A few years ago when everyone was writing about the smart home, I wrote In praise of the dumb home, which is so well insulated with temperatures so stable that a Nest smart thermostat would be bored stupid.
But designing the walls for a dumb home is not as simple as simply adding insulation; you have to be smart about managing moisture and condensation. You want to stop air movement but not moisture movement. It’s also smart to use healthy materials that don’t burn easily or are full of flame retardants, or are made from fossil fuels.
That’s why this “Smart Wall” developed by Lucas Johnson of 475 High Performance Building Supply and Andrew Legge of Havelock Wool is so interesting. Lucas explains:
Most every wall out there is built with toxic materials and ends up being too vapor retarding and even vapor-closed. Such walls can trap moisture and may become exceedingly difficult dry out. If a building wall or a house can’t dry out, it can lead to all kinds of bad things.
Wool is terrific insulation; ask any sheep. Andrew Legge describes it:
It is scientifically understood that wool manages moisture against 65% relative humidity, irreversibly bonds with formaldehyde, nitrogen oxide and sulfur dioxide, and the trade is responsible for the sequestration of 525,000 tons of pure, atmosphere-derived carbon. It is also entirely renewable and sustainable in its creation, a great insulator that has evolved over thousands of years, and compostable at the end of an extended useful life. These claims are fact, not conjecture or marketing speak.
Unlike other insulations like cellulose, there is no borax needed to control rodents and fire. And instead of giving off toxic chemicals, it actually sequesters them. But I will sheepishly admit that I have not been a fan of wool insulation, in the belief that it actually has a high carbon and water footprint. Years ago, Colin wrote:
In New Zealand, home to 45 million sheep (to under 5 million people), more than half of the nation's greenhouse gas emissions come from their livestock; the methane that the sheep so conspicuously add to the atmosphere has a global warming potential of 21, compared to (a much smaller) 1 for carbon dioxide. Water, the world's most precious resource, plays a big role, too, from raising the sheep to cleaning the fiber; it takes approximately 500,000 liters of water to manufacture a metric ton of wool.
We will look into that question in a separate post, and besides, the wall is more than just wool insulation. From the inside, there is a service cavity behind the drywall for wiring, so that electrical boxes and wires do not have to penetrate the membrane. Instead of an old-school vapor barrier it has what is now known as a “smart vapor retarder.” Alex Wilson of BuildingGreen explains that it changes with the seasons:
The goal is low permeability in the winter when humidity is low but it’s critically important to block moisture flow and prevent condensation, and high permeability in the summer when humidity is higher and you want drying potential to both the interior and exterior.
I do not understand the physics of this, even after watching the video a few times, but evidently it is not just an attribute of the new high tech materials; according to Alex:
It turns out that the plain old kraft paper facing on fiberglass batts has this variable permeability property—as leading building science expert Terry Brennan explained to me. As humidity increases (in the summer), it becomes more permeable to moisture, while in winter, when the humidity drops, it becomes less permeable and a better vapor retarder. Terry describes it as “poor man’s vapor retarder.”
On the outside of the exterior structural sheathing there is a layer of wood fibre insulation, which is vapour open, allowing moisture to pass through. This is not used much in North America but is such a neat product, made from waste wood fibre, sequestering carbon. The entire wall has and incredibly low embodied energy and is almost edible, the ingredients are so healthy. I believe that this is really important; 475 High Performance Building Supply is kind enough to quote me:
The answer to our predicament today, is not to throw off technology and return to the Primitive Hut, but instead, is to synthesize our understanding of natural systems, and selectively use technology that provides great benefits with manageable environmental impacts. Lloyd Alter, writing in Treehugger, notes that our current obsession with energy efficiency and high-performance, as realized in the Passive House Standard, is not enough.
It has to be healthy and have a low embodied energy as well.
This Smart Wall is actually perfect for the Dumb Home or the Primitive Hut. It is worry-free, won’t hold water and will probably last forever. As they say at 475:
The Smart Enclosure acknowledges its profound relationship with the outside environment and the occupants within. This system is made of efficient, resilient and sustainable products resulting in buildings that are better for people and for the planet.
I have written before about the search for the perfect wall; this may be it.