March 19, 2018

Project Spotlight: Three Tree Builds High Performance House Without Sheathing!

When it comes to building a wood framed house, one of the typical ingredients we have come to expect is exterior sheathing. It’s the way most houses achieve shear strength and provide a nail base for siding materials. But while familiar, it’s a recipe that can prevent robust drying of the assembly, raising the risk of moisture damages and mold.  The formaldehyde-based glues often found in sheathings like plywood or OSB can additionally inhibit great indoor air quality. And significantly, all that sheathing simply is not needed – it’s just more stuff.  Hence, it’s no surprise that some builders are looking to avoid sheathing altogether. But how can that be?

How can an assembly without sheathing achieve sustainable and durable high performance standards?

At 475 High Performance Building Supply, we’re always challenging builders to build to best practices. One of our favorite walls is a sheathing-less double stud wall, as described in our blog post The Double-Stud Wall Simplified: Low Cost, High Performance.

With this system, you build a house out of 2x framing lumber, fibrous insulation, Solitex Mento Plus weather resistive barrier outboard with back vented rainscreen and inboard, Intello Plus or DB+ smart vapor retarder, with a service cavity and finish.  That’s it..

These Pro Clima components together with sheathingless wood frame construction provide optimized insulating and moisture-control conditions that enhance sustainability and comfort while eliminating unhealthy sheathing products. And when flipped onto the roof, the same components can tackle the even thornier issue of roof moisture control without the mother of unhealthy and unsustainable building products – closed cell spray foam.

Three Tree Home Performance based in northern Vermont is a versatile home energy efficiency company that “uses building science to make homes safer, healthier and more efficient”.  An opportunity for a new house without sheathing client came to them from a client, and Three Tree Home Performance owner Dave Powers was happy to oblige. He had used Pro Clima’s airtight systems on the enclosure several projects, and was keen to try them in the minimal way we suggested on our blog.

The Project Story in Three Slide Shows:

The result is a sheathing-less house that brings the highest performance European airtightness and vapor control technologies to a New England single family home. The photos from this project tells the story, with annotations added to the following three slide shows.

Slide Show 1: Exterior airtight weather barrier, from roof to walls

Starting from bottom, first course of Solitex Mento Plus is rolled out across trusses, followed by 4’ long 2×4 vertical battens. Unlike for conventional roofs, care must be taken to step only on structural members. As shown, the standard 59” wide Mento Plus is preferred so the membrane can be applied upward in manageable installments (118” double wide Mento Plus is also available). Staple fasteners are ideally limited to beneath membrane laps for a leak-proof temporary roof, but otherwise staples in field can be taped with waterproof Tescon Vana.

Second course of Solitex Mento Plus seams are taped with Tescon Vana, followed by 59” battens (width of Mento roll) aligned above the first course. For this project, horizontal battens were added for a metal roof, but sheathing could be added for asphalt roof. The horizontal battens also add a ladder-like lattice for easier movement. The Pressfix squeegee tool is optimal for pressurizing the tape for a permanent bond, especially where the reinforced membrane has no support across gaps between trusses, rafters and studs.

Tescon Naideck double-sided butyl tape is used for sealing screw and nail penetrations that fasten the battens through the Mento Plus to the structure below. The 2” wide tape is first applied to the backside of a batten, then the release paper is removed before final fastening. Naideck completes the temporary waterproofing on a fully strapped roof. Mento Plus has 3 month UV exposure rating, so final roof can wait.

Unfastened “flying” battens are pushed into place below the horizontal battens, halfway between the vertical battens, to maintain generous rainscreen gaps above the vapor open Mento Plus. This is especially important in cases where the roof is insulated with dense-pack cellulose and the membrane pillows up. Best practice is to orient a 2×3 on edge (or true 2×2 shown here) to also create a drainage channel that pulls any potential moisture away from the penetrations.

As on the roof, Solitex Mento Plus was used as the airtight sarking membrane for the sheathing-less vapor open walls. As at the roof, all seams were taped with Tescon Vana – and pressurized across unsupported studs with Pressfix tools. Taped airtight connections were made to the roof Mento Plus and foundation to complete the pre-window exterior air barrier.

Slide Show 2: Interior airtight smart vapor retarder and dense-pack cellulose insulation

The interior 2×4 framing of the double stud wall was added inboard of the 2×6 exterior framing. Also note that porous netting was added at each truss to partition truss bays prior to cellulose dense-packing to make it easier to achieve uniform density.

Pro Clima DB+, our reinforced paper-based airtight smart vapor retarder, was a perfect fit for this super sustainable, all-wood project. It was instead of Intello Plus across all wall and roof interiors, and taped with Tescon Vana. Horizontal strapping at 16” o.c. was added per our best practice service cavity recommendation. The interior wall partitions were added after the DB+ and service cavities to ensure continuous interior airtightness and vapor control.

Though proficient at dense-packing directly behind Intello Plus from previous projects, on this project Three Tree decided to first net and dense-pack with Insulweb before installing DB+. Like Intello Plus, DB+ is resilient enough for direct dense-packing, but experience and equipment can make the difference for a perfect installation.

Slide Show 3: Full window integration and details

At .56 ACH 50, this project achieves Passive House airtightness with some of the most sustainable building components now available in North America. In the end, we can’t solar panel our way out of the climate crisis. It’s enclosure-first projects like these that will get us closer to global carbon reduction goals. With their choice of the lowest embodied energy insulation and cladding materials – cellulose instead of spray foam, and metal instead of asphalt roofing – this project also boldly leads the way to a low-carbon future.

 

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12 Responses to Project Spotlight: Three Tree Builds High Performance House Without Sheathing!

  1. Ken March 20, 2018 at 11:43 am #

    Great project

    Was there a second layer of horizontal strapping applied on the exterior on top of the diagonal layer? The nail pattern on the vertical siding appears to be horizontal.

    With the spike in sheathing prices, plus the increased breathability of the assembly (and speed/ lower cost) makes me really want to incorporate this into my next new build.

    • David Powers March 21, 2018 at 9:13 pm #

      Yes, there is a second layer of horizontal strapping. This was a bit tricky. I would have preferred for the second layer to be fastened directly to the framing but was not practical with the first layer on the diagonal. The two rarely lined up over a framing member. I felt it would have been prone to leaking if a screw or nail spanned the void created by the 1st layer then punctured the Mento. The second layer is only fastened to the first layer. This is all rough cut strapping that is around 1″ – 1 1/4″ thick.
      I would have preferred to not do the first layer on the diagonal but is was required by the engineer.

  2. Buyani Nyoni March 20, 2018 at 12:07 pm #

    Is there a reputable reference for the argument that metal cladding is lower-carbon than asphalt? Or do you mean that it is just more easily recycled at the end of its life I wonder?

    • BOTTEGA miscellanea March 20, 2018 at 12:46 pm #

      I wondered the same thing, but then thought…the metal roof allows them to make the connections on the strapping where asphalt shingles would need sheathing. I wonder how the cost savings by eliminating the sheathing works out to the increase in metal roofing material. I love these articles, by the way!

    • John March 21, 2018 at 12:38 pm #

      I don’t have a specific link to send, but I think you’ll find: even if you assume similar amounts carbon resources go in to the production and shipping of metal vs asphalt – asphalt is a petroleum product, metal will last decades longer before requiring replacement, and metal is fully recyclable at the end of life.

  3. Ben Graham March 20, 2018 at 3:47 pm #

    is anyone concerned that the metal strapping seems to be right in plane of where moisture would most likely collect?
    I have always been concerned that the metal won’t last as long as the wood and you end up with a shaky house. Robert Riversong has been doing this sheathingless thing in Vermont for years. Would love to see some non-metal bracing and then I would feel more comfy in a wind storm.

    • Tim March 21, 2018 at 2:57 pm #

      Notice the metal strapping is actually on the inside (the dry side) of the Mento. It should be weather protected and not have corrosion issues.

    • David Powers March 21, 2018 at 9:02 pm #

      I do believe it is possible the metal strapping could experience some condensation. It is 16 Ga. 3″ galvanized steel. I would think it would have to be some severe conditions over a long period of time to cause any corrosion. The fasteners for the steel strap is what weighed on my mind in terms of rust.
      A majority of the bracing for this house is let-in 2x bracing and the diagonal strapping. The metal was only used in some key areas as specced by the engineer.

      Riversong certainly has a lot of experience building without sheathing. It’s nothing new.

  4. Danny Buck March 22, 2018 at 4:31 am #

    What is the floor system and does it have insulation?
    What is the longevity and maintenance requirements for the wood cladding?
    What is the exterior surface of the stem and how is that flashed to the cladding?
    Is the diagonal wood wall bracing let into the studs?
    Very well written and photographed article.

    • David Powers March 22, 2018 at 12:12 pm #

      Floor system is a frost protected shallow foundation. 8″ of EPS underneath, 6″ XPS perimeter and 2.5″ EPS wing.
      The back vented vertical grain S-P-F will last decades. There are barns all throughout New England with original vertical grain siding 100+ years old. Our last three HP new builds all have had vertical siding. Seems to be popular, also cheap and locally sourced. In this case, the homeowners are skilled and sided the house themselves.
      No Stem wall. A polyethylene mesh embedded in a siliconized acrylic coating is bonded to the Mento along the bottom of the wall then flashed over the perimeter foam down to below grade. We typically use aluminum flashing for this but I dont always get to make all the decisions.
      Yes, all the wood bracing was let into the studs.

  5. Michael April 17, 2018 at 9:59 pm #

    Great article and discussion … but, though I have not utilized these products, my concern is determining the most likely point of failure and in my opinion that is the unsupported taped seams between the sheets of Mento. Being unsupported it is unlikely that the tape bond will be as strong or consistently adhered as a rolled seam over a solid substrate. In addition the lack of backing support for the Mento would make it susceptible to movement based on the wind effects on the air pressure in the rain screen cavity, probably not at first since the dense pack will put the Mento under tension, but over time I suspect it will stretch/flex and the dense pack will compress. Any resulting flutter in the Mento has a chance of causing the taped seams to fail over time. Optimally the Mento would be installed in a shingled manner and taped, but without the sheathing as solid backing I would argue that the Mento should be installed with all seams over framing, in this case parallel to the wall studs and roof trusses. In addition I would recommend using the Naideck double-sided butyl tape between the layers of Mento, in line with the framing, as this will also provide redundancy in sealing any fastener penetrations.
    This is a great web site and source for intelligent information and discussion, thank you for your efforts.

    • John April 18, 2018 at 10:22 am #

      Thanks for the kind words on the article, Michael. We appreciate your engagement with the material.
      We’d agree that someone attempting this method take a greater amount of care than the average user to ensure seams are bonded. That said, we find that simply applying the mento tightly and using the PRESSFIX tool is all that is needed to create a proper bond. If you were using a roller with this method, it would not work the same way – the PRESSFIX is a must. Although not a requirement, if you did want some adhesive redundancy you could apply DUPLEX tape as you lap each layer of MENTO very affordably. Interesting idea with the NAIDECK double-sided tape to do double-duty. Typically we just recommend it to be used under roof battens for extra security in sealing penetrations.
      We’d be happy to work with you on details, Michael. We think the sheathingless method has great potential for creating affordable high-performance assemblies. Thank you for adding your thoughts to the discussion.

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