Wool gathering

Singlespace, as described in the essays up until this one, is a building with a concrete floor and wall, and extruded polystyrene as insulation. Both of these materials involve a lot of embodied energy, and hence are far from carbon neutral. They're also not local - they don't occur naturally on site, but need to be transported in. I need to use these materials because I've embedded the house into the hillside, and it's a damp hillside.

But, as Pete pointed out, I don't need to earth-shelter the walls. I could have a walkway round the back of the house. Then it could be drained much more conventionally, and the wall wouldn't have to resist the pressure of either earth or water. So the wall could be much lighter. If there were problems with it, access to repair it would be easy.

More significantly, it could be timber - and timber does occur on site (although if I plan to build this year I won't have seasoned timber of my own and will have to use 'imported' timber). Even 'imported' timber has a far lower energy cost than concrete.

But even more radically, if I use a timber wall with air outside it, I don't need the polystyrene insulation whose primary virtue is it works when buried in damp earth. Instead, I can stuff the wall, between two skins of timber, with wool. Ordinary sheep wool.

Now that makes a huge difference. We now have fifty ewes (and a hundred lambs) grazing the farm. We have wool. We don't have enough wool - my estimate is I'd need about 400 fleeces - but Galloway produces masses of wool, and under current market conditions it has little value. Farmers burn fleeces to get rid of them. I can get enough fleeces for little cost.

There are problems with this solution. Concrete is such amazingly cheap stuff that, even though the polystyrene insulation is enormously more expensive than wool, using a wooden floor and wooden walls adds at least 10% to the total cost of the structure. Second, there are problems with invertebrate infestation in wool insulation - there are solutions to this, but I need to research them. Third, extruded polystyrene, as well as providing insulation, protects the waterproof membrane from mechanical damage. Without it, something else would be needed - but old carpet would do.

But finally and most seriously, there is the problem of condensation in the roof. Extruded polystyrene can be put outside the waterproof membrane; it works when wet. Wool insulation doesn't, so it must be inside the membrane. This, too, can be worked around. If there's an airgap - 50mm would do - between the insulation and the waterproof membrane, with a controlled draught, it should clear condensation. But this makes the roof much more complicated to build.

You'll gather from the tone of this essay that I'm not yet persuaded of this solution. I love the idea of an almost zero carbon dwelling. I love the idea of sourcing my materials in my native landscape. Romantically and ecologically, it is undoubtedly the right thing to do.

But two things make me cautious. The first is cost. I really am very short of money to build anyway. Adding 10% really is a big deal. It means I'm going to have to cut corners in other areas. But the other is reliability. Provided the membrane is not breached, the concrete structure will need no maintenance in my lifetime. Of course, if the membrane is breached, then that's a disaster, but it's a reasonably unlikely disaster. By contrast the wooden structure will likely - almost inevitably, in fact - require regular maintenance as I age. The maintenance will be relatively easy to do, but it will need to be done.