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Flatrock Passive: Framing and Air Sealing

Exterior walls are framed with 2x8s and insulated with fiberglass batts and exterior foam

Exterior walls at Flatrock Passive are framed with 2x8s on 24-inch centers, insulated with a combination of fiberglass batts and rigid EPS foam. An interior 2x4 service wall also is insulated with fiberglass.
Image Credit: All photos: David Goodyear
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Exterior walls at Flatrock Passive are framed with 2x8s on 24-inch centers, insulated with a combination of fiberglass batts and rigid EPS foam. An interior 2x4 service wall also is insulated with fiberglass.
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.)

The sub-slab vapor barrier runs beneath the bottom plate of the exterior wall and the area is air sealed with butyl flashing tape and acoustical sealant.

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.


  1. Reid Baldwin | | #1

    Winter moisture issue?
    I am trying to reason through whether there is a winter moisture accumulation concern with this wall design. With 3" of EPS and 11" of fiberglass, I suspect that the interior surface of the EPS will be colder than the interior dewpoint much of the winter. That would concern me if I were building this, but I don't trust my building science knowledge enough to tell someone else that it is a problem. Does the lack of wood at that surface make it ok? Does the OSB sufficiently slow interior moisture from reaching that area? Is EPS permeable enough to make it ok?

  2. GBA Editor
    Martin Holladay | | #2

    Response to Reid Baldwin
    You raise an important issue. Let's look at the facts.

    First, what's the climate zone? Most maps put Newfoundland in Climate Zone 7.

    Second, what is the usual recommendation for this type of wall in Zone 7? As I explained in one of my articles on the topic (Combining Exterior Rigid Foam With Fluffy Insulation), the exterior rigid foam layer for this type of wall in Zone 7 should have an R-value equal to at least 43% of the total R-value of the wall.

    What's the situation at the Flatrock Passive House?

    Total wall R-value is R-49.88.
    Three inches of Type I EPS might have an R-value as high as R-13.
    That means that the R-value of the EPS layer represents 26% of the total R-value of the wall -- significantly less than the 43% minimum that is recommended.

    The risk is somewhat lessened by two factors:
    1. The interior OSB is a vapor retarder that limits outward vapor drive during the winter.
    2. David Goodyear is the type of homeowner who will probably do his best to avoid high indoor relative humidity levels during the winter.

    When we make decisions on wall assembly specifications, we are always balancing costs with risks. This wall is somewhat risky. I like to err on the side of caution, so I wouldn't recommend this type of wall.

  3. Expert Member
    Dana Dorsett | | #3

    Most people desiging Passive Houses simulate the assemblies
    In the right hands WUFI is a useful tool for figuring this stuff out. As noted:

    *OSB is a Class-II vapor retarder when dry, and is a fairly powerful protection if the wall is air-tight.

    *EPS gains performance at colder temperatures- it's R-value at outdoor temperatures that matter is significantly higher than the 75F mean temp tested & labeled R, reducing the number of condensing hours.

    *Type-I EPS (the density used in this assembly) is still a Class-III vapor retarder at 3", with more than 2x the vapor permeance of the OSB sheathed interior. With a rainscreen construction it can dry to the exterior.

    I wouldn't be at all surprised if a WUFI sim showed the risk to be quite low.

  4. DAVID GOODYEAR | | #4

    Winter moisture issue?
    Hi Reid,

    Hygrothermal modelling show this wall to perform well in our climate (Martin pointed out zone 7, but the eastern part of Newfoundland is in zone 6). My understanding is that it performs well with Type I EPS when compared against type II, due to higher permeability and therefore higher outward drying potential. The blower door test came in significantly less than the standard so I expect that bulk air carrying moisture into the exterior wall cavity will not be an issue especially under conditions of balanced ventilation. Under current circumstances, I expect that the effect of vapor drive, being significantly less than that of bulk air carrying moisture, to not really be a problem in the winter. In addition, interior relative humidity during the winter is often uncomfortably low ie 20%-30% depending on ventilation rates, etc. which further decreases risk again.

    One of my mandates during this build was being able to source local methods and materials. I really wanted to do a dense pack cellulose wall without rigid foam but lack of resources (ie no installers) made the original wall system impossible. I did raise questions about this wall when we decided to move in this direction. I was assured that this wall system was previously modelled and has already been used in Nova Scotia (Also zone 6) for some time. Surprisingly, although our winters are longer, their winters are colder and temperatures actually fall below our local winter averages...thanks to being surrounded by the Atlantic ocean and our proximity to the Gulf Stream. If anything, I would expect the wall to be riskier in Nova Scotia, and riskier again in places above zone 6 as Martin pointed out.

    Although the recommendations shown by Martin illustrate that the wall doesn't meet the percentages (for zone 6 I believe its around 35%), hygrothermal modelling and reality seem to indicate that the wall performs well.

    This does, however, have huge implications to the homebuilder: Don't try to build walls with beefed up insulation without some sort of climate based modelling, or a set of prescribed rules that work. You could create a masterpiece that leads to catastrophe.

    Thanks Martin for chiming in!

  5. DAVID GOODYEAR | | #5

    Most people desiging Passive Houses simulate the assemblies
    Dana, you are right. WUFI showed there was little risk with this wall. I have not simulated this myself but the designers confirmed that the wall system performs well with Type I eps and a 3/4" rainscreen.

  6. davorradman | | #6

    Spruce cladding question
    I find wooden cladding beautiful, but it's prohibitively expensive where I live, in central Europe.
    But, only Arch and other even more expensive woods are used for this.

    So I am intrigued by the spruce cladding.

    Is the American spruce different variety than European? or is it just better because of the conditions it has grown in (colder climate)?

  7. GBA Editor
    Martin Holladay | | #7

    A good discussion
    Thanks to everyone who contributed further information to this discussion. I don't doubt the statements and conclusions shared by David Goodyear.

    When I give advice here on GBA, I tend to be conservative. It's really important to encourage GBA readers to choose robust wall assemblies with low risk -- in part because it's hard to anticipate possible job site errors, and in part because future occupants may operate a house at high levels of indoor relative humidity.

    One option for advice-givers (people like me) is to say, "Well, you can always perform a WUFI simulation." But the majority of WUFI simulations, unfortunately, are garbage -- because WUFI simulations require lots of training and experience to get right.

    I don't doubt that the Passive House consultants hired by David Goodyear know what they are doing. But GBA readers should be cautious before adopting the "just do a WUFI simulation" approach.

    For more on this topic, see WUFI Is Driving Me Crazy.

  8. GBA Editor
    Martin Holladay | | #8

    Response to Davor Radman (Comment #6)
    I don't know whether the spruce species sold in Europe have similar characteristics to the spruce species sold in the U.S.

    In North America, several spruce species are used for lumber, including red spruce, black spruce, white spruce, and Engelmann spruce. Each of these species has somewhat different characteristics.

    For exterior use, either red cedar or white cedar would be considered to be more rot-resistant that most spruce species. That said, plenty of builders install spruce siding. The key to siding longevity is a high foundation (to reduce splashback), wide roof overhangs, and a ventilated rainscreen gap between the siding and the WRB.

  9. DAVID GOODYEAR | | #9

    Response to Davor Radman
    the species in NL is black spruce. Due to a short, cool growing season, it is a relatively slow growing tree with tight rings. It is the only species sawn locally and really the only wood siding used. It is fairly dense compared to other softwoods and rates fairly high on the janka scale. not sure how rot resistant it is but I know it maintains color and looks beautiful for many years under the right conditions and needs very little maintenance despite what people think.

  10. Jon_R | | #10

    Diffusion ratio
    As others point out, the wall has more water vapor diffusion to the exterior than from the interior. This offsets the increased risk from less than recommended levels of exterior foam. How much offset is unclear - there are cost and build-ability benefits if someone comes up with a simple guideline (ie, either not WUFI or a simple, conservative, verified WUFI model). Somehow this was possible for external foam as a % of total R, despite issues like micro-climates.

    It should be clear that unfaced EPS on the exterior outperforms foil faced polyiso at the same actual R value (cold climate).

    My guess is that building pressure (effecting exfiltration) also (in addition to indoor humidity) plays a large role. Ie, slightly depressurize all Winter and some normally risky walls would work well.

  11. Reid Baldwin | | #11

    Thanks for the explanations
    I suspected that this might be a case where the assumptions that go into the minimum foam thickness recommendations didn't apply for some reason. Thank you for educating me, and anyone else that reads the comments, about what those reasons are in this instance.

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