Envelope: Frontenac House’s “Perfect” Walls

Frontenac House, our small one-storey build on a rural lakeside property, went up so quickly it was as though it was built by magic.

Our builder, Michael McGonegal, as well as our two clients, were eager to get this house up and running, and made the process look so easy! But in reality, building this house took a lot of hard work, quick thinking, and careful execution.

This house was a great exercise in balance.

Because of the property’s rural, off-the-beaten-path location, we had to be flexible and creative with certain choices. Some materials we were set on using were unavailable in the area, which meant quick unexpected decisions needed to be made. On the other hand, other aspects of the house had to be immaculately planned and followed out exactly, like the placement of toilets, showers, and sinks, which had to be measured out on the property before the house was in order to correspond to the drainage systems being installed in the engineered fill laid on top of the bedrock, far before the structure was even built! But balancing imaginative creativity and meticulous measurements is really what being an architect is all about. Thankfully, the basics of our structure, envelope, and insulation, are a bit more straightforward!

In our last blog post on Frontenac house, I talked about its conventional stick-frame construction, its slightly less conventional wood framed floor, and its contemporary “flat” roof. This post, I’ll fill in the blanks of the structure with insulation and talk about its “Perfect Wall” assembly.

Frontenac House was designed to be incredibly airtight and incredibly well insulated.

Our air-tightness comes from the innovative ZIP System wall sheathing by Huber Engineered Woods, a plywood sheathing with an airtight rubberized coating and a built-in vapour permeable water barrier. We were set on using ZIP System wall panels, but were concerned it would be difficult to find in the rural area, like many of the other materials the building had been designed to use. Huber’s ZIP System is made in the US, and their products are only just beginning to make their way north of the border, so we were thrilled when we found out Michael had used it before and knew exactly where to source it.

These sheets of ZIP System wall sheathing simply have to be taped together to create a seamless, airtight sheathing fastened onto the structure’s wooden studs. ZIP System sheathing is air-closed, but vapour open — it’s airtight, but it “breathes” in terms of vapour.  Vapour is controlled by the polyethylene sheet stapled to the inside face of our batt insulation. After our ZIP wall sheathing structure was assembled, we then insulated both within the wood studs as well as on the exterior of the structure.

We used 2 inches of Roxul mineral wool rigid comfort board as our exterior insulation, and 5.5 inches of mineral wool comfort batt for our insulation set between the wooden studs. We’ve mentioned before how much we like Roxul Mineral Wool; it’s much less damaging to the environment than other forms of insulation like spray foam, it’s made of completely recycled materials from the mining industry, it has no polluting aspects and is healthy and easy to install. We are always happy to use it, and this time was no exception! All this mineral wool gave us an R30 wall.

Joe Lstiburbek aptly calls this type of advanced air-tight framing with exterior insulation the “Perfect Wall”, which has all four principle control layers (a rain control layer, an air control layer, a vapour control layer, and a thermal control layer) on the exterior of the structure, all capped with an aesthetically pleasing cladding whose only jobs (besides looking nice) are to act as a rain screen and an ultra-violet screen. We pretty much followed a perfect wall assembly exactly, with our ZipWall sheathing as our air barrier, our interior and exterior insulation as thermal control, and the polyethylene sheet as our vapour control.

Think of putting on a fluffy parka when it’s cold outside – though that’s hard to imagine this time of year!

Where the ZipWall is the airtight layer of the parka, the outer shell, all our insulation acts as the fluffy down that the parka is filled with.

Our “Perfect Roof” is quite similar to our “Perfect Walls”, just turned horizontal. Our roof joists are covered with sheathing, topped with a fully adhered tight vapour and air closed membrane and exterior mineral wool insulation right on top of the flat roof structure. The only difference between our roof and walls was our interior insulations; Instead of Roxul batts, we opted for a vapour open, 1/2 lb open-cell foam sprayed between roof joists.

Insulating the roof structure’s exterior had one other great benefit. As we said last Frontenac blog post, no “flat” roof is truly flat. All flat roofs need a slight slope, to help snow, rain, and leaves on their descent to the ground. And our roof’s slight slope wasn’t part of the roof’s structure — it came from our exterior insulation. We used Roxul Batts which were custom cut for the roof and came with a built in slope. This tapered mineral wool took out the guesswork of our “flat” roof.

Finally, our roof was to be topped with modified bitumen – or so was the plan. A mod-bit roof, comprised of black tar sheets which are torched together, is pretty customary for flat roofs. It’s fairly inexpensive and airtight. However, we weren’t particularly thrilled with having to use mod-bit, because the only colours you can get it in are black and dark grey, and both colours are great at absorbing heat all year round, which is a real problem in the summer. However, due to our budget, it seemed like the only option.

However, our builder was unable to find anyone in the area that could install a mod-bit roof, which turned out to be a blessing in disguise. After brainstorming, we came up with an alternative that was even less expensive and even more beneficial to the structure than mod-bit!

We settled on a TPO (Thermoplastic Polyolefin) roof. TPO is a single-ply, reflective roofing membrane which, instead of being torched together, is polymerized (or chemically bonded) together. TPO has pretty similar characteristics to mod-bit, but with one huge advantage – it’s white instead of black. As white reflects light and heat, our TPO roof does a great job in bouncing heat off the roof, so our insulation has to do slightly less work. Our mod-bit crisis actually gave us an even better roof than we had planned for.

Next in this house is discussing its extremely efficient systems – then the interiors, and then move-in! It’s hard to believe it’s happening so fast.

2 Comments

  1. Steve Chace May 5, 2017 Reply

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    • Author
      Solares May 26, 2017 Reply

      Thanks so much for the feedback, Steve! We’re glad you’re enjoying our posts!

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