Since the snow started earlier than expected, the roof is too slippery to work on, so the crew have instead been starting on the wall insulation.
For those who have been paying attention, you will recall that we are mainly using German-made Schneiderholz rigid wood fibre insulation boards. These are basically mashed up and reconstituted spruce and pine from sustainable sources. They have a high R-value of near 4.0, and we are using two layers on both walls and roof: a thick inner layer of 240mm (9 1/2″) covered by a thinner outer layer of 40mm (1 1/2″). The outer layer also has a relatively water-proof coating. The whole wall / roof assembly will be over R50 (that’s a real testable R-value, not a grand claim). The additional benefit of wood fibre is the relatively high thermal mass, which will make the house cooler in summer. This will be particularly important in the roof. Finally, it is also air-tight but water vapour-permeable.
However, due to the logistics and costs of transporting such bulky material, it was only cost-effective to get a single 40′ container load of insulation shipped from Germany. This left us with a shortfall of some 50 m2 (about 540 sq ft) in the inner layer. The question was what to use to make up the shortfall, and where it would go. We considered various options, including:
- Buying more wood fibre insulation from a Canadian supplier. This was rejected because we are short of funds, and it would have cost about 3 times as much as from the German supplier, even including the costs of shipping. In any case, this would also be imported. We are left wondering, once again, why Canadian companies can’t (or don’t) make competitively priced, high-performing and sustainable wood-based products in a country which has so much timber.
- Framing about 2/3 of one of the larger walls and using Roxul mineral wool batts or something similar. However, having specifically built a house that does not conventional framing, it seemed silly to have to use it non-structurally for the insulation. We’d also had to have made Larsen trusses to avoid cold-bridging. It just seemed like a lot of extra time and effort. Note that we are not against Larsen trusses at all in general; they are a great solution if you are going to build a highly insulated stick-framed house.
- More Expanded Polystyrene (EPS) panels, as we used in the foundation. EPS is not the most environmentally friendly substance, but its better than Extruded Polystyrene (XPS) and it actually has a relatively low embodied energy because it is mostly air. It insulates very well (Type 2 has an R-value of near 4.0). You can also get EPS panels cut to the exact size you want, relatively cheaply. There are some other disadvantages: it doesn’t have a high thermal mass compared to other materials and it is relatively vapour-impermeable (although still highly air-tight). This would make EPS a bad choice for the roof, but using it as a base layer for the wall insulation, all around the house, would not be so bad, because the vapour would still be able permeate upwards through the whole of the rest of the walls and roof. Two layers of 2′ x 8′ x 9 1/2″ SR.P 200 (Type 2) EPS panels from Styrorail, who made our foundation insulation, would be enough to make up the shortfall.
So that’s what we decided to do. And it is so easy to work with, it was pretty much done in an afternoon. One potential issue was what to do if water was somehow trapped behind the panels, so to cope with that unlikely eventuality, we cut tiny channels in the base of the panels. And it isn’t at all affected by the snow which, right now, is rather important…