Envelope: Kawartha Highlands’ Hurdles

As an architecture firm dedicated to sustainable residential design, we can’t strive for only ‘passable’.

Every project we design must be absolutely perfect, and not only aesthetically -We fill tiny leaks and holes that might be invisible to the naked eye, we source the best, most effective, and in many cases, most environmentally conscious materials that might be forever hidden under drywall, cladding, or paint. But, as a small firm with lofty goals, every house must set a precedent. Even with especially difficult builds, our projects’ energy efficient goals make it near impossible to take an easy way out or stick to much less complicated, conventional building practices.

In an ideal world, our whole team would all be equipped with the same expertise.

In an ideal world, building an effective double-stud wall would be in every builder’s repertoire. But unfortunately, sustainable building strategies have not completely crossed over into mainstream construction. Plus, a Net-Zero home is an unforgiving one – all aspects of design and construction must be meticulously executed. Even to a builder well-versed in sustainable houses, a double-stud wall construction is ultra-challenging, especially if they’ve never done it before!

As you might recall from Kawartha-Highlands’ last blog post, the structure’s outer frame was fairly conventional, which made construction simple and straightforward – everything from its concrete footings and 6-inch thick foundation walls to its 2×6 exterior wooden stud walls was cut-and-dry.

On the exterior of our 2×6 stud walls went our OSB sheathing, which was then lined with Tyvek’s HomeWrap, a vapour-open weather protection sheet which protects from air and water, yet lets vapour escape which prevents mould and rot in our walls. On the exterior of our Tyvek HomeWrap, we installed vertical wood strapping, which our final wall cladding, steel siding, was affixed to, running perpendicular to our strapping.

On the interior our 2×6 stud walls was 7.5 inches of space, filled with dense pack cellulose insulation. Our insulation amounted to 13” in total – 5.5” in between our 2×6 studs, and the 7.5” of uninterrupted space between stud walls. This gave us R60 walls.

Here’s where we hit some serious turbulence.

On the interior of the insulation, a peel and stick air barrier, our BlueSkin VP100, was supposed to be affixed, between our insulation and OSB sheathing that came before our interior 2×4 stud walls. In order to be truly airtight, the air barrier needs to be seamless, around the entire house – so that includes affixing and sealing the air barrier around the foundations, ceiling, and around each window and door frame, to create one singular, continuous membrane. These areas are so important to focus on because they’re the most difficult spots to ensure an airtight seal, because of the irregular shapes of corners and crevices.

In a stick frame house, the air barrier is usually placed overtop of the house’s sheathing. However, in a house with this much insulation, in order to be properly effective, the air barrier should be placed much further inside the whole structure. That way, if there’s any air leakage, it will be less likely to create condensation, which can become trapped within the insulation and cause mould and rot. Condensation, a serious problem when found within house walls, arises when warm interior air quickly escapes into much colder outside air. If the air barrier was placed closer to or on the house’s exterior, air escaping outwards through leaks would hit the cold air barrier and instantly turn to condensation and start to rot out the walls.

As we’ve said before, air-tightness and insulation is absolutely critical for a well-performing house.

The more air that escapes a house, the more energy it must use in order to keep the interior livable, by heating, cooling, humidifying, dehumidifying… the list goes on. Plus, if an air barrier isn’t continuous , it is basically useless. Like a parka in the winter – if you use it unzipped and open, it really isn’t able to do it’s job, no matter how downy or puffy it is.

Both of these facts makes our air barrier an extremely vital element of our house’s construction. However, without any prior knowledge or expertise in energy efficient design, one can’t truly understand just how important a structure’s air barrier really is to the overall design and performance of the house.

Not only does BlueSkin have to be applied fastidiously to walls, corners and door and window frames, but BlueSkin also strongly recommends that their air barrier is fastened with a primer in cold weather, or else it won’t adhere properly. This phase of construction was taking place in the winter, and since our house was to be net-zero, this wasn’t just a recommendation for our build – it was an absolute necessity. Our air barrier needed to be as strong and airtight as possible.

However, when we came to our first blower door test, after our air barrier, slab, and doors and windows were installed, we were shocked at the numbers – and not in a good way.

They were much worse than we had expected – coming in at 4ACH. Our air barrier test was done by our trusted Greg Labbé, of BlueGreen Consulting Group. With his expertise in measuring air tightness, and our deep understanding of the home’s comprehensive design, we were able to assess the situation and fully clarify the problem. Together, we came to the conclusion that the air barrier wasn’t properly attached. We suspected that it had been installed without primer; there was no other possible reason the house was performing so poorly. Our design had been perfectly calculated, planned and laid out. With the air barrier unattached, it was blowing freely within the cavity, not properly secured to the walls or the other elements of the air barrier like the foundation, walls, ceiling, and window and door frames, and letting in huge amounts of outside air. Plus, due to our building’s sequencing and structure, the air barrier is difficult to access, making it extremely hard to access and fix.

So, we did the only thing we could do: not panic. We began to brainstorm.

Hypothetically speaking, once our cellulose insulation was packed in, the pressure could push our air barrier back into place, but we didn’t want to count on it. With our knowledge and experience, we know that it’s never too late to make a good air barrier! So we decided to cut our losses and abandon the Blueskin air barrier, opting for a new, clever, yet unconventional plan: to create a new air barrier on the interior side of the interior wall.

We turned our 6 millimetre polyethylene sheet (which was already acting as our vapour barrier) into our air barrier. Then, we made our builder seal every single stud with acoustical sealant, from top to bottom. We made sure all our doors and window sills were sealed, and where our studs met our floors and ceilings. We did the same thing in our attic, where our OSB (which was supporting our blown-in cellulose insulation) was taped to the underside of the ceiling. We made our builder execute it exactly to our standards, regardless of how tedious or difficult the process of fixing it was. With our understanding of this project’s building practices and our builder’ co-operation, we began to slowly get this project back on track.


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