I’m very keen on the idea of building a oak-framed house, preferably one with lots of big windows and the posts and beams exposed. Oak is beautiful, sustainable, and strong, but the question I’ve been struggling with is, what do we use to fill in the gaps between the oak posts and beams?

Cob is, of course, an option, but if we end up with a plot that doesn’t have the right sort of subsoil, or not enough subsoil, to work into cob then we’d have to bring some in, which would leave a big hole somewhere. Cob is also very labour-intensive, and cob walls end up very thick - 450 - 600 mm thick, according to the Devon Earthbuilding FAQ. I really don’t want the walls of my house to drown out the oak frame and I worry that perhaps cob might do that (but I’d welcome any cob experts who want to weigh in on that point). It also takes quite while to dry out, which slows the build down.

Straw bales are another good, eco-friendly option, but they suffer from the same wall thickness issue as cob. Straw bale walls tend to be about 450 - 500 mm thick, which is similar to cob and again, seems to my novice eye to be a bit thick.

In July’s Self Build & Design, I discovered what might be the answer - hemp and lime (HL). Unlike cob and straw, HL isn’t a load-bearing material, but if we’re using a timber frame, then we don’t need the infill to be load bearing. Hemp and lime is a good insulator, requiring no additional insulation and no cavity, and it’s environmentally friendly - sequestering away over 110 kg of CO2 per m3 of wall covered. It also allows for much thinner walls of just 300 mm, and can be clad in anything from render to timber to masonry. Like cob, HL gives you lots of thermal mass, i.e. it warms up in the sunshine and releases that heat when the house cools, a sort of passive solar heating.

In terms of construction, whilst you can go the complicated way and spray the stuff on, you can more easily build HL walls by using shuttering, i.e. put up sheets of plywood and pour the mixture into the gap. According to Self Build & Design, it’s not the cheapest material at about £68/m2, but then you do save money because you don’t need to buy additional insulation.

Like many ‘alternative’ building materials, there are issues getting planning and building control officers to accept it, as many of them are not up to date with unusual building materials. But Hemcrete, produced by Tradical, have been through all the tests and can produce the relevant certificates.

Obviously I still have a lot to learn about building materials, but I’m really rather attracted to HL. If you have opinions on cob, straw and HL, do leave a comment!