The Building Envelope: Our Third Skin

Our clothing is our second "skin" and our home's envelope is our third "skin." Each must be semi-permeable and able to breathe. 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 plastic.

[Editor's note: Robert Riversong, a Vermont builder, continues his 10-part series of articles taking design and construction to what he sees as radical or "root" concerns. Enjoy--and please share your thoughts. – Tristan Roberts]

6. Envelope – shelter from the storm, our third skin

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.

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 to fabricate a 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 virgin 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 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 2x4–8' stick has 52% more endgrain compression resistance and 78% greater resistance to buckling than a KD 2x4, so it can easily substitute for a KD 2x6.

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 2x4 inner load-bearing frame, stabilized with metal T-bracing, a 2x3 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

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 2x4 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 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.

Images, in order: Cordwood infill Cordwood and earthen floor Bau-biologie chart Larsen truss Riversong truss Double wall

1.    Context – land, community & ecology

2.    Design – elegant simplicity, the Golden Mean

3.    Materials – the Macrobiotics of building: natural, healthy and durable

4.    Methods – criteria for appropriate technology

5.    Foundations – it all starts here: how do we begin?

6.    Envelope – shelter from the storm, our third skin

7.    HVAC – maintaining comfort, health and homeostasis

8.    Energy & Exergy – sources and sinks

9.    Hygro-Thermal – the alchemy of mass & energy flow

10.    Capping it All Off – hat &  boots and a good sturdy coat

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

Robert Riversong has been a pioneer in super-insulated and passive solar construction, an instructor in building science and hygro-thermal engineering, a philosopher, wilderness guide and rites-of-passage facilitator. He can be reached at HouseWright (at) Ponds-Edge (dot) net. Some of his work can be seen at BuildItSolar.com (an article on his modified Larsen Truss system), GreenHomeBuilding.com (more on the Larsen Truss), GreenBuildingAdvisor.com (a case study of a Vermont home), and Transition Vermont (photos).