Wednesday, March 10, 2010

Building with hemp.

Hey man, hemp is so cool! But if you remove the active hippy ingredient THC (tetrahydro-cannabinol) from your variety of cannabis sativa (so that the police will let you grow it) and stuff it into the walls of your house you get a very useful insulating affect, certainly enough to keep a couple of ageing hippies warm on a cold night.

Hemp is a great plant to grow, as it’s a low maintenance but very abundant crop. A method of building pioneered in France is to mix the chopped up stalk—“shiv” in English and chenvottes in French— with building lime to create a porridge that you pack between shutters, which dries to give you a solid wall that is both insulating and very breathable.

A couple of houses were built using hemp lime in an affordable home project in Suffolk, England in 2001, their construction and performance monitored by the Building Research Establishment and compared to equivalent more conventional brick-built houses in the same development. (The Haverhill Hemp Houses–BRE). The thermal performance was apparently better for the hemp houses and outperformed calculated expectations. If I understand this correctly, there are two reasons for this: 1.) the walls have a mass to them that can store heat so that the houses were slightly warmer for the same amount of heating energy and 2.) hemp/lime walls are extremely breathable and the reduced humidity in the building leads to a reduction in the ambient temperature at which one feels comfortable .

Apropos of needing to rebuild the wall around a new (recycled) front entrance door and learning about the material with an eye on our anticipated self-build ecohouse, we tried the technique out. The photos show the new front wall under construction, a handful of hemp shiv, and Gabrielle tamping the mix between wooden shutters.

Maurice Lebret, a local farmer, now produces hemp for use in building. He supplies sacks of chopped shiv and along with bags of lime but has been thinking along loose fill lines too. Knowing how cellulose (shredded old news-papers) is blown into spaces in walls and ceilings, he has innovated a method to do that with hemp fibre. It’s an agricultural machine used for distributing animal feeds inside barns, adapted with a fill hopper and a blade to chop up the hemp fibre. Micehl proudly claims that it's the only one like it in the world! Christophe Le Petit, the guy who installed our grey water plant filter and pond, invited me round to see the machine in action.

He’s currently building a timber-framed house for his family. The structure is clad on the outside with sheets of OSB to provide racking strength, and he has tacked a breathable membrane onto the inside to complete the “boxes’ into which he blows the loose-fill hemp fibre. The photos show Christophe loading the hopper (as I try to snatch a photo while holding the other end in place) and then him aiming the tube into the spaces in the wall through a hole cut in the membrane, which is then sealed with tape when full.

One caveat I read in Tom Woolley’s Natural Building: A Guide to Materials and Techniques is that in an example of loose-fill hemp being used to insulate a floor, “the hemp has been allowed to get slightly damp and become infested with paper lice … most natural and cellulose materials if allowed to get damp can breed paper lice, which, while having no serious health effects, can be a nuisance.” Tom is an advocate of hemp lime walls, see Hemp Lime Construction: A Guide to Building with Hemp and Lime Composites. by Rachel Bevan and Tom Woolley, a comprehensive technical guide to this method. See also The Green Self-Build Book by Jon Broome, pages 91-95.