From natural materials to open building, this wall will work for generations.
Settle in, wall nerds, and see how Hans Porschitz of Bensonwood builds a modern wall for wood frame construction. It looks like a normal wall that might be in any home, but it is very different, and there is a lot to learn from it.
For instance, the wall is clad in ZIP sheathing, which is, on its own, pretty water resistant. In fact, to test it, Bensonwood built a garden shed sized building out of it, taped the joints, and left it out in the rain for five years.
Many building codes today require continuous insulation on the outside of the studs to reduce thermal bridging; Bensonwood uses Steico wood fibre insulation boards instead of the foam most builders use, because it is made from a renewable resource and is vapour-permeable.
The wall is insulated with cellulose blown in through holes in the bottom plate; soon they are getting a new machine that will fill the whole wall before they stick on the interior sheathing. I have always worried about cellulose and how it stands up to moisture; Hans says they tested it many times, and with a wall that is permeable to vapour like theirs is, it just dries out. My other worry has always been about vermin nesting in it, and he just rolled his eyes, given that the wall is built so tightly that there is no way that vermin could ever get into it.
On the inside face of the wall, where most builders have drywall, they have flake board which is taped. I asked Hans why they didn’t have the traditional poly vapour barrier and he said that the board was a “vapour control membrane.” They then nail on wood strapping to hold the drywall, creating a gap where all the electrical wiring goes.
This is part of what Tedd Benson calls OpenBuilt, based on the concept of Open Building, first described by Dutch architect John Habraken. It recognizes that some components in buildings age more quickly than others; for instance in my own home, it was wired with knob and tube wiring in 1913 and redone in Romex over the years, with the last knob and tube circuits being removed in 2015. The walls and floors had to be ripped apart to get at it.
In a Bensonwood house, you can pull off the baseboard to get at the wiring and then you can pull it down from the electrical box through the conduit. I suspect that in the next ten years we will be converting to direct current and a lot of people will be wishing they could do this. It also is much better for the insulation and moisture control, as there are no penetrations of the
vapour barrier vapour control membrane.
So what is so wonderful about this wall? It is almost entirely built of natural and renewable materials. It is sealed tightly, water-resistant materials, but has no impermeable plastics that might trap water inside. The open building design means that it won’t have to be ripped open to modify services or interior and exterior finishes. Tedd Benson says it will last two hundred years, and I believe him.