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Community and Q&A

External Comfortboard: vapour/air barriers

Graham Brockley | Posted in General Questions on

Hello!   I live on Vancouver Island in Marine climate zone 4.  I am building a shop 20′ by 50′ with a simple shed roof oriented south that will carry 40 solar panels.  There will be no windows.  Trusses with a 3 in 12 slope have been designed. 
If we do 2 by 6 construction with external Comfortboard 80, what would you recommend as far as placement and type of air/vapour barrier?  I plan on 3/4 inch plywood on the interior walls and rockwool bats.  I am also planning on running wiring through conduit on the internal surface of the wall to prevent any unnecessary intrusions through the internal surface.
The Rockwool installation manual recommends: (Exterior to interior)
• Cladding • Drained/vented cavity • Wood strapping, screwed through insulation • ROCKWOOL COMFORTBOARD™ 80 rigid insulation (thickness to meet R-value requirement) • Vapour-permeable sheathing membrane (air-barrier) • Sheathing (plywood or OSB) • 2×4 or 2×6 wood framing with ROCKWOOL COMFORTBATT® insulation • Polyethylene sheet vapour barrier (cold climates only) • Gypsum board and interior finish

It seems strange that they want a poly sheet inside; this goes against everything I’ve read about external insulation.  Is it different for this material because it is vapour permeable?  I guess these walls would dry to the outside?

I was thinking a combined air/vapour barrier on the sheathing under the Comfortboard would be best instead of a vapour permeable sheathing membrane –  perhaps a paintable product?    
Thanks.  I’m a home owner, not an experienced builder and will be getting help with the contruction from an experienced builder but I wanted to try to get these questions answered before we submit drawings.

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Replies

  1. Jon R | | #2

    You can use this calculator for wall vapor retarder/barrier decisions.

    https://www.appliedbuildingtech.com/fsc/calculator

    Canadians usually know better, but if you are considering a Class III vapor retarder with minimal external insulation, study wall "MI3 T" here (page 18, > 26%).

    or

    "the walls with Class III vapor retarders and the lowest levels of foam required by code fail the ASHRAE criteria by a much higher margin than the rest." Page x.
    https://www.nrel.gov/docs/fy13osti/56709.pdf

    1. Jeremy Puzycki | | #6

      Great links, Interesting reading, Thank you.

  2. Peter_R | | #3

    I'm using a similar assembly on a house we're building outside of Vancouver. We are using a "Class II" vapour retarder latex primer on the drywall in place of the poly sheet (this is nice since we need to prime the drywall anyways and I don't like poly as it limits the inward drying potential). But it wasn't exactly easy for me to get my building permit: I had to hire a building envelope engineer, who completed a Schedule B, in order to satisfy my jurisdiction's building department that what I was proposing indeed met code.

    My understanding is that if we were following the latest IRC, that even the class II vapour retarder on the drywall could be avoided, as the plywood sheathing with, e.g., a liquid WRB (we're using Prosoco) by itself is sufficient for our climate zones with the right ratio of external-to-internal insulation. But speaking with my Building Envelope engineer, my understanding is that this won't fly in BC at the moment and you need at least a Class II vapour retarder on the warm side of the wall. I'm a newbie so don't take what I'm saying as authoritative.

  3. Expert Member
    Malcolm Taylor | | #4

    Peter,

    I think you have summarized the situation perfectly.

    Graham,

    Peter has summarized the Part Nine requirements. Depending on the occupancy, they may or may not apply to your shop. Practically however, there is no issue with using interior poly in the assembly you are considering. All the insulation is permeable enough to dry to the exterior. From a moisture control viewpoint, it is very similar to the wall assemblies used in 99% of residential buildings here on the island, without any issues.

    1. Graham Brockley | | #5

      Perfect, thanks. I guess it would be more of a concern if one was using XPS as the external foam board. I'll just follow the instructions!

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