Choosing a Wall Assembly Design
I would appreciate some help from the community here!
We’re a few weeks away from breaking ground on a new home build. Up until about a month ago, the idea of insulation and house performance wasn’t really on the forefront of my mind (long story, don’t ask!). Anyway, so my wife and I have laid out the architectural drawings for the house we’re going to be building with the assumption that we have 2×6 exterior walls. The house will be constructed in Kimberley, BC, CZ 6B. Design values shown below:
January 2.5% design dry bulb temperature °C -25
January 1% design dry bulb temperature °C -27
July 2.5% design dry bulb temperature °C 31
July 2.5% design wet bulb temperature °C 18
Annual total degree days below 18 °C 4,650
Originally, I was generally content with the idea that we would easily achieve BC Step Code 3, and likely land in Step Code 4. Now, I’m thinking that I’d like to aim for Step Code 5 since my builder can likely get an ACH of <1.0 without much difficulty and we are considering solar, given our house aspect and climate. https://dosdesigngroup.com/bc-energy-step-code-requirements/
In going down this rabbit hole, I’ve read nearly every GBA and BS article I can find about different wall assemblies. With this context, I’m aiming for a wall assembly that’ll get somewhere around the low R-30s.
My builder is open to different framing concepts as he has used many of them. I am trying to evaluate which is “best” and integrates with other house systems most easily. I think I am down to three concepts and would like input/thoughts.
Option 1: 1/2″ drywall, 2×6 stud, plywood, barrier, two layers of 2″ comfortboard, 1×4 furring strips, siding, Roxul inside the wall cavities
Option 2: 1/2″ drywall, 2×6 stud, plywood, barrier, two layers of Gutex Multitherm 60, 1×4 furring strips, siding Roxul inside the wall cavities
Option 3: 1/2″ drywall, 2×4 double stud wall (12″ out to out), plywood, barrier, 1×4 furring strips, siding, dense pack insulation (cellulose?) inside the double stud wall
* I think this is a tried and true method and I feel good about having no moisture issues for our climate.
* Roxul is the most fire resistant of the choices and we live in an area subject to fire
* 4″ exterior insulation will remove all thermal bridges
* 4″ exterior insulation will tie in nicely with the 4″ thick (each side) ICF basement forms
* Only comfortboard 80 appears to be available now, whereas 110 would be better for the purpose of installing the 1x4s
* Installing the comfortboard presents the most challenges with respect to insulation, cladding, windows, etc.
* It seems that comfortboard is in limited supply, everywhere, currently
* From a pure wall assembly material cost, I think this is the middle option. I think extra costs will add up for screwing around with windows and siding.
* I have concerns with the suitability of fiberboard. It seems like it’s well tested in Europe, but is newer to the scene in North America. I am unsure how well it would perform here.
* I know they say it’s fire resistant, but I have my doubts
* 4″+ exterior insulation will remove all thermal bridges
* 4″+ exterior insulation will tie in nicely with the 4″ thick (each side) ICF basement forms
* Only Gutex Multitherm 60 is available right now, and in limited quantity
* Installing the Multitherm shouldn’t be too bad with respect to insulation, cladding, windows, etc. It’s dense and I’m sure long fasteners on windows would allow the windows to be installed as outties, whereas comfortboard would be an inbetweener.
* From a pure wall assembly material cost, I think this is the most expensive option. I think costs of installing siding and windows is a few % more expensive than Option 3, but it’s the same technique by and large.
* I think this is a tried and true method in general, but am worried about cold sheathing issues here. I know I’ve seen newly constructed homes around here where there is clearly no exterior insulation and you can see every stud through the siding on cold mornings.
* There will be many thermal bridges, notably at top and bottom plates
* Transition to ICF will be ugly
* I have concerns over long term performance of dense pack (fill in the blank). I know it is highly installer dependent, but it just seems like this is one of those things that in 20 years, we’ll look back and see gaps at the top plate and question why it ever seemed like a good idea. Alternative would be to install Roxul batting instead (defeating some of the wall cost efficiency)
* Installation of this wall system is definitely the easiest.
* I like the idea of really deep windows and sills
* From a pure wall assembly material cost, I think this is the cheapest option
* Since our house was “designed” (by myself and my wife) assuming 2x6s, we would lose 6″ along all perimeter walls on the usable space. It’s probably not a big deal in the grand scheme of things.
I do think that material availability is a big thing. It would suck to delay the house build by months b/c of waiting for insulation to show up. I am half tempted to use Option 1 and replace the comfortboard with 4″ of XPS (I know, not a good product). It seems that people no longer like to use XPS or rigid foams (environmental + sensitivity to moisture buildup/vapor barrier). I think with 4″ of it, there would be no issues there and the sheathing would always be warm and protected.
I really liked this article:
and related to it, the building technique highlighted by the case study:
I am just not certain the thermal bridging at the plates would work out well long term and whether cold sheathing is okay. A lot of articles have said: “yeah, we have cold sheathing in our CZ 6 area and there has never been a problem”, so I’m trying to make sense of it all.
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