Tuesday 8 March 2011

Clutching at straws

In my last essay I discussed the possibility of using wool as insulation. After all, as I said, we have wool. The problem with using wool as insulation in the walls is that it has no structural strength and is not weatherproof; it must be packed into structural, weatherproof boxes. The wool is virtually free, but the boxes come expensive.

Well, OK, we do have wool. But we also have straw (and a square baler to bale it). This year we're growing three acres of barley on my croft, and thirteen acres across the farm as a whole; and no-one else is going to be bidding for the straw, except as horse bedding. Straw bales provide both structure and insulation. Indeed, the first straw bale houses, on the north american prairie, also had turf roofs, so there's no doubt whatever that they can sustain the compressive load. The insulation values - tested, again, over a century of prairie winters - are very good.

In straw bale construction, you need, of course the bales. In order to tie them strongly together, you need a certain amount of steel rod - about two or three metres for each square metre of wall. And you need a rodent-proof render on both inside and out. On top and bottom you also need something rodent-proof; boards are possible, but so is render. All this is cheap - as cheap as or cheaper than concrete blocks.

The bales must of course be used after the harvest - so September - and must be laid and rendered dry. Dry weather is not guaranteeable in Galloway autumns, so there would be benefit in having the roof up first. Furthermore if the roof isn't up first, there isn't a lot of time to get it up before winter closes in. Of course, if the roof is to be up first it must be supported on pillars. Of course, those pillars could be temporary - my proposed roof structure is after all, as Pete objected, floppy, and so will accommodate a slight movement as the pillars are removed (provided that it is slight!).

All this begins to look very like a plan. It is cheap - actually the cheapest structure I've so far costed. It's also extremely 'green' - low energy, low carbon cost. I continue to assert that the zero carbon house is impossible to achieve - this one will have some sort of plastic membrane on the roof, and will have glass windows; furthermore a certain amount of cement and quicklime will be needed for foundations and render. But this is probably as close as it is possible to get. And it makes use of materials - timber, straw, wool - which we produce on site. I think I have a plan.

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