06. Envelope

Our Third Skin


Corwood Masonry House with Round-Log Frame

What began, centuries ago, as a simple structural envelope with a rudimentary weather barrier, the bone, muscle, fat and skin of our habitations have evolved into rather sophisticated containers that are expected to perform a number of essential functions and remain durable over time. But they rarely resemble any organic natural entity.

What are the essential functions of a home’s envelope? Structure, weather-resistance, thermal resistance, thermal capacitance, moisture resistance, air resistance – and a conditional separation between the inside and outside environments. Conditional, rather than absolute, because the outside environment is not (or better not be) antithetical to life, as would be outer space or the bottom of the sea — each of which requires absolute isolation in order to maintain a livable interior space. While the outdoor environment may not always be comfortable, it is never-the-less the matrix which birthed us, which nurtures us, controls the expression of our personal DNA blueprints and our evolution as a species. We have not evolved to live within an isolation chamber or a picnic cooler – a hermetically-sealed house.

Cordwood Masonry Walls, Earthen Floor

Cordwood Masonry Walls, Earthen Floor

Nature is Semi-Permeable

Every boundary in the natural world is semi-permeable, including our cell walls and our integument – our skin. We have learned that we are far more comfortable in a breatheable parka than in a rubber or plastic raincoat, and we still struggle in vain to fabricate materials more comfortable than wool or cotton. Our clothing is our second “skin” and our home’s envelope is our third “skin.” Each must be semi-permeable and able to breathe. And each better supports the life they enclose the more they are composed of the natural materials with which our physical bodies evolved and to which we have adapted over many millions of years.

This puts my philosophy of building at odds with much of the so-called “green” building movement, which relies heavily on non-breathing, non-natural and ecologically-harmful plastics, including but not limited to poly sheeting, bituthene membranes, rigid foam board and spray-in-place foams. But it makes it more consistent with the Bau Biologie (biology of the house) and natural building movements, both of which support human and ecological health and measures of sustainability to a far greater degree.


The Skeleton

The “bones” of a house can be logs or heavy timbers or light sawn-lumber repetitive framing, or earth or stone. Sticks, stones and dirt are the traditional materials from which we fashioned human shelter. Earthen walls work best in dry desert-like climates, where earth predominates and thermal mass may be more important than thermal insulation. Stone is appropriate in mountainous or coastal locales where the bones of the earth are easily available and winds are harsh. But in much of the temperate zone of the earth, at least where we have not already cleared the forests, wood is the most available, abundant and useful material – and it’s generationally-renewable if we care for our woodlands.

There are many ways to engineer and assemble wood into a structural frame. Traditional timber framing has experienced a modern revival, and is appropriate as long as it does not require a secondary, non-structural frame or foam plastic panels in order to complete the enclosure, and is not so expensive to produce that it supports merely a niche market. The most ecological way to wrap a timber frame is with straw bales and earthen/lime plasters. It is also one of the most healthful envelopes for its occupants. Another healthy envelope that meets high standards for hygro-thermal performance (more on this in essay #9) is a cordwood/masonry structure, which can be made even more healthful and green by substituting either lime mortar or cob (clay, sand and straw) for the Portland cement mortar. One of many advantages to this technique is that it creates the bones, the meat, the fat and the skin all in a single low-tech process. It can, like straw bales, be either structural or infill. Either lime-treated sawdust or borate-treated cellulose (shredded newsprint) can be used as insulation between inner and outer wythes of mortar.

Rough, Green & Strong

But most of today’s homes are going to be constructed more conventionally of sawn lumber. Except for the restrictions of building codes in some jurisdictions, there’s no reason that local, rough-sawn lumber, either air-dried or green, can’t be used in place of often-imported kiln-dried wood. I’ve been building from rough-sawn and green lumber for more than 20 years. The lack of a grade stamp should not be an issue, since rough lumber is so much stronger than the milled “nominal” equivalent. A full-dimension 2×4–8′ stick has 52% more endgrain compression resistance and 78% greater resistance to buckling than a KD 2×4, so it can easily substitute for a KD 2×6.

RS v. KD Section

With smaller and stronger sticks, it’s easier to design double-wall envelopes that use no more material than a conventional frame. In 30 years of building variations of double-wall houses, I have found that the most cost-effective, resource-efficient, labor-efficient, thermally-efficient, and hygrically sound wood-framed envelope is a modification of the Larsen Truss (which was developed for retrofits), which has become known as the Riversong Truss. My system uses a 2×4 inner load-bearing frame, stabilized with metal T-bracing, a 2×3 outer chord gusseted with 1x4s and supported at both foundation and rafter tails. Combination sheathing/siding of pre-finished novelty drop spruce (pattern #105) over housewrap, air-tight drywall inner skin and dense-pack cellulose fill completes the package.

Many Ways to Skin a House

Larsen Truss Frame

Larsen Truss Frame

Rough-Sawn Riversong Truss Wall

Rough-Sawn Riversong Truss Wall

But there are many ways to design a double-wall or deep-stud envelope, including in-line or staggered studs on common or separate plates, load-bearing at either the inner walls or the outer walls (or split: floor-bearing inner and roof-bearing outer, which keeps the band joists inboard of the insulation), I-joists as studs, or sheathed stick frame with Larsen Trusses. When I design homes now for other builders to erect, I typically use the split-bearing double 2×4 24″ on center frame with staggered studs and exterior plywood sheathing. This not only meets code requirements but is only a variation on the conventional techniques that most framers already know.

For an exterior, breatheable skin, I prefer pre-finished latex-stained solid wood horizontal shiplap or beveled lap siding. If sheathing is included, it is either diagonal boards or plywood, never moisture-vulnerable and poorly-breathing OSB (or its ZIP-wall variation). Interior breatheable skin is drywall with air-tight acoustic caulk or gaskets (as well as air-sealing between each framing assembly), either finished with latex vapor-retarder primer or coated with gypsum, earth or lime plaster. Insulation is always dense-packed cellulose, blown dry behind the drywall (though insulweb is an option). The final element is some form of extra thermal mass within or contiguous with the conditioned space.

Local, minimally-processed lumber is the “skeleton”, the “skins” are wooden boards and gypsum, the “fat” is recycled newsprint with boric acid (fire retardant, mold resistant, insect-proof, vermin-resistant, air-resistant, and sound-proof), and the “meat” is the thermal mass. Together, and composed primarily of nature’s gifts, these create an organic integument that meets both the requirements of a good building envelope and the demands of a finite planet. 


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by Robert Riversong: may be reproduced only with attribution for non-commercial purposes

8 Responses to 06. Envelope

  1. Frank Lee says:

    what’s your input on SIPs ? specifically closed cell polyurethane

    • Riversong says:

      SIPS can mean either Structural Insulated Panels, meant to be wall structure, insulation and both interior and exterior skin – or stress-skin panels, which are similar but not intended to be structural.

      The latter typically is installed as a “wrap” around a timber frame. Traditional timber frames can be beautiful, but are quite expensive and generally require more board feet of lumber than a “stick-built” frame. One advantage to the stress-skin panels is that they can wrap a timber frame relatively quickly to protect the investment from the weather. But sealing the joints, and cutting and framing door and window openings can be a somewhat toxic experience and take a good deal more time than what might be expected.

      My two primary issues with any kind of foam-based panel system is that it uses a high-embodied energy (a petrochemical) material to save heating and cooling energy, so there is a payback period that can be significant. There is also a relatively high global warming contribution in the material. And I don’t trust the longevity of chemical bonds for a primary structural element.

      My greatest concern, however, is that a foam-wrapped house is what I call a “Hermetic” house – hermetically sealed and effectively an oversized picnic cooler, not intended to support biological life. Every “skin” of a living creature – from our integument to our clothing to our shelters – should be able to breathe.

      There were no petrochemicals on earth until mid-20th century, and life has not evolved to be compatible with them. There are now more than 80,000 different petrochemicals, most of which do not biodegrade, remain toxic to life and potentially bioaccumulate in the food chain.

      So I never use foam panels in construction and do not recommend them. It is a profoundly unnatural way to create human shelter.

  2. Blake Olson says:

    How about Roxul rock board as sheathing. I know it’s industrial containing trace amounts of formaldehyde, but so do plywood and Sheetrock; and it breaths with a perm. of around 30.

    • Riversong says:

      Roxul mineral-fiber board, even in its most dense varieties, is thermal and acoustical insulation but NOT structural or nailbase sheathing, and the urea-extended phenol formaldehyde is not “trace amounts” but used in the manufacturing process as the fiber binder (1-6%).

  3. Thomas says:

    How do you feel about sheep’s wool as insulation? And how does it compare to mineral wool?

    • Riversong says:

      As anyone who has worn wool clothing on a cold wet day knows, it is one of the best natural insulators. And, as anyone who has lost good wool clothing to moths knows, it is vulnerable to insect damage. Just as with cellulose and recycled blue jean insulation, this problem is addressed by treating the fibers with borax (which also makes it fire-retardant and fungus-resistant).

      The primary problem with sheep’s wool as building insulation is availability and cost (and, I suspect, it’s very difficult to cut, just as are blue jean batts). It’s generally only the dirty first shearing that is recycled into insulation, as the better-quality subsequent shearings are too valuable as a clothing fiber.

      But that first shearing has to be thoroughly cleaned of both lanolin and all the other contaminants – from sheep dung to the organophosphates used in sheep dip to control parasites, which adds cost and industrial processing.

      Mineral wool is generally more available and is one of the few non-combustible and insect-proof insulation materials. But all batts suffer from the problem of imperfect installation, which leaves gaps, depressions and air-flow channels which can significantly reduce its effective R-value in an assembly.

  4. shaun boyce says:

    How does the diagonal sheathing work to mitigate heat loss if there are gaps, as it’s not considered an air barrier? Does your method rely on the thickness of the walls with cellulose insulation and air sealing the interior dry wall to prevent this?

    • Riversong says:

      The purpose of the sheathing is ONLY to provide structural bracing to the frame and to serve as nailbase for the siding. As I made quite plain in this article, an exterior skin (particularly in a northern climate) must be able to breathe (water vapor) and is NOT part of the air barrier, which ideally should be at the interface between the warm, moist interior environment and the structural/thermal layers – in other words, on the inside of the insulated wall system, which is where properly-taped drywall will do the job perfectly.

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