Hello GBA community,

I am in the process of designing a small cabin (<600sqft, conventionally built – this is a tiny home in size only) 30 minutes north of Kingston Ontario in climate zone 6 with about 3,300 HDD (5,940 HDD F). The climate is quite wet – precipitation occurring quite frequently throughout the year (1100mm) and is classified as Dfa by the Köppen-Geiger system.

Since the building footprint is quite small I want to invest in a well insulated wall system. After reading many of the articles on GBA, I am exploring a variation of the 2×6 advanced wall with structural sheathing on the inside and continuous mineral wool on the outside. While I do not plan on certifying the building as a passive house, I do plan on applying the same principles. I plan on using closed-cell spray foam as part of the roof insulation, and since the trade will be on-site I am also considering using it in the walls to help with air sealing and increasing the R value. Yes, I know using it between studs has been called a waste time and again – but I’m willing to accept the extra cost on such a small project.

From inside to outside the assembly is as follows:

-Drywall (1/2″)
-Horizontal strapping forming a service cavity (2x6s ripped in half)
-OSB as structural sheathing and the vapour barrier with Prosoco joint & seam filler  (3/4″) 
-2×6 Studs with:
    -Closed-cell spray foam (2″ of R6/inch SOYA Heatlok HFO using a very low impact blowing agent)
    -R22 24OC mineral wool sized for 2×4 bays
-Exterior sheathing – either thinner OSB or insulating fibreboard as long as it is suitable for the comfortboard fasteners
-Tyvek house wrap (using cat5 seems overkill since I will already have the spray foam air sealing)
-Two layers of R6 comfortboard exterior mineral wool insulation
-Vertical strapping (1″ nominal drainage plane)
-Exterior siding

My rough calculation of the whole wall R value estimates a value of about R40.
Assuming 19% framing at R6.6 and 81% cavity at R22+R12 and then R12 for the continuous comfortboard.

Are there any glaring issues with this assembly? I am least versed with vapour and air barriers, the inside face of the OSB should stay dry and resist condensation. everything exterior to the spray foam should be able to dry outwards in warmer months. I admit that there are redundancies in the assembly, but as a pilot I am a fan of them.

Thank you in advance for your time and comments!

Emils