So last Friday I got a chance to tour the Niagara EcoHome with Ian Weir. Ian is the president of NatureBuilt, a company which makes Structural Insulated Panels using straw as an insulating material.
The technology is pretty cool: rather than building a straw wall and then plastering it when it's upright, you pour the plaster into a horizontal mold, then pack the bales in place, then smush more plaster on top. Once it's dry, you stand the panel upright, and hey presto, a section of wall. Panels can be as big as 8 x 10, and they're 16" thick. With an inch of plaster on either side, and a wooden frame, they're fully structural.
Like all SIPs, one of the advantages is that they're built in a controlled environment and then brought to the site en masse, so when you start building the house goes up very quickly -- just a few days and you're ready to put the roof on.
Unlike many green technologies, these walls also claim to be cheap -- comparable to stick frame but achieving R35+ with no thermal bridging and considerable thermal mass.
I'd looked into them a few months ago, and was intrigued but really didn't feel like driving to Welland to see the model home. But I had to go to Vaughan on Friday, which was halfway there, so I figured I may as well take the opportunity to have a look.
I was quite impressed. The system has many of the pros and cons of straw bale builds -- among the positives, thick walls make for impressive sound barriers; plaster tends to give a room a nice acoustic warmth; and air quality feels good. Thick walls can also make a house feel dark, particularly if the windows are undersized. In this home they've used good-sized windows, and expanded the flow of light by shaping the walls away from the windows. Unfortunately, the rounding gives it a bit of a Hobbit house effect, something we've disliked in other straw bale homes we've seen. Ian assured me that they can achieve a squared off look, or do an angled cut on the sides of the windows.
Because the panels aren't able to contain windows, window and door sections are framed conventionally. This is one of the potential downsides of this system, it seems to me: you're going back and forth between different wall types, between straw bales and conventional framing. The problem with that -- and here I'm wading into territory where I'm a bit murky -- is moisture. Straw bale walls, as I understand it, are meant to be vapour permeable (some call them breathable, but that seems to imply they're porous. It's not that air passes through them, but moisture can, very slowly.) That's why there's no vapour barrier, just an inch of plaster on either side. A conventional wall, though, has a vapour barrier. So you now have a six foot section of wall with no vapour barrier and straw insulation, adjacent to a few feet of wall with vapour barrier and window. So what happens to the moisture? Seems like there's the potential for problems there.
There are also issues with the look of the wall where SIP meets conventional framing. There's going to be a seam visible, which means you either have to plaster over both SIP and conventional wall (adding another step that requires skilled labour) or find some other way to bridge the gap. Ian said you can actually drywall over the entire interior wall, but I wonder if that doesn't work against the whole breathability issue. Then again, it would add more thermal mass to the interior, at a fairly low cost (you're bringing in a drywaller anyway to do all the interior walls.)
There's much more information at the Naturebuilt site. CBC also did a piece on the overall project -- ignore some of the "gee whiz" tone of the reporter!
Update: Ian responded to some of my questions. See his response in the comments section, below.