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Choosing a Wall Assembly Design

kenmoremmm | Posted in Green Building Techniques on

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.

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

Option 1:
* 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.

Option 2:
* 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.

Option 3:
* 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:

A Case for Double-Stud Walls

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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  1. gstan | | #1

    kenmoremmm: Greetings - sounds like you have done your homework - let's see if I can help. There is a better way using option 3. The better way is to ignore the popular wisdom of filling the double stud wall with cellulose or other blow in and instead install a layer of foam in the center between the stud walls.
    This is what results:
    Option 3: 1/2″ drywall, 2×4 double stud wall (12″ plus or minus), structural plywood, WRB, 1×4 furring strips, siding . Inside the double stud wall we have Mineral wool insulation in both stud walls with a 4 inch layer of foil faced polyisocyanurate in between the two stud walls.
    This will completely eliminate concerns about cold sheathing, moisture build inside the walls, and settling associated with cellulose and other blow in insulation products.

    A little discussion: First, Polyiso is, from what I read, the most environmentally friendly foam product available - second, the foil face provides the best air/moisture barrier available (for any amount of money) - third, the actual R-Value achieved with this configuration will exceed R-40. The two stud walls provide protection for the foam from physical damage, insect infestation, and fire. The R-40 value is calculated this way: The mineral wool filled stud walls are assumed to be R-10 each (if you could achieve a perfect installation job they would be R-12.25 - it's never perfect!), the Polyiso when it's new will have an R-value of approx. 6 per inch for R-24 - as it ages it will gradually decline to a final value of around 5 per inch, so you will end up at about R-20.
    Polyiso is known to lose R-value at low temperatures (There is at least one brand which claims that it doesn't). Most research shows an R-value drop of around 1 per inch at temperatures below -1 C (30f) so, even if you were to assume twice that, you would still have a total value of R-36 for this wall at the lowest projected temp.
    This wall is relatively easy to construct ( Yes - I have actually built one). The moisture barrier ( the foil face) is approx. 1/4 of the way through the total thermal resistance of the wall which means the temperature of the condensing surface (the foil face) will always be above the dew point - You will not get any moisture accumulation from condensation in this wall!
    I have designed, built and lived in three super insulated houses - lived in them for more than 26 years ( all in climate zone 6) - tried several wall types and insulations they all work but vary in expense and construction difficulty - if a wall exists that is more stable and easier to construct than this, I am not yet aware of it. GOOD LUCK!

  2. kenmoremmm | | #2

    Thanks for the input. What does the detail look like at the top and bottom of the foam sandwich that will prevent thermal bridges or a big disconnect between air barriers? How do you maintain continuity of the air barrier? My understanding of most zone 6 builds is that you want a breathable wall. Seems like the sandwiched foam would be the opposite.

    1. gstan | | #3

      The idea of "breathable" has ,I think, been largely discredited in recent years but let's talk about it. In general, moisture consists of water molecules suspended between air molecules. If the temp of the air changes the total number of water molecules that can be held in suspension changes - when the temp goes down the amount of water that it's possible to suspend goes down. Air currents in walls (through insulation) carry the water molecules in them - the force which drives the air currents is produced by changes in air temperature from one side of the wall to the other - if the temp at any point in the wall drops to or below the dew point water molecules will start to stick to any available surface (foil faced insulation, wood studs, fluffy insulation fibers, etc.). If the temp later rises this surface water starts to evaporate - that is it is carried off by air currents. In the wall I recommend, the foil face acts as an absolute barrier to both air currents and water. So the polyiso portion of the wall is totally isolated from moisture problems - that only leaves the fluffy insulation between the studs. Fluffy insulations have varying properties when it comes to water absorption and evaporation - mineral wool (Roxul is the popular brand name) has almost zero absorption and therefore evaporation is efficient - others such as cellulose have very high absorption rates and
      evaporation takes a really long time. In general, the more water any substance has in it the faster the heat transfer across it - insulations slow heat transfer - you don't want them wet!! Houses that "Breathe" allow air currents to flow and moisture to transfer which allows heat transfer. This is all a rather simplistic way to put things but gets at the general principals. Last in cold climates (zone 6 and above) in the winter the total amount of water in the air is very low even when the measured relative humidity (as reported on the news) is very high - when you bring outside air in (for fresh air) and then warm it up the relative humidity goes way down inside the house because the total number of water molecules per cubic foot is still the same but the number that could be held in suspension has gone way up with the rise in temp. This is why houses are often too dry for comfort in really cold weather - it is also why you won't get any
      moisture build up in the inside stud wall - and for the same reason not in the outside stud wall either.
      The detail at the top and bottom of the sandwich: This is a problem to be solved by the architect, designer, builder , etc. for each house. I will explain what I did but that may not be your preferred solution. In my house I built the external stud wall first, put on the roof and then attached the polyiso to the inside of the external stud wall. The next step was to attach polyethylene sheet to the bottom of the roof trusses and tape it to the foil face of the polyiso. After all that, I constructed the interior stud wall as just one more interior wall. The floor in that house was a 4" floating slab contained within (but not attached to) the stem wall- both the slab and stem wall were insulated
      by 4" of XPS. Your contractor and/or building dept. may balk a solution like this - if so you will have to find another. AGAIN GOOD LUCK.

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