Sometimes builders interested in sustainable design and construction suffer from a serious malady. It doesn’t have a name, per se, but could be accurately described as “wall waffling.” It is a condition whereby they convince themselves that there must be a new kind of highly efficient wall system which nobody else has thought of yet. It is fast to build, uses commonly available materials sparingly, provides a high-r-value per unit of thickness, and performs durably for their specific hygrothermal climate. They will endlessly pen out iterations of differing arrangements of materials, hoping to finally find “the one.” Basically, they think they can reinvent the wheel.
I suffered from this obsessive affliction myself over the course of about 2 years, while our family noodled away on the interior design and elevations of our house, waiting to start construction. I looked at 6″ of exterior mineral wool, modified I-joist Larsen truss walls, conventional exterior bearing double-stud walls, ICF’s from footing to trusses, and just about everything else in between. What I finally (and perhaps foolishly) settled on was a modified version of a double stud wall. While working with Peter Amerongen of Habitat Studio Design & Workshop in Edmonton, Alberta as a young carpentry apprentice, I learned of the great speed, efficiency and low cost that simple double stud 2×4 walls filled with dense-pack cellulose in a super cold climate with no seismic concerns could be built. They were dead simple, fast to build, and never got wet.
2 Layers of 3″ Exterior Roxul
But then I moved to Vancouver Island, British Columbia and started working with Pheasant Hill Homes. While we always try to build to a higher energy efficiency standard, and have even had the opportunity to build a couple of “near Passive” houses, the obvious wall assembly just didn’t seem to present itself so easily. With a cold (but not that cold) climate and an extremely rainy and humid coastal setting, high r-value walls were a very finicky beast to build in a way that wouldn’t leave them sopping wet after a few winters. As a company we are practiced and comfortable with applying up to 3″ of exterior mineral wool insulation, which is fantastic from a building science and assembly durability perspective. But as the carpenter that has had to put the stuff on the wall, get all the flashings integrated properly, install strapping that is flat and co-planer with all of it’s surrounding rainscreen straps, I know just how slow, laborious, fussy and downright itchy it can be.
Photo Credit: Building Science Corporation
So for my own house, I was going to redesign that wheel and finally get it just right! Or so I thought.
I eventually settled on a modified double- stud 2×4 wall (see my plans here: Praxis Section Drawing.pdf) based on Joseph Lstiburek’s design (an esteeemed Canadian Building Science engineer that is a principle at Building Science Corporation).
Here’s a link to his design for comparison.
Essentially, I decided to build a 12″ thick wall, with an inner 2×4 structural/bearing wall sheathed with 1/2″ air-sealed plywood sheathing on the exterior face. The interior side of this wall also houses the wiring and occasional plumbing (for laundry and fridge water lines), so acts as a service cavity, a practice common to Passive House construction. Because the plywood on the outside of this wall acts as the primary air barrier, none of the outlets, switches or plumbing services breech this air barrier. Once these rough-in services are installed (and the framing is inspected by the engineers and municipal officials), the stud spaces are insulated with typical fibreglass batts. Outboard of that interior bearing wall is a 4″ wide space that encircles the entire home on both levels, and is virtually free of all framing materials. It is completely insulated with R-20 mineral wool batts, and serves very effectively as a complete thermal break. The only interruption of this space is the 3/4″ plywood spreader plates (which tie the inner and outer walls together at top and bottom), the plywood window and door liners, and the subfloor sheathing which ties the entire frame to the floor system.
Sequence Showing Wall Construction
Outside of the inner wall and the insulation cavity between, there is a second 2×4 outer wall, which serves strictly as a housing for more insulation and a structure to support the wall cladding. This wall has a layer of ttaped exterior gypsum sheathing on the exterior face, then housewrap, and finally 3/4″ rainscreen strapping and cladding to finish it all off.
I will now craftily make it appear as though I made a smart choice by telling you all the reasons why this was a good wall design for our project. First, it is highly insulated, with actual wall thermal performance of just under R-39. Second, because I was building it with the help of my Dad, one other carpenter for part of the time, and occasional help from friends, it was very inexpensive in terms of labour. In fact, mostly free. Thirdly, the material costs were quite low, as we installed basic mineral wool batts ourselves in the outer wall and between the two walls as we framed them on the ground before standing. The rest was just 2×4 material, and 3/4″ plywood (which we ripped completely out of materials we had saved/reused from the foundation forming). And finally, the wall dries extremely well in either direction based on what the conditions require at any given moment. In a very humid coastal environment that gets lots of rain, this is probably one of the most important elements to ensure with your wall design. Slapping plywood on the exterior, while making life MUCH easier later on when installing rainscreen and flashing, would have meant that we had a vapour retarding plywood layer near the inside of the wall assembly, and on the outermost surface (plwyood just qualifies as a Type II Vapour retarder). A double vapour retarder condition like that sets an assembly up for mold, rot and eventual failure because when moisture does get into the system, it has no expedited way of getting out in either direction.
In contrast, by using totally vapour open insulation (fibreglass and mineral wool batts) throughout,
by installing our vapour retarding layer (the plywood sheathing- which can still dry slowly to the interior if need be) on the warm 1/3rd point of the wall, and by utilizing incredibly vapour permeable exterior gypsum and housewrap on the outermost faces, we have a wall that will generally resist moisture migration from the interior of the home into the wall cavities, yet will dry with incredible ease to the exterior/cold side. Because plywood has the unique ability to become more vapour open when it’s moisture content increases, the assembly will still permit drying into the interior of the home during those few times of the year that the outside conditions are both very warm and super humid. To see my Air Barrier Section in detail click here: Praxis- Air Barrier Section.pdf
Crucial to the success of this wall is also its airtightness, which we ensured by taping the seams of the plywood sheathing on the exterior face of that inner 2×4 wall (with 3M All Weather Flashing Tape), as well as by taping the seams of the exterior gypsum sheating with a vapour permable air sealing tape (Siga Wigluv). Both tapes are high quality acrylic tapes, designed specifically for air sealing assemblies. The exterior gypsum air barrier layer was sealed to the foundation wall (which is in turn sealed to the underslab poly).
It was also sealed to the topmost spreader plate (which is in turn sealed to the air barrier on the underside of the roof trusses. The other air barrier (the plywood sheathing on the inner wall) was sealed to the spreader plates at the tops and bottoms of walls, as well as to all of the plywood window and door liners (which were sealed in turn to the windows and doors after installation with foam rod and Quad Max OSI caulking). When we did a mid-construction blower door test (to check the home’s airtightness), we had a result of 0.26 air changes per hour at a 50 Pascal pressure difference. The world’s most stringent energy efficiency standard (Passive House) requires no more than a 0.6 ACH50 rating (which is over twice as leaky as our house), and an average new home built to code has an air leakage of around 5 ACH50, which is about 20 times more leaky than our house. So, it all worked out very well in the end in terms of the quality of thermal design, and the effectiveness of the air sealing.
Sounds like a perfect wall then, right?! The wheel has successfully been reinvented! We don’t have to think about it anymore! No more poring over Green Building Advisor forums and articles on the weekend, squinting at Bulding Science Corp case studies at 2am, or scratching out endless concept drawings in the truck at the end of a day’s work. Phew!
Yeah. Right. Let me tell you now while I would NEVER build this wall assembly again.
While great for my family and I, free labour is thankfully a thing of a bygone era. And clients don’t generally have the skill, desire, or frankly the hours in a day to build and insulate their own walls. That’s why they hire good homebuilders with trained carpenters to build their homes for them. While carpenters aren’t reknowned for their outrageous wages, framing two walls, each with its own layer of sheathing, air sealing two complete barriers in that wall assembly and ALSO insulating
the interior cavity and every stud bay in the outer wall takes time. A lot of it. It took about 1.5 months for the equivalent of two full time carpenters to frame the first and second story walls of our house. JUST the exterior walls. Standard 2×6 exterior walls for 2 carpenters on a house with the same design would have taken two carpenters about 1/3rd that time.
Secondly, it was complicated. At the start we were missing air sealing steps so frequently in the complicated interface of materials to be sealed, that I sat down and wrote out the THREE, SINGLE SPACED PAGES of step-by-step instructions to build and air seal a single length of the double stud wall before it was ready to be stood into place.
Only after building these walls every single day for about 3 weeks could I confidently do it without thinking I might have missed something. High efficiency building needs to be simple, and easily performed by every decent carpenter out there, or it is a wasted mental exercise. An elderly technician from Pro-Clima once decreed at an air sealing workshop: “The merit of an assembly should be determined by the elegance of its design, NOT the perfection of it’s execution.” Basically, an element should be so easy to build/air seal that any idiot can do it properly, not just the world’s greatest craftsperson under the most ideal conditions.
But the most important reason I would never build these walls again? Because each time we pre-insulated, air-sealed and stood a wall up, it just sat there, completely exposed to whatever mother nature had up her sleeve until we got the trusses stood and the roof membrane on.
Even though we were building in July and August, the driest months of the year on Vancouver Island, I cannot explain to you the mystical tarping rituals we had to employ at the start and finish of each day to expose then re-cover our work. I cannot describe to you the traumatic psychic fatigue I endured laying awake each night, anxiously straining to perceive the fearful pitter-patter of raindrops. I cannot tell you how emotionally disheartening it was, after making it through weeks of construction keeping our walls totally dry even through an occasional downpour, just to have a clog in a tarp drain cause a failure and flood on the first floor of our house the very last night before the roof membrane was completed and the house was dried in. The resulting removal and replacement of exterior drywall and wet rockwool is still something that conjures cold sweat and mental expletives in me.
Was the wall cheap for us to build for ourselves? Yes. Is is super-insulated with great moisture management capabilties. Absolutely. Will it make our house one of the most efficient and comfortable homes on the continent for the rest of our lives? 100%. But is it something I would ever suggest we build again for friend or enemy? Not a chance. The highly complicated construction process, the extended labour time to insulate and air seal during the framing process, and above all the risked liability of having the walls suffer moisture damage during construction just means they are unsuitable for general practice.
So, it’s a back to the drawing board. That’s one of the great things about hiring a homebuilder with an interest in energy efficiency. We can’t stop thinking about it. It’s what keeps us up at a night, but in a good way. We dream about how things can be better, more efficient, more rewarding for us to build and clients to live in.
But you know what? I had a thought for a new wall design the other day, and I think this might just be the one . . .