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Unvented loose-fill cathedral roof in Zone 9

lairdyherbert | Posted in Green Building Techniques on

Not to beat a dead horse anymore than we need to (!), but I’m embarking on a foam-free build in zone 9, with a cathedral ceiling that will be insulated with a loose fill batts of sheeps wool.

I’ve gotten lost in the debate about loose fill unvented chathedral roofs, although I understand that is isn’t advisable, and very clear attention to detail is required. The hiccup is that the house is clad to weather, and the roof is continuous down the sides of the walls (no eaves) and there is a only a 3/4″ air vent between sheathing and cladding, vented out the ridge cap on the roof from the bottom of the wall. It’s completely finished on the outside, so whatever safety nets I can do can only happen from the inside.

My question is, what can I do to set myself up for success? internal air gap between sheathing and insulation, that goes nowhere? or an internal air gap continues down to the bottom of the wall? or is a 3/4″ gap enough and with excellent vapor barrier detailing it will likely be fine.

Here is the roof structure, from the outside in:

1) Steel roofing tiles
2) 3/4″ air gap (furred with 1×4)
3) #15 felt
4) 1×6 planking on a diagonal
5) Trusses
6) Vacuum insulated Panels (VIPS) between trusses
7) loose fill with sheeps wool batts
8) Db+
9) 2×4 service cavity (for electrical etc)
10) 1×6 pine boards

Thanks for your feedback in advance!

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  1. Jon_R | | #1

    What are your air barriers? Just Db+ that isn't well supported on either side?

    Where is this building?

    1. lairdyherbert | | #2

      Taped DB+ is the interior air barrier, with 2x4 service cavity so that I'm minimizing any punctures in the barrier. The DB+ will be secured to the bottom side of the trusses, which is infilled with the sheeps wool and the VIPs. It's in the Yukon, Canada...

      1. GBA Editor
        Martin Holladay | | #3

        The Yukon is a cold and unforgiving location. I can't give you much advice, other than to tell you that your decision to omit soffit vents, a ventilation channel, and ridge vents seems to me to have been a big mistake.

        1. lairdyherbert | | #4

          Sorry martin, I do have a 3/4' ventilation channel which vents out the roof vents and at the bottom of the building (it's on posts - no concrete foundation. it's a continuous and smooth vent path up the walls and out the roof... There's no eaves or soffit because of the design of the building. I was careful to leave a vent channel, but I couldn't do any more than 3/4" for a somewhat complicated reason which is why I'm a bit nervous....

          1. lairdyherbert | | #5

            I tried to use a permeable membrane (felt) and planking (I wanted to avoid ply and potential mold issues there) which has lots of gaps between each of the planks - hoping that any potential elevated moisture will move to the roof vent through these layers. With careful air barrier detailing I just don't know if the roof vent is big enough or if the theory is tested enough to be doable up here...

          2. GBA Editor
            Martin Holladay | | #7

            There are three factors which slow down the ability of your ventilation channel to remove moisture from your roof sheathing:

            1. The ventilation gap isn't very high -- building scientists recommend 2 inch high ventilation channels; U.S. code requires a 1 inch high (minimum) channel; and you chose 3/4 inch.

            2. The ventilation channel isn't open at the soffit -- so air has to travel farther to make its way from the bottom of the wall to the ridge.

            3. You installed asphalt felt between the ventilation channel and the sheathing -- and the asphalt felt is a vapor retarder that slows useful diffusion.

  2. Jon_R | | #6

    At this late stage, I'd air seal very well, test with a blower door, keep the interior relative humidity low and install some sensors. Reduce interior pressure (reducing or stopping exfiltration) if you see a problem.

    Make sure that the lower wall vents can be reached by the wind (no snow blocking them).

    Not clear to me that you wouldn't be better off with a better supported, lower perm interior side vapor/air barrier - like taped plywood plus polyethylene.

  3. GBA Editor
    Martin Holladay | | #8

    Jon is right about the potential for air leakage. At the top of the assembly, you'll have lots of air leaks -- board sheathing plus asphalt felt is leaky. At the bottom of the assembly, all you have is your European membrane (DB+).

    That membrane better be installed impeccably. I would verify the performance of your ceiling air barrier with a blower door if I were you.

  4. lairdyherbert | | #9

    Jon and Martin, thank you so much for your input. I'll move forward carefully, and definitely do a blower door test.

    I'll also get some sensors, think more about the membrane options, and keep an eye on things closely. I might explore ways of increasing air flow at the bottom of the wall too if moisture levels get too high.

    Either way - I appreciate the expertise and insight - this is a great source for knowledge and advice.

    Thank you!

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