LEED Platinum V4 Wall Design: Building Science
The wall we designed for the Edelweiss house, Canada's first LEED Platinum V4 Home is best categorized as a 'REMOTE' wall, a high performance building envelope developed by the Cold Climate Housing Research Centre in Alaska. REMOTE stands for 'Residential Exterior Membrane Outside-insulation TEchnique'.
Remote walls are framed the way you would be used to seeing it done with 2x6s and sheathing; the difference is that vapour and air control layers are moved to the exterior of the sheathing, then additional insulation is installed outside of that.
Stud cavities are still insulated with batts, but the majority of insulation is on the outside of the sheathing, this way the vapour control layer is sandwiched between layers of insulation where it can perform best in a wildly varying climate like we have in Canada.
There are many advantages to this system - it provides much greater thermal protection and energy efficiency, better drying capabilities and protects the air barrier from the assault it would normally receive from wiring and other penetrations when we rely on an interior polyethylene as an air and vapour barrier. See our pages on vapour and air movement & the difference between air & vapor barriers to get a better idea of how this works, and why it makes so much sense to build this way in cold climates.
1. After the concrete slab had sufficiently set, we began wall framing. We were provided with detailed framing plans courtesy of Kott Lumber, with each wall planned down to the smallest framing member, including a cut list with the length of each piece. This made the framing process go fast and efficiently and reduced our waste, as the wood order matched the requirements of the walls.
Kott also provides Chain of Custody documentation for FSC certified wood (Forest Stewardship Council). FSC wood is guaranteed to come from sustainably-managed forests and offers additional LEED points.
2. Sheathing was installed leaving a 1/8th inch gap at the seams to provide additional drying capability of the walls along with providing room for sheathing to expand without buckling. This protects our exterior air barrier membrane (Delta Vent SA) and prevents any risk of walls warping.
The weak points in any air barrier are the penetrations that come with attaching it, or attaching other components to it. At the recommendation of Cosella-Dorken technicians we stuck on a small piece of Delta Flexx-Band (black squares) which will self-seal the holes where screws from strapping would puncture the Vent SA.
3. Window openings were built out to allow for the additional exterior insulation. The Delta air barrier was wrapped around the entire window box and into the interior.
4. Trusses were dropped on top after walls were completed. The trusses we needed for this house were enormous by most standards, for a number of reasons: the long span from north to south (35 feet), the shallow roof pitch (2/12), and the added weight that would be coming in the form of soil for our live roof (green roof). This gave us a deep cavity for insulation, with Roxul batts totalling R95.
5. The height of the south wall along with the large openings to facilitate natural light and passive solar heat gain meant the headers we needed were massive, making these walls absolutely un-liftable by mere humans, so we enlisted the help of a boom truck to get them standing.
Kott lumber also engineered the walls, so it was quite handy to have that done by the same company that would provide the necessary lumber.
The Ecohome R.E.M.O.T.E. wall
The difference here compared to what is commonly done with the REMOTE wall technique is that we used Roxul ComfortBoard IS instead of polystyrene rigid insulation (EPS or XPS - see comparisons here) .
We chose this option for a variety of reasons. Roxul has high embodied energy, but the final product is effectively inert, with characteristics that will remain unchanged with the passing of time. This is not the case with polystyrene, which over time will off-gas its blowing agents and cause material shrinkage, reduced thermal performance and the release of climate changing greenhouse gases. Read more here about how to choose the right rigid insulation board.
Furthermore, Roxul, or now Rockwool, contains no fire retardant chemicals such brome-based HBCD (Hexabromocyclododecane, spelling test to follow), a persistent bio-accumulative chemical found in EPS and XPS that is connected to a disturbing amount of human health issues.
Avoiding thermal bridges:
When one section of a wall conducts heat much quicker than another, we refer to it as a thermal bridge, and that can greatly reduce the overall R value of a wall. Virtually every thermal bridge in this wall assembly was identified and eliminated in the design phase.
Thermal breaks in exterior insulation:
It would have been quicker for us to attach the entire mass of exterior insulation with longer screws and only one course of strapping, but using THERM software, our engineer determined that the cold transmitted through the metal screws would have reduced the effective R-value of the exterior insulation by about 1/3. Double strapping and screws that don't span the entire wall eliminates that potential thermal bridge without having to invest in expensive insulated attachments.
Something we often hear when discussing this house, is that this sure is a lot of insulation. Thanks, we agree! It certainly can look like overkill when compared to the under-insulate homes that are typically built, as housing developers rely on building code minimum standards to determine efficiency. As performance isn't always high on the radar of home buyers, builders make more money that way, but leave homeowners with unnecessarily high heating bills for the rest of their lives.
In contrast, a hyper-insulated house will save occupants a lot of money over its lifetime, but even on the short term can actually be cheaper to own and operate from the moment you move in, as the added construction costs are offset by greatly reduced operating costs.
In our next post we will outline the reasoning behind this wall design, associated monthly costs, and the concept of the 'sweet spot', which is the optimum level of insulation for a home, proper orientation, and designing for your local climate. There isn't one coat that would serve you well in every part of Canada, houses are the same.
The following diagram details the building envelope design of our LEED V4 Platinum certified Demo House. The drawing is not to scale, but performance values are accurate. Additional note: stud cavities are filled with R24 Roxul mineral wool batts.