Tuesday, May 8, 2012


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.)
Exterior of the house. The walls have been treated with a skin of stucco. Note that they've decided to use the transition from SIP to conventional wall sections as a design feature, emphasizing the transitions with slightly different wall thicknesses.

Interior. The window ledges are deep but not outrageous. The rounded effect of the walls is something I'd prefer to avoid. This was one of the first houses built with this system, even though it's just opening as a model home now (delays in other aspects of the build, I believe). Ian said they've learned a lot about interior finishing since this one.
There are still a few questions to answer, but it does seem interesting.
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.

1 comment:

  1. I sent some of my questions regarding moisture to Ian. Here is his response:

    "With the integration of conventional and straw bale, we are trying to create a continuous air barrier on the outside and continuous vapour barrier on the inside. We do this by adding a flap of 6mm poly on the inside edge of the panel and typar on the outside edge. This allows us to tie in to the conventional framing.

    Now, of course, for the bale section, the wall is actually vapour permeable. So any moisture that were to get into it is allowed to wick to the outside. Since straw doesn't have a condensation point, no vb is required. For the framed section there is a condensation point and we therefore need the vapour barrier over those sections.

    When talking about breathability, drying of the bale wall will most certainly happen towards the outside and not so much to the inside. Therefore, although it would be best to have the wall fully breathable on both interior and exterior skins, it is much more important that the exterior skin remain breathable. For this reason you could get away with drywalling the inside and reducing the walls breathability. In my opinion, I would say that it is best to not drywall and to keep the wall as a plaster wall and either trim it out or finish it with a finish coat of either lime or clay plaster.

    For the outside, when you hang siding, you would fur out the siding to leave an air gap behind the siding to allow any moisture behind to dry out. This is pretty well standard (or should be!) even for conventional builds."