How to make a hempcrete wall

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I am not quite sure that “smokeable” is a good advert for a building material.


So what happens when if the house catches fire? Very happy fire fighters? Whole neighborhood gets high?


the fire resistance is probably not worse than compressed and plastered straw bales. an Austrian fire test gave a resistance class of F90 (in a fire stable for at least 90 minutes, in Austria and Germany the required class for dwellings)


It is quite difficult to burn stacks of office paper too. You have to get the oxygen in somehow, and as the main additive is clay, when the outer layer burns it leaves behind a fire resistant ash. Without enough wind to blow the ash away, it keeps oxygen from getting in.
There’s now a steel designed on the same principle - it’s used on a bridge near us. It produces a very close grained solid rust which makes oxygen penetration hard, something like the way aluminium forms an adherent oxide that prevents further oxidation.
tl;dr the plaster on the straw bales is an important part of the fire retardancy.

You can buy hemp clothes here. Hemp is grown in the EU which has a very low cannabinoid component but which produces high strength natural fibres. It costs more than cotton but is a lot more environmentally friendly.


Seems to me I’ve been hearing stories about “Hemp is the environmentally-friendly low-cannabinoid material of THE FUTURE!” for decades now. Is it really just PR that’s holding it back?

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I think it has been the efforts of the US government to stop people growing it, and the strength of the cotton lobby, plus the agrobusiness lobbies. Hemp unlike cotton requires little insecticide and fertiliser. It is simply too easy to grow to find favour.
According to some accounts, Columbus’s secret technology to reach the Americas was hemp rope, which could last a Transatlantic voyage. Francois Rabelais, writing in the mid-1500s, described it as a truly miraculous substance that could be used to make medicine and cloth.


Since I live in an 1880-era adobe, this is an interesting subject. You can’t just slap insulation on adobe because a moisture barrier causes condensation at the adobe/insulation interface, which over time destroys the adobe.

If fibercrete (hemp, cornstalks, whatever) is vapor-permeable but insulating it could be combined (fibercrete over adobe) to produce a combination with the insulation properties of the fiber and the thermal mass of the adobe. Which would be a serious benefit in large parts of the world.


Try researching ‘cob’ building, also papercrete. The basics seem pretty universal: dirt with a heavy clay component or Portland cement or slaked lime to act as a binder, sand, and a shredded fiber to act as another type of binding agent. The main comparison between straw bale construction or adobe is that you want to let this formula breathe so it doesn’t get moldy, so go light on surface treatments if you use them at all.

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