Monday, February 5, 2018

Perfect Walls

Katy Hollbacher, who founded Beyond Efficiency provideing technical consulting services for high-performance buildings since 2009,
shared this post with me. I pass it on because she has refined wall systems that are the most effective to date for our Coastal California climate. (Plus the mountains, too.) Let's use this technology.

Perfect Walls*, on the Coast and in the Mountains
A 100-year strategy you can take anywhere.
by Katy Hollbacher
There are a number of perfectly acceptable ways to design a wall that has excellent thermal performance and air tightness while also being moisture-safe and durable. But terms such as vapor barrier, drainage plane, air tightness, and breathable are often misunderstood and misapplied, sometimes resulting in risky walls—or safe walls that are overly complex and expensive to build. Following are my favorite assemblies, both using "outsulation" rather than double-stud walls. These foam- and plastic-free walls can dry both to the inside and outside and will last 100 years while maximizing thermal comfort for occupants, minimizing the use of chemical-laden materials, and prioritizing low-embodied renewable materials. It's a win-win-win!

[cladding and fasteners not shown]
Q. I've never done continuous exterior insulation! How do you install cladding?
A. For a stuccoed California wall, just attach lath directly through the insulation. For other cladding materials, first attach furring strips. Yes, the fasteners can get intense when you're going through 6" of insulation...but it's doable! Mock up a wall and give it a go, and read this Building Science Corporation report to boost your confidence. The furring not only allows cladding attachment as you may be accustomed to but will greatly extend the longevity of wood or fiber cement cladding thanks to the rear-ventilated gap it creates.

Q. Won't the outsulation compress and make my stucco crack and siding sag?
A. No. Under service loads, deflections in the insulation (even rigid mineral wool) are a small fraction of that inherent in the wood-framed structure itself. Don't fret, just walk through this presentation by RDH Building Science.

Q. Why not Tyvek or building paper for the WRB?
A. Liquid-applied or fully adhered WRB products reign supreme over floppy sheets that don't seal at nail holes and don't double as an air barrier. Do your own "bathtub test" with a variety of WRB products: prepare your specimens, pound in some nails, then fill the tub and see who wins. This is not an unfair test—these are pressures that real walls face when the wind and rains are raging.

Q. Don't you need another WRB over the cork?
A. No. The material at risk in this detail is the plywood, which in this assembly is kept warm and cozy from the outsulation, and kept dry from the WRB that's sheltered from abuse by the outsulation (Hooray for outsulation!). Like most other rigid exterior insulation products, cork doesn't care if it gets wet. Flash your windows at the sheathing and WRB layer as you've always done—this gives you "innie" windows, which look nice and provide built-in protection from rain and sun.

Q. Why not blown-in fiberglass?
A. I have nothing against fiberglass (though I do take issue with batts of any material—unless I'm the one diligently installing them :). I just happen to like cellulose better. It's post-consumer recycled content; specify an all-borate treated product for fire and pest resistance and you've got a super-green, low-embodied energy, high-performance product. Great stuff, but what's extra special about cellulose is its ability to absorb and re-release water vapor. This attribute actually buffers the wall from moisture damage by reducing the amount of water vapor available to condense on cold surfaces (although with "perfect walls" we can worry less about that thanks to the outsulation).

Q. Why not sheep's wool insulation? Why not foam? Why not mineral wool? What about fire-rated walls? Why not OSB? Why not double-stud walls? What...
A. Depending on one's preferences, project budget and other constraints, the "perfect wall" for another person or project will inevitably be different. But that's OK; differences are what makes the world such an exciting place. I do have answers to these questions, and more details and caveats about the information shared—so contact us to debate my proposed walls and learn more.

    * Term inspired by Joe Lstiburek. Thanks Joe!

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