As mentioned in part 1 of the series this wall detail takes a high vapour permeance or breathable approach in contrast with a more vapour closed system such as traditional 6 mil poly, spray foam insulation, or SIPS panel. In an everyday example it is similar to choosing a breathable Gore-tex or Tyvek jacket over a fully waterproof rubber or latex one.
The sequence of assembley allows for easy access for sealing of the air barrier membrane and the vapour barrier is solid (plywood) avoiding the difficulties in sealing and movement of the traditional 6 mil poly. 1) a 2x4 stud wall is framed on conc. foundation 2) plywood is attached to the stud wall without gaps 3) air barrier membrane is draped over the plywood and sealed at edges with specialized tape 4) two layers of roxul insulation are installed with 5) 2x4 vertical strapping holding the insulation in place 6) fibre cement siding forms the finished cladding 7) Drywall is installed on the inside face of studs
1) 2x4 Stud Wall. The 2x4 stud wall is framed traditionally providing for a common and cost effective installation. 2x4s utilize less wood than the code standard 2x6 while providing adequate strength.
2) Plywood is installed over the 2x4 stud wall and form the vapour barrier and sheathing in a single step. The plywood is sealed at seams with vapour barrier tape. This vapour barrier is much more breathable than the traditional poly and more so than the code requirement. This allows for the wall to dry to the outside and inside should any moisture enter. As a solid sheet product this vapour barrier avoids the difficulties in 6 mil poly construction where care must be taken to ensure proper sealing with sealants, lapping at edges, solid backing at electrical boxes and other penetrations, and post construction movement during pressurization and depressurization due to wind pressure. An inherent property of plywood is its natural tendency to increase its vapour permeance when wetted. This allows it to dry itself out more rapidy if it ever gets wetted through condensation or rain penetration at window openings or other gaps.
3) Siga Majvest (http://www.siga.ch/en/product-overview/majvest-detail.html) is the air barrier membrane which is more vapour open that the code requirement in the BCBC allowing for greater drying capacity. As the air barrier is applied to the outside over plywood the installation is no different that the traditional installation of tar paper or Tyvek. Siga is taped at edges and seams to form a strong air barrier.
4) Roxul exterior semi-rigid insulation is used in two layers. These layers are lapped at edges so there is a lesser chance of gaps between insulation and avenues for water penetration. Roxul is made from rock and slag absorbs less than 1% of water and does not harbour mould according to their website http://www.roxul.com/products/commercial/products/roxul+comfortboard+cis. Partial assembley of the 2x4 strapping allow for install of the Roxul (see construction sequence image).
5) 2x4 borate treated vertical strapping is installed over the rigid Roxul. Long screws penetrate the strapping, plywood, and 1.5" into the studs. The screws are installed traditionally with a common drill & bit and according to the installation crews are easy to install since the majority of the intervening material is Roxul which takes very little effort to penetrate. The only trick is the inability to pull out screws that miss their studs for fear of introducing penetrations to the air and vapour barrier. These screws are left in place.
6) Fibre cement cladding is to be added over top the 2x4 strapping. Nails penetrate the strapping only so no holes to the air & vapour barrier are introduced and reducing any small thermal bridging.
7) Drywall is installed on the inside face of studs as per typical residential construction.
This and similar exterior insulation systems for passive house are considered one of the most durable due to the solid plywood. It has seen application in Alaska as part of the REMOTE wall (http://cchrc.org/docs/best_practices/REMOTE_Manual.pdf) although this system utilizes an exterior 6 mil poly vapour barrier over the plywood and rigid EPS or XPS insulation (vapour closed).
Double wall is another system used for passive house. Typically this system utilizes a double 2x4 wall with a intervening space between the stud walls. This space is filled with insulation (typically blown cellulose) providing the additional insulation value without thermal bridging in the wall. A 6 mil poly vapour is typically used on the inside face of the inside stud wall with plywood or osb backer at electrical boxes and other penetrations. This is a good wall which utilizes conventional skills the trades already have and has a good track record for high performance with proper workmanship. Because the wall cavity is also used for plumbing and electrical runs the poly vapour barrier must be penetrated many times so the key in achieving the air tighness required here is proper sealing at penetrations. There have been questions on durability of the vapour barrier due to post construction use (picture hanging) and pressure differentials dislodging the poly vapour barrier. Nonetheless, this method is tried and true and has achieve net zero designations. The strongest advocate I'm aware of for this technique is Peter Amerongen (http://habitat-studio.com/?page_id=2).
SIPS panels consist of rigid insulation typically EPS/XPS sandwiched by OSB. These form a prefabricated panel which is often delivered onsite and assembled with a crane within a few days. This system is vapour closed due to the rigid insulation and the OSB. It is considered very durable and has a long history especially with Timberframe builders who have used the panels as infill between posts and beams and more currently outside the timbers as a continuous wall. Services can be run within pre-drilled holes in the insulation or within a backup stud service wall. To my knowledge this system works well but is considered more expensive than other systems. This is a good, durable, vapour closed system that has seen a long history use.
Vapour open exterior insulated wall as in the Langley home is described in the Building Enclosure Design Guide (HPO 2011) and Guide for Designing Energy Efficiency Building Enclosures (FPInnovations 2013) both by RDH. The wall design borrows from the North Shore Passive House in construction by the Econ Group (http://www.econgroup.ca/north-shore-passive-house/) and progress photos of the North Shore home are available at http://www.econgroup.ca/north-shore-passive-house/. This system borrows the durability of the REMOTE wall and other exterior insulation wall types common to commercial and multi-family construction. Its' durability comes from the ease of installation on a solid vapour barrier and requires little additional skill from typical construction. It introduces an increased vapour permeance that allows the wall to breathe and dry out while maintaining a very tight envelope. The increased vapour permeance is not represented in the Building Code and results in greater reliance on consultants and their expertise.
Each of the wall types discussed above have a long history in construction in various forms each with their advantages and disadvantages. With the increased need for more energy efficient construction the budding popularity of high-efficiency wall types is certainly a good thing. Time will tell if the single family home will latch onto one of these systems to the exclusion of the others as has generally happened in the past or if it will embrace multiple wall types.