A Vapour Barrier? That’s Smart

While installing a vapour barrier is smart, it’s also a requirement of the building code – and for good reason!  Without air-tightness, all that insulation is for nothing, at least in terms of energy conservation.  A drafty wall assembly is also an invitation for moisture buildup from condensation, which leads to a drastically reduced efficacy of insulation, mold growth and rot.

In a Passive House, the vapour barrier has the same job, and is installed in the same place (on the warm side of the wall assembly), but with three subtle, yet  important differences.  The first is detail; we’re going for a very tight building envelope, and achieving 0.6 ACH @50 Pa (or better!) requires a very high level of detail in terms of planning and installation.  The second and third are permeability and strength, hence the title of this post, but with a slight change in punctuation:  “A vapour barrier that’s smart“.

Raised Heel Trusses

Raised Heel Trusses

As the picture shows, we have specified an engineered truss design with a rather substantial 32″ raised heel at the wall. A “regular” truss would allow for about 6-8 inches of insulation at this location. We’re looking for full-depth insulation everywhere.  That amount of insulation (we’re blowing in cellulose) introduces a couple challenges, namely mass and reduced airflow.  This is where smart vapour barriers outperform the standard 6-mil poly. We’re using a product which has 2 layers;  the first is a fabric which greatly improves its strength, and the second is a membrane.  Limiting airflow through insulation is what makes it effective.  With this much insulation, it also means a reduced ability to dry out, should moisture be present.  It’s not that we’re planning for a leaky roof – moisture will always be a factor; take a look in your attic after a snow-storm with winds.  Vents will allow snow into the attic and when it melts, evaporation is how it dries. With 32″ of insulation there’s very little airflow, so we’re relying on a smart membrane to assist. What makes the membrane smart, is its ability to become vapour-open in the presence of moisture.

Specialized tapes for specific purposes

Specialized tapes for specific purposes

The fabric layer is required to resist stretching under the weight of all that insulation.  We were also concerned about the possibility of staples being ripped out over time, creating holes in the vapour barrier.  To that end, we decided to use a super-sticky reinforced double-sided tape in place of staples. Should the weight of the insulation force the tape to let go, we won’t have to worry about a thousand holes in the membrane.

At this point, we’d normally install metal resilient-channel to the underside of the trusses.  This gives us something to attach drywall to at 16″ O.C. (trusses are typically 24″ O.C.) and helps protect the drywall joints from truss-lift in the dry season.  We chose to do things a little differently; in place of resilient-channel, we installed 2×3 on its edge.  This is much stronger (better support for the vapour barrier and insulation) and provides a 2.5″ deep service cavity for all the electrical work.  Our electricians did not poke a single hole in the vapour barrier.  The plumber is another story, but that’s not his fault; in Ontario we’re not allowed to use air-admittance-valves (aka “cheater-vents”) in new construction.

Placing gaskets to prevent V-B leaks

Placing gaskets to prevent V-B leaks

We went a little O.C.D. on the 2×3… to ensure the long screws didn’t result in leakage through the vapour barrier, we chalked lines and placed foam gasket tape at all intersections of the 2×3 and trusses.  The scaffold took on a new “look” with all those gaskets stuck to it. With one person pushing the scaffold and two people placing, it was done in no time.

2x3 service cavity up, walls in place

2×3 service cavity up, walls in place

Installing the 2×3 also went well with teams of two.  We chose to pre-drill the 2×3 so the wood screws had more truss to bite into, drawing the 2×3 in and compressing the gaskets – no leakage there!

We were also careful to eliminate leakage potential around the perimeter.  This being a SIP house, the inside OSB sheathing is our vapour barrier for outside walls.  Where the walls and ceiling V-B meet we applied a generous bead of polyurethane caulk and then nailed strapping to the wall (through the calking bead) to support the weight of insulation.

This method is strong and air-tight, but means we’ll have to be careful when screwing drywall to the 2×3… keeping screws to a minimum where the ceiling meets internal walls.  18 inches should do the trick – allowing the ceiling to flex a bit at those points.

V-B is calked and strapped to the walls

V-B is calked and strapped to the walls

Curved walls? A Passive House deserves architectural details too!

Curved walls? A Passive House deserves architectural details too!


Window Dressing

After a few hiccups with our supplier, we finally have all the windows installed.  It’s amazing how much of a difference that makes to the environment inside the house. I love fresh air, but when it’s blowing the plan-set all over the living room, there can be too much of a good thing.

installed - awaiting detailing

installed – awaiting detailing

The installation process went well. Removing an operator here and there helped lighten the load a bit;  tri-pane windows are noticeably more heavy than their double-pane counterparts.  Installation started out the same as any other house… a level, some cedar shims, fasteners and special low-expansion foam (which remains flexible – this is important!).  The first photo shows an installed window in the SIP wall assembly.  What you can’t see (translation: what I forgot to take a picture of) is that each window opening was first detailed with a trowel-on elastomeric water-proof membrane, also called the “Weather-Resistive Barrier” or WRB.  The most common WRB’s are of the “house-wrap” variety, of which Typar and Tyvek are the most popular. The main detriment to wrapping a WRB around a window/door opening is that it makes it difficult to air-seal on both sides of the membrane.  With a trowel-on membrane, it is sealed to the substrate (frame opening) when applied and the window is subsequently sealed to it; far less opportunity for air leakage.  The other detail missing is the outermost 2×4 window buck hiding just behind the inside edge of the window frame. 2013-11-15 11.11.40

Though not significant, that 2×4 does represent a thermal bridge in the window installation so we chose to isolate it by wrapping the interior of each window opening with polyiso insulation.  Two beads of spray-foam and a bead of calking ensure it also becomes part of the vapour-barrier continuation.

Air-sealing tape applied

Air-sealing tape applied

Finally, an application of air-sealing tape between the window frame and the foil-faced polyiso completes the air-tight seal.

These pictures also give you a glimpse of the 2×4 service partition. That partition is currently being well-utilized by the electrical, plumbing and HVAC subcontractors, and we’re happy to see that, knowing the alternative would be to carve holes and channels into the OSB vapour-barrier.  We have chamfered the sides and top of the service partition around window openings as an architectural detail; this also improves the field of view and the amount of light allowed to enter the rooms.

Sustainable Kingston Event – Nov 14th

Sustainable Kingston (http://www.sustainablekingston.ca) is hosting a pair of evening talks, each one featuring a keynote address from a well-known author, and an opening presentation from a local Sustainable Kingston Community Partner.  The first event is this evening, and I’m quite excited to be presenting on the topic of Passive House and an overview of this build.  Here are the details:


Sustainable Kingston, in partnership with the City of Kingston, is excited to announce the first two speakers in the upcoming Sustainable Kingston Speaker Series. Pamela Blais, author of Perverse Cities, and Tom Rand, author of Kick the Fossil Fuel Habit, will be featured in two events held in November. Each event will start with a talk by a local sustainability champion who will highlight the great work being done in our own backyard. Come early to mix and mingle with other community groups who will have booths set up, including the Loving Spoonful, Wintergreen Renewable Energy, and more. This speaker series is another community action showing Sustainable Kingston’s commitment to the implementation of the Sustainable Kingston Plan.THURSDAY NOVEMBER 14th –

Opening Speaker, Local Sustainability Champion: Chris VanderZwan of New Leaf Custom Homes will be speaking on innovation, building Kingston’s “Passive House” (PassivHaus), and climate change.

Keynote Speaker: Pamela Blais, the author of Perverse Cities, makes the case that accurate pricing and better policy are fundamental to curbing sprawl and shows how this can be achieved in practice through a range of market-oriented tools that promote efficient, sustainable cities.


Doors open at 6:45PM, event runs from 7PM – 9PM.

Event will be held at Portsmouth Olympic Harbour, 53 Yonge Street, Kingston, Ontario.

Event is free. Please RSVP to info@sustainablekingston.ca or to 613-544-2075 x 3 so that we can determine attendance and appropriate room set up.


Hosted by Sustainable Kingston