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Climate zone 4 cathedral timber frame insulation

DaveHope | Posted in Energy Efficiency and Durability on

Hello, 

I am designing and building a timber frame cottage in Victoria, BC. so climate zone 4, and I’m in the woods, so even damper than average for the area.

I am trying to stay away from foam products, and use mineral wool as much as possible.  

The ceiling of the cottage will be 2×6 T&G fir over the rafters, with a “blackout layer” above that (possibly roofing felt unless this causes vapor movement issues).  

It will be a standing seam metal roof, using a cold roof  system.

I’m thinking that all insulation on the outside of the wateproof membrane is the way to go, but how to achieve that without foam panels seems to be the challenge.

Any advice pointing me down the road is appreciated

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Dave,

    The question of building an unvented roof with out foam is the subject of Martin's current blog: https://www.greenbuildingadvisor.com/article/filling-rafter-bays-with-fluffy-insulation and several recent questions. I don't see either a practical or code compliant way of doing it, but maybe other posters have some ideas.

  2. Jon R | | #2

    Build a vented cold or warm (insulation above structural elements) roof. I'd use a fully adhered underlayment to get good air/water sealing.

  3. DaveHope | | #3

    Thanks for the responses,

    I checked out the blog article...it raises some alarming concerns regarding settling of all types of fluffy insulation...which would no doubt be worsened by the 12/12 pitch I have planned.

    It looks like rigid foam is the way to go in order to prevent any settling issues....I was also thinking that possibly comfortboard, but I assume it would be succeptible to air wash if installed external to the air water seal.

    Here are my revised thoughts: 5/8 ply sheeting with an impermeable air/water adhered membrane, 3 staggared layers of foam held in place with 2x4 pressure treated strapping on 24" centers, then another layer of 5/8 ply, high perm membrane then standing seam roof with a continuous ridge vent and soffit vent.

    The potential problem I see with this configuration is in the attachment through to the first layer of plywood, and whether the self healing characteristics of the membrane can be relied upon afte being screwed through.

  4. Expert Member
    Michael Maines | | #4

    Dave, your situation seems made for wood fiber insulation. Gutex is the best-known brand, but there are others: https://www.greenbuildingadvisor.com/article/fiberboard-insulation-developer-takes-step-forward.

    Small Planet Supply in Vancouver has some Agepan in stock, last I knew: http://www.smallplanetsupply.com/.

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