Editor’s note: Kent Earle and his wife, Darcie, write a blog called Blue Heron EcoHaus, which documents their journey “from urbanites to ruralites” and the construction of a superinsulated house on the Canadian prairies. GBA first posted a blog about their decision not to seek Passivhaus certification in May 2015, and later posted a blog about how the couple decided to heat their house.
After deciding on the mechanical system of the house, we needed to choose what type of wall system we were going to use. As I’ve learned, with an Eco-house, there’s more than one way to skin a cat! (Who came up with that saying?!) Again, it was a matter of weighing the advantages of each option and ensuring that our contractor felt comfortable with whatever system we decided on.
Passivhaus tends to utilize a double-wall system, although there is no set way to do this, as long as you meet the Passivhaus criteria.
Double-stud walls aren’t new. There are many houses from the 1970s that utilized a system with a 2Ã—6 wall with studs 16 inches on-center (o.c.) parallel to a 2Ã—4 interior wall with studs 16 inches o.c. Nonetheless, the biggest concerns in ensuring an exceptional envelope for a Passivhaus or any other superinsulated home are thermal bridging and airtightness (and to a lesser degree the overall R-value).
Thermal bridges provide an easy pathway for heat to flow out of your home. In a conventional single 2Ã—6 wall, this happens every 16 inches as the 2Ã—6 piece of wood is connecting the inside to the outside without a “thermal break.” This is why a 2Ã—6 wall insulated with R-19 insulation has a whole-wall R-value that is a lot less than R-19.
Airtightness refers to the leakiness of your house. While thermal bridging can be limited by proper design, airtightness can only really be ensured by paying attention while on site, building the house. Airtightness is tested with a blower door, with results often reported in terms of “air changes per hour at 50 pascals of pressure difference.” As previously mentioned, the Passivhaus standard requires a maximum leakage rate of 0.6 ach50. The Canadian R-2000 standard requires at most 1.5 ach50.
R-value is of course also important, but not as important as reducing thermal bridging and ensuring excellent airtightness. This is because R-value is a rating of the “effectiveness” of the insulating materials. You could have a so-called R-50 house, but if it is leaky and has thermal bridging it will not function like a “true” R-50 house.
New options have emerged
In the last 10 years, there have been numerous high-performance, high-tech wall systems that have been developed, including insulated concrete forms (ICFs – concrete poured into thick pieces of foam) and structural insulated panels (SIPs – OSB laminated to the inside and outside of a big piece of foam), both of which systems almost eliminate thermal bridging.
Passivhaus buildings use a variety of approaches, although many that I’ve read about use some form of double-walled system, sometimes with SIPs on the outside and 2Ã—4 timber framing on the inside. An 8-inch-thick SIP is about R-33 on its own, so add that to the R-11 of a 2Ã—4 wall and you get a well-insulated house with minimal thermal bridging.
We had already decided that we would be best served, given our rural location and 55 kilometer distance from town, to pay our contractor to be at the site working – instead of driving back and forth with lumber. We wanted to utilize a company that could provide either a prefabricated wall system (which could be erected very quickly on site) or a “kit” (with all of the wood cut to size, ready to be put together like a model). Because we have a large shop on site, the materials can be all shipped at once and stored inside. Also, this significantly reduces on-site waste and chance of error.
One of the companies we looked at was Pacific Homes out of Victoria, B.C., a company that our builder has worked with on previous projects. This company produces a “Smart Wall” system – a prefabricated timber wall that eliminates thermal bridging and significantly increases R-value. A standard 2Ã—6 wall is R-19. The 2Ã—6 Smart Wall is R-31.
We compiled a list of attributes for each option that we felt were most important in our decision-making process:
- Ease of construction
- Construction labor time
- Material waste
- Total cost (time/money/energy)
Here are our options
The options for walls were as follows:
1. Pacific Homes 2Ã—6 Smart Wall with 4 inches of rigid foam (EPS) on the outside.
This option was appealing due to its low cost and simplicity. However that simplicity quickly got tossed as we are quite certain that we want to use cedar siding on the house. Trying to secure cedar siding to foam is not possible without significant strapping and labor to ensure everything is kept in place. This would work for stucco, but it would not be ideal for us. The wall system has an R-value of about R-45. Airtightness might be less than other options given that this is really a single-wall system.
2. Pacific Homes 2Ã—6 Smart Wall with offset 2Ã—4 standard wall at 16 inches o.c.
This is a simple option as well. The outer wall would be prefabricated and shipped. Given the prefabrication, the house could be framed with roof, windows, and doors installed in two weeks (the same as Option #1). The interior framing could be done after, and standard batts could be used inside. The good thing about this (compared to Option #1) is that the plumbing and electrical would not pass through the 2Ã—6 outer wall, thereby eliminating potential air leakage. The cost of this one was quoted at about $5,000 more than Option #1, due to the extra 2x4s and insulation batts. R-value for this option was R-41.
3. Fourteen-inch-thick ICFs.
ICFs use expanded polystyrene (EPS) forms with concrete poured into them. The system is appealing for a few reasons: no thermal bridging, super-strong walls (providing disaster protection), good R-value (R-48), and good airtightness (except in the corners and around openings, which of course need to be sealed as with any other system). ICFs are a bit controversial, though, and quite a bit more costly in terms of time, money, and energy. It is a labor-intensive approach and we simply did not think it would be worth it in our case.
4. Eight-inch SIPs with 2Ã—4 standard wall at 24 inches o.c.
SIPs are touted as an energy-conscientious option that can be installed extremely quickly. A 2000-square-foot house can be erected in two days. SIPs use two sheets of OSB laminated to a slab of EPS foam. The panels are very strong and do not require further framing. The R-value of an 8-inch wall is R-33, so combined with a 2Ã—4 wall at 24 inches o.c. you get R-44. I thought this would be a pretty excellent option. SIPs are only marginally more expensive then a standard wall system (and when you factor in the reduced labor cost, the difference is negligible) and are quite a bit less than ICFs. Unfortunately, SIPs have been found to have some pretty serious problems with moisture buildup, airtightness problems, and early decay. None of those sounded good to me. Sorry, SIPs — not for us.
5. And the winner: the Deep Wall System
One of the engineers on our team had worked with a group from Edmonton, Alberta, who utilized the “Deep Wall System.” This wall assembly was used on the Riverdale NetZero house in Edmonton. I had never heard about this, but was intrigued.
Essentially, this system uses two parallel stud walls: a 2Ã—4 wall 16 inches o.c. on the outside and a 2Ã—4 wall 24 inches o.c. on the inside. A 3/8-inch-thick piece of OSB is cut 16 inches inch wide for the top plate and bottom plate. The 2x4s are spaced and secured to the plates with a 3/8-inch OSB sheet on the outer wall. Essentially you make a box with a mesh on the inside.
The 16-inch cavity is filled with blown-in high-density cellulose. This gives an incredible R-value of 56! As if that’s not impressive enough, the material cost of building the wall is about the same as a standard 2Ã—6 wall (not including cost of extra labor time for framing, mind you). The airtightness on the Riverdale NetZero house was 0.59 ach50 and at the Mill Creek NetZero house was 0.36 ach50. Amazing.
As I later found out, Rob Dumont — the energy guru and one of the creators of the Saskatchewan Conservation House (the house that inspired Wolfgang Feist and led to the German Passivhaus Institut) — developed and used this exact method for his house in Saskatoon. He built his house in 1992 and at the time it was considered to be the most well-insulated house in the world. The airtightness was tested at an incredible 0.47 ach50. Why didn’t they just tell us that in the first place!? There would have been no decision-making necessary. We would have just done what he did. We will still have a company cut all of the lumber to size and ship it as a package. Although this system will take a bit more time to complete (due to framing labor), the advantages of this wall system for us far exceeded the other options.
I’m super-excited about our superinsulated and locally developed wall system.