.5” Owens rigid codeboard R7.5 under (inside) the roof sheathing, spaced by 1.5” spacers from eaves up to ridge vent for air flow. Creating a baffle out of codeboard essentially, increasing insulation at the same time.
Installed 6” rock wool R22 under (inside of) the Owens codeboard.

Rvalue between rafters 29.5 (Not accounting for thermal bridging).

I didn’t caulk anywhere yet and there are  gaps between the rafters and edges of the codeboard, and right at the peak also.
should I make this layer airtight? I assumed not as the existing baffles were simply loose stapled on and not airtight.
the rockwool is normal friction fit.

should I put vapour barrier poly under inside of the rockwool before finishing?

I had intended to finish with t&g 1×3. But reading further the recommended route appears to be drywall first.

comments?

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Replies

  1. Expert Member
    Akos | | #1

    If you are in Zone6 area, you would need a bit more rigid insulation (50% of assembly R value) for condensation control to avoid a warm side vapor retarder. Too late now, but the better use of the rigid would have been under the joists. The reduced thermal bridging would have gotten you a higher R value in assembly plus it can also be your main air barrier.

    With your current assembly, you will need a warm side vapor retarder (you can use standard poly or one of the smart vapor retarders). You can combine your vapor barrier and air barrier if you put up OSB instead of the drywall. This is about 2 Perm, so not quite code but should work well enough. You can tape the seams of it as your main air barrier and it makes a pretty easy surface to nail the T&G up against.

    If you have any light fixtures, this is a good time to replace the device boxes with air tight ones (plastic ones with a flange). This is much easier than trying to air seal existing ones.

  2. Ben Gowland | | #2

    Akos,

    Thanks for all that.
    I could easily install some more 1” or 1.5” Rigid Owens insulation over the inside now, covering the edges of the rafters to tackle the thermal bridging. That would be what.. R36 or so in the bays?.

    Then the situation would be: roof deck, 1.5” airspace, 1.5” rigid insulation R7.5, 6” rockwool, 1.5” rigid insulation.

    Then strapping, long screwed through the rigid insulation into the rafters and then my finish layer of osb or drywall screwed into that? Should work? If I can get away from poly but use the airtight boxes you mention it should be ok?.

    Ben

  3. Expert Member
    Akos | | #3

    Adding the insulation underneath now is not really worth it, my point was that in your initial assembly, having the rigid under the rafters would have been less work and resulted in a higher R value assembly.

    The roof as you have built is pretty much most of your energy savings. As long as your warm side air barrier is good, you should have no problems.

    Just make sure you detail the drywall or osb well and seal up your device boxes. If you go with drywall, make sure to install a vapor retarder as well.

  4. Ben Gowland | | #4

    Ok I got ya.
    Lastly then, would you recommend that I seal up the gaps that are around the edges of the rigid insulation where it’s fit into the bays between the rafters? I see some info that says the baffle layer should be airtight and some info that it shouldn’t be.
    It’s not long to pull down the rockwool to make the rigid layer / baffle airtight to the rafters.

    1. Expert Member
      Akos | | #5

      I doubt that is worth the work. I would personally leave it unsealed as it allows a bit more drying for any moisture that makes it through the ceiling.

      You only need a good warm side air barrier for a roof to work well, focus your efforts on that.

  5. Ben Gowland | | #6

    Yes, that was my thought too.
    Ok, thanks again for all that.

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