Image Credit: All photos: David Goodyear Where an interior structural wall meets the exterior wall, air-sealing must take place before the connection is complete. Here, the author primed surfaces and then applied an air-sealing tape that connects the OSB sheathing with the slab. The dark blue strip is the edge of the sub-slab vapor barrier. Where this interior structural wall meets the exterior wall, air-sealing details had to be taken care of before work could continue. Seams in the OSB sheathing were sealed with acoustical sealant and 3M flashing tape firmly pressed into place with a J-roller. At the base of the wall, the author connected the sheathing with the sill seal under the bottom plate, then added a second layer of sill seal. The weight of the wall will compress the layers and complete the seal. 3M flashing tape seals the OSB air barrier to the top plate. This can't be done after the second-floor joists are set in place. A window buck installed in an exterior wall. The 1 1/2-inch layer of foam behind the 2x4 rim helps reduce thermal bridging in the assembly.
Editor’s Note: This is one of a series of blogs by David Goodyear describing the construction of his new home in Flatrock, Newfoundland, the first in the province built to the Passive House standard. The first installment of the GBA blog series was titled An Introduction to the Flatrock Passive House. For a list of Goodyear’s earlier blogs on this site, see the “Related Articles” sidebar below; you’ll find his complete blog here.
We started framing! We hired a local company, KeoCan, owned by Patrick Keogh. They were keen to take on our job and we were happy to get started. Having a poured floor to work on is a luxury to these guys. Normally when they start framing there is nothing but foundation walls to work on.
The exterior portion of the thermal envelope is pretty conventional: a 2×8 stud wall framed at 24 inches on center. This is as advanced as it gets. Although framers here have lots of conventional experience, there is little to no advanced framing experience in the residential construction industry. This is unfortunate since using fewer studs decreases costs and also saves energy and resources.
The battle in my head on this has been won… and lost. Using my house as an experiment for OVE [optimal value engineering] framing would probably use up more energy in headaches than the energy savings I would gain! I figure it is a win/lose (or lose/win situation) whichever way you look at it.
Seriously though, I took some time to analyze the R-values. The wall is composed of 1/2-inch spruce cladding, a 3/4-inch air space, 3 inches of Type I EPS, 2×8 (24-inch o.c.) stud wall filled with R-31 fiberglass compressed to about R-27, OSB (our air barrier), R-15 fiberglass batts in a 2×4 service wall, and gypsum wall board. The calculated R-value is about 49.88 according to this great little calculator. Changing the exterior 2×8 stud wall to 16-inch o.c. emulates an increase in the stud fraction and gives an R-value of 48.95. The U-factors for these assemblies are 0.0200 and 0.0203 respectively.
Another way to look at this is that heat transfer rate has been reduced by 98% or 97.97% respectively. This illustrates a point. Adding a few studs here and there to keep your framer and the local officials happy is no big deal. It needs to make sense, of course, but adhering to local practices may not be the end of your energy-efficient building. If window headers require double jacks, then double jacks it is… (If there are any questions, you should contact your Passive House consultant.)
The walls on the main level are all 9 feet high. Since 2×8 lumber is not a conventional wall building component, pre-cut lengths aren’t available. This means that 10-foot lengths have to be cut to the proper size. This leads to some waste, but since it’s all kiln-dried softwood it will make for great fuel in my Walltherm wood stove this coming winter. It’s not a great use of lumber, but at least it won’t be thrown in the landfill.
Being in a windy location means that the structure needs bracing, bracing, and more bracing. Windy is normal here. When the sun comes out, it’s windier! The wind speed reached a sustained 50 km/hr (31 mph) yesterday. Today the weather is forecast to be sunny and 26° Celsius (79°F). The wind speed today is expected to gust at 70 km/hr (43 mph) by 6 p.m. Bracing the walls has been a bit of a challenge since there are not very many places to brace from.
Building the air barrier
The typical air barrier in our neck of the woods is polyethylene plastic. Although people call it a vapor barrier, it’s primarily an air barrier. Passive houses also have air barriers and vapor control layers. In our house that air barrier/vapor control layer is 7/16-inch OSB applied to the interior of the 2×8 stud wall.
In a code-built home, the vapor barrier is usually an afterthought, patched in like a quilt after insulation has been completed. Often, there are wall intersections where air sealing would be impossible after the fact. It takes some proactive measures with placing an air barrier behind the stud and sealing appropriately before the interior partition is erected. Air sealing really should be implemented incrementally as the structure is erected. Mindfully determining how one detail will tie to the next is important if the structure is to be airtight (i.e. 0.6 ach50).
On the main level we have one interior structural wall that intersects an exterior wall (its structural purpose is to support the second-story floor system). My air-sealing plan mapped out how to deal with this intersection. The wood surface, slab, and slab vapor barrier were primed with Blueskin primer. A bead of acoustical sealant is laid along the slab and another bead at the intersection of the OSB and the vapor barrier. Blueskin butyl flashing tape is then applied to the slab, rolled back to the corner and up the wall. At the wall intersection, the Blueskin is bedded in a bead of acoustical sealant. (See iIages #2 and #3 below and the drawing below.)
After after placing the Blueskin lightly in place using hand pressure only, we rolled it out under pressure with a rubber J-roller. This stuff has an amazing amount of holding power when used with primer and is impossible to remove, so it was necessary to mark out a starting position on the slab to ensure the Blueskin wouldn’t be seen once baseboard moldings are on the wall. This air-sealing detail seems to be a fairly good redundant system. If the slab were to crack anywhere and compromise the Blueskin seal to the slab, I hope the bead of acoustical sealant will still be intact.
We applied acoustical sealant and 3M tape to the joints in the sheathing, bedding the tape with a roller (see image #4 below).
Insulating the garage
When the house was designed, I decided to beef up the insulation in my garage and porch. Most people park cars in their garage. That’s where I park my tools. I operate a small woodworking shop and do most of my woodworking in the winter months so I wanted a space that was going to be more energy-efficient than my current garage.
This means two things: The structure needed good R-value with minimal thermal bridging, and it had to be somewhat airtight… with the exception of the leaky garage door, of course! So we opted to do the same as the house (i.e. 3 inches of Type I EPS insulation applied with horizontal let-in 2×4 strapping) to mitigate thermal bridging.
Since I would be doing most of the air sealing myself, I decided to use acoustical sealant on all OSB joints and 3M 8067 tape applied over it. This is not hard work: Caulk the joints with acoustical sealant, tape with 3M, and roll under pressure with a J-roller. It’s tedious work, though, mainly because the tape is so tenacious, and it can become quite hard to handle with all the wind! Acoustical sealant is normally messy but with the wind, the strings of acoustical sealant go everywhere, so it was important to release pressure on the caulk gun to minimize dripping after a run of caulking. It took a little over one hour to air seal a 9-foot by 24-foot wall.
Since the sill will act as the primary air seal to the foundation, I wanted it to perform well. I came up with this detail myself. I am hoping that it will outperform using the sill gasket alone. I am using Owens Corning FoamSealR; it’s pretty much the only locally available sill gasket. It is fairly thin (3/16 inch) and if the foundation wall has any unevenness, it hardly seals at all.
At 12 cents a foot, this stuff is cheap, so I figured I’d just use more of it. Doubling it gives it a thickness of 3/8 inch, but the stuff compresses really easily so the weight of a wall will pretty much flatten it. FoamSealR was stapled to the bottom of the pressure-treated wall plate. Then the OSB and gasket are primed with Resisto primer. Resisto Redzone (bitumen-based tape) was then stuck to the OSB and wrapped down onto the sill gasket. This seals the OSB to the bottom of the sill gasket. Another sill gasket is then placed below the previously applied gasket, sandwiching the Resisto Redzone in a sill gasket sandwich (see Image #5 below). The idea is to let gravity do the rest. The weight of the structure will compress the Resisto tape between the gaskets and the gaskets will deal with any uneveness between the walls.
All the garage and porch walls will be finished in the same fashion. I figure the air-sealing details for the garage and porch will cost between $300 and $400, not including the labor to install (which is free if you do it yourself!). This is really pocket change compared to the price of a house, so I figured it made sense to do it.
Window bucks and rim joist air sealing
As described in the previous post, air sealing should be incremental; that is, carried out as we go to ensure the airtight barrier is continuous throughout the whole structure. The next step in the process is to ensure that the rim joist space at the top of the first floor is airtight and can be connected to the interior OSB sheathing. Since the second-floor joists are laid on top of the double top plate of the wall, there needs to be some way to connect the OSB to the top plate and to be able to caulk the space with acoustical sealant.
Once the joists are in place it would be too late to do this. The designer’s construction details for the rim joist space rely on two parts: Acoustical sealant and tape-sealing the OSB to the top plates and then spray foaming the rim joist space. It took a little thought on my part on how to accomplish this. I decided to use 3M 8067 flashing tape. It has a split back, so I was able to tape the top using half the width of the tape while leaving the paper backing on the other half and letting it hang down over the top plate (see Image #6 below).
I also taped any butt joints out to the edge of the plate where the rim joist will sit. I used a J-roller to apply firm pressure to the tape to ensure good adhesion. Once the joists are installed and the house shell is complete, the surface of the rim joist space will be sprayed with foam out over the tape. This will provide a fairly airtight seal.
While the framers were moving along with construction, I figured it would be a good time to map out the window buck construction. I decided to try two different methodologies. One was to frame the buck as a single unit and push it into the window opening. The other was to frame the buck in place. With foam-sheathed walls (3 inches of Type I EPS), the bucks needed to be about 10 1/4 inches deep (the 2×8 stud, plus the 3 inches of foam).
I ripped sheets of 3/4-inch spruce plywood, then crosscut the pieces to fit the narrowest dimensions of the buck. The butt joints of the bucks were nailed and glued, then a 2×4 rim was screwed around the perimeter. The 2×4 rim acts as a backer for nailing the window nailing fin and the Type II EPS (1 1/2 inches thick) was glued to the back of the 2x4s and then the buck was inserted into the opening until the foam was flush to the building (see Image #7 below).
Framing the window buck in place takes about the same amount of time and sometimes requires two sets of hands. My experience, although limited, tells me that framing the window bucks and then inserting them works well for smaller windows. For larger windows, the buck will need to be framed in place.