Continuous Exterior Insulation Rain Screen Details
Editor's note: This post is one of a series by Eric Whetzel about the design and construction of his house in Palatine, Illinois, a suburb of Chicago. The first blog in his series was called An Introduction to a New Passive House Project; a list of Eric's previous posts appears below. For more details, see Eric's blog, Kimchi & Kraut.
Continuous insulation and double-stud walls both are options for high-performance walls, but we decided continuous insulation (CI, for short) made the most sense to us. Continuous insulation has its own challenges, especially in terms of air and water sealing details around windows and doors. Still, it intuitively seemed the way to go.
For anyone interested in the performance of various wall assemblies, this paper from the Building Science Corp. is an excellent place to start. Or you can check out Hammer and Hand's evolving wall assembly strategies. And here's a mock-up wall assembly by Hammer and Hand showing many of the details we incorporated into our own house:
While many believe a double stud wall simplifies much of the framing, we decided that a continuous insulation approach, which in theory should better manage seasonal moisture changes inside the walls, was worth the extra effort.
Two continuous layers of mineral wool
Based on the drawings from our original builder, Evolutionary Home Builders, which recommended 3 3/4 inches of rigid foam, and the recommendations of both PHIUS and Green Building Advisor for our Climate Zone 5 location, we decided to go with 4 inches of Rockwool Comfortboard 80 on top of our Zip System sheathing.
In the Chicagoland area it's still a struggle to find builders or subcontractors who are knowledgeable about, or even interested in, "green building." Even though PHIUS has dozens of certified builders and consultants listed for Illinois and the larger Midwest region, it's unclear just how many of them have worked directly on an actual Passive House project.
Until there's more demand from consumers, or the building codes change significantly, it's difficult to imagine the situation improving much in the near future. This is unfortunate since particularly here in the Chicago area, or the Midwest more broadly, homes could really benefit from the Passive House model (or something close to it, e.g. The Pretty Good House concept) to cope with our weather extremes.
Many lookers, few takers
In our own case, when I think of all the individual trades we had to hire, a siding contractor was far and away the most difficult to secure. Because of the level of detail involved before the siding itself could be installed, it was a real challenge even to get quotes. As things turned out, we had nearly 20 contractors (a mix of dedicated siding contractors and carpenters) visit the job site before we received an actual estimate.
Many of those who visited the job site expressed genuine interest, most going so far as to acknowledge that this kind of wall assembly made sense and would probably be mandated by the residential code at some point in the future. But almost without exception they would disappear after leaving the job site—no bid forthcoming, and no response to my follow-up phone calls or emails.
Clearly they were terrified, not without justification, to tackle something so new, viewing our project through a lens of risk rather than as an opportunity to learn something new. From their point of view, why not stick with the type of jobs they've successfully completed hundreds of times in the past? It also didn't help that I was a first-time homeowner/GC, rather than a GC with a long track record of previously built homes in the area.
In addition, not only is continuous insulation over sheathing a novel concept in the Chicago area, especially in residential builds, including a ventilated rainscreen gap behind siding is almost unheard of. Typically, fiber cement lap siding is installed directly over housewrap (this can be observed directly on hundreds of job sites across the city and suburbs).
While there are any number of certified LEED projects in our area, and even some Passive House projects (both residential and commercial), for the most part consumers are still largely unaware of Passive House or other "green" building standards like Living Building Challenge. Clearly "green" building, let alone Passive House, has its work cut out for it here in the Midwest if it ever hopes to have a meaningful impact on the construction industry.
Installing the Rockwool insulation
Mike Conners, from Kenwood Passivhaus, was nice enough to recommend Siding and Window Group, which definitely got us out of a jam. Thankfully, Greg, the owner, was up for the challenge and was nice enough to let us work with two of his best guys, Wojtek and Mark.
Wojtek and Mark dropped off some of their equipment at the site the day before they were to start work on the house. This gave me a chance to go through many of the details with them directly. Although a little apprehensive, they were also curious, asking a lot of questions as they tried to picture how all the elements of the assembly would come together. In addition to the construction drawings, the series of videos from Hammer and Hand on its Madrona Passive House were incredibly helpful.
Also, this video from Pro Trade Craft helped to answer some of the "How do you…?" questions that came up during the design and build phases:
As sophisticated and intricate as some architectural drawings may be, in my experience nothing beats a good job site demonstration video that shows how some newfangled product or process should be properly installed or executed.
On the first day, while Wojtek and Mark installed the Z-flashing between the Zip sheathing and the foundation, along with head flashings above the windows and doors, I started putting up the first pieces of Rockwool over the Zip sheathing.
We found it easier to embed the metal flashings in a bead of Prosoco's Fast Flash. Once in position, an additional bead of Fast Flash went over the face of the flashing, ensuring a water tight connection between the metal and the Zip sheathing.
We installed the first layer of Rockwool horizontally. The second would be oriented vertically. This alternating pattern helps ensure that seams are overlapped.
We didn't worry too much about the orange plastic cap nails missing studs since they were sized to mostly end up in the Zip sheathing. In the end, only a couple of them made it completely through the Zip without hitting a stud.
Before installing the bottom row of Rockwool we used shims to create a slight gap between the Rockwool and the metal Z-flashing on the foundation insulation. This will give any water that ever reaches the Zip sheathing a clear pathway out.
In a pattern that would repeat itself with each layer of the remaining wall assembly, Wojtek and Mark would carefully think through the details as they progressed slowly at first, asking questions as issues arose, before getting the feel for what they were doing and eventually picking up speed as they progressed around each side of the house.
Dealing with door and window openings
Working through the many details with Wojtek and Mark — the majority of which occur at junctions like windows and doors, the top and bottom of the walls, along with mainly outside corners — was both collaborative and deeply gratifying. They demonstrated curiosity and an ability to problem-solve on the fly, and they clearly wanted to do things right, both for me as a customer and for the house as a completed structure (it felt like both aesthetically and in building science terms).
They never hurried over specific problem areas, arrogantly suggesting they knew better, instead they patiently considered unanticipated consequences, potential long-term issues, and actively questioned my assumptions in a positive way that improved the overall quality of the installation. This mixture of curiosity, intelligence, and craftsmanship was a real pleasure to observe and work with.
If a GC built this level of rapport with each subcontractor, I can certainly understand a refusal to work with anyone outside of their core team—it just makes life so much easier, and it makes being on the job site a real pleasure.
For the second layer of Rockwool, Wojtek and Mark tried to hit only studs with the 3-inch Trufast MP-3000 screws. Screwing into the studs with these fasteners, in effect, became a guide for accurately hitting studs with the first layer of strapping.
If our lot had been larger, we would've gone with a completely detached garage, but unfortunately it just wasn't an option. We left a gap between the garage and the house so both layers of mineral wool could slip between the two, keeping the thermal layer on the house unbroken.
Installing battens to create a rainscreen
Initially we were going to use two layers of 1×4 furring strips (also referred to as strapping or battens). The first layer would be installed vertically, attaching directly over the 2×6 framing members through the two layers of Rockwool and the Zip sheathing. The second layer would be installed horizontally, anticipating that the charred cedar siding would be oriented vertically on the house.
But as the second layer of Rockwool went up, Wojtek and Mark pointed out that putting the siding in the same plane as the Rockwool/metal flashing on the basement foundation would be needlessly tricky. Maintaining a 1/8-inch horizontal gap between the bottom edge of the vertical siding and the metal flashing on the foundation around the house would be nearly impossible, and any variation might prove unsightly.
As a solution, we decided to use 2×4s for the first layer of strapping. This allowed the siding to be pushed slightly out and farther away from the Z-flashing covering the face of the Rockwool on the foundation. The bottom edge of the siding could be lowered. Wojtek and Mark also found that the 2×4s were easier to install than the 1×4 furring strips so that they didn't overly compress the insulation (an easy thing to do).
Unfortunately, increasing the overall wall thickness with 2×4s meant having to use longer Fastenmaster Headlok screws (it would also cost us later when it came to the siding on the north side of the house—more on this later). Apart from this change, the additional overall wall thickness mostly just increased the air gap in our rainscreen, which arguably just increased potential air flow while also expanding the drainage plane behind the eventual siding.
In one of the Hammer and Hand videos Sam Hagerman mentions that at least 1 1/2 inch of screw should be embedded into the framing (excluding the thickness of the sheathing). But when I asked a Fastenmaster engineer about this directly he recommended a full 2 inches of the screw should be embedded into the framing in order to avoid any significant deflection over time.
As a result, we ended up using 8 1/2-inch screws. The screws work incredibly well, requiring no pre-drilling. They're fun to use with an impact driver (keep your battery charger nearby). Along with the plastic cap nails and Trufast screws, I think we ended up with less than a dozen fasteners that missed the mark for the entire house—a testament to Wojtek and Mark's skill. I was able to seal around these errant fasteners from the inside with a dab of HF Sealant.
During the design stage, using these longer screws prompted concerns regarding deflection, but based on this GBA article, data provided by Fastenmaster, along with some fun on-site testing, the lattice network of strapping (whether all 1×4s or our mix of 2×4s and 1×4s) proved to be incredibly strong, especially when the siding material is going to be relatively light tongue and groove cedar.
Once the 2×4s were all installed vertically through the structural 2×6s as our first layer of strapping, Wojtek and Mark could install the components of the rainscreen, including the Cor-A-Vent strips at the top and bottom of the walls, as well as above and below windows and doors. In combination with the 2×4s and the 1×4s, this system creates a drainage plane for any water that makes its way behind the siding, while also providing a space for significant air flow, speeding up the drying time for the siding when it does get wet.
In addition to the Cor-A-Vent strips, we also added window screening at the bottom of the walls just as added insurance against insects. We noticed that on the garage, even without any insulation, the Cor-A-Vent didn't sit perfectly flat in some areas on the sheathing. Since the Rockwool on the foundation, now covered by the metal flashing, was unlikely to be perfectly level, or otherwise true, along any stretch of wall, it made sense to us to double up our protection in this way against insects getting into the bottom of our walls at this juncture.
Wojtek and Mark also did a nice job of taking their time to shim the 1×4 layer of furring strips, thus ensuring a flat installation of the charred cedar. This really paid off, not only making their lives easier when installing the tongue and groove cedar, but also providing aesthetic benefits in the overall look of the siding.
This was especially true on the north side of the house, which has the largest area of charred siding with almost no interruptions, apart from a single window. It's also the tallest part of the house, so without proper shimming the outcome could've been really ugly. Instead, once the cedar siding was installed it was impossible to tell there was 4 inches of Rockwool and two layers of strapping between it and the Zip sheathing.
Detailing around windows and doors
Things got somewhat complicated around windows and doors, but once we worked through all the details for one window it made the remaining windows and doors relatively straightforward.
In the photo below you can see all the elements coming together: the window itself, the window buck covered with tapes for air and water sealing, the over-insulation for the window frame, the Cor-A-Vent strip to establish air flow below the window and behind the cedar siding, along with the strapping that both establishes the air gap for the rainscreen while also providing a nailing surface for the siding.
Once most of the siding was complete around each window, but before the 1×6 returns to the window frames went in, we installed a metal sill pan at each window. The pan slid underneath the bottom edge of the aluminum clad window frame and then extended out just past the edge of the siding (I'll post photos of this detail in the next blog post about installing the charred cedar).
Here's an article from the Journal of Light Construction discussing a couple of options for trim details. And here's a detailed slide presentation by Bronwyn Barry regarding details like these for a Passive House wall assembly: Sills and Thresholds – Installation Details.
Many of the same details were repeated at the top and bottom of our two doorways. Once a dedicated metal sill pan was installed (after most of the siding was installed), it felt like we did everything we could to keep water out.
In the next blog post I'll go through the details for the top of the ventilated rainscreen when discussing how the charred cedar siding was installed.
Other posts by Eric Whetzel
- The Blower Door Test
- Choosing and Installing a Ductless Minisplit
- Installing an ERV
- Choosing Windows
- Attic Insulation
- Installing an Airtight Attic Hatch
- Air Sealing the Exterior Sheathing
- Installing a Solar Electric System
- Prepping for a Basement Slab
- Building a Service Core
- Air Sealing the Attic Floor
- Ventilation Baffles
- Up on the Roof
- A Light Down Below
- Kneewalls, Subfloor and Exterior Walls
- Let the Framing Begin
- Details for an Insulated Foundation
- The Cedar Siding is Here — Let's Burn It
- An Introduction to a New Passive House Project
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Source: https://www.greenbuildingadvisor.com/article/urban-rustic-exterior-insulation-and-a-rainscreen
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