Help arrives at hemp building

On the last day of winter we began to share our hemp building skills. Tony, who we had met at Klara Marosszeky's Hemp Building Workshop in October 2011, joined us for the weekend. Braving an afternoon cold front that significantly dropped the temperature the three of us managed to do 18 mixes of hemp walling, working in some tricky areas under the noggins.

Saturday brought improved weather and further reinforcements, friends Martin and Nerida and Tony's son, Joel and Joel's partner, Alice. Our helpers picked up the mixing and tamping skills quickly and did a great job. As the walls got higher we used firstly milk crates and a plank as our scaffold and then a plank on a some steel stands. To fill and tamp the walls as they got higher we had to be able to both see down into the form work and be able to reach a hand or tamper down to even out and tamp the layers of hemp lime mix.

Hemping higher up the walls

Working in a section with several closely spaced studs

Once again we were working under window sills and noggins, but changes to the placement of form work made the work easier. The form work was at or just below the sill or noggin on the outside and about 12cm below the sill or noggin on the inside. This created enough room for an arm to reach under the noggin, but also allowed the hemp to be built up to and under the noggin on the outer side with the inside to be filled when the form work is moved up. Work was slow where there were lots of closely spaced studs, but difficult to get at sections were easier with one person spreading and tamping the hemp lime mix from the outside and another working from the inside.

Tamping under a window sill

Window sill - sloped down on the outside to shed water

There was great camaraderie and we set a new daily record of 26 mixes, bringing our overall total to date to 102 mixes. To achieve this we had five people working all day and extra two people for half a day, however when we had seven people two were moving form work up while the rest mixed and tamped the hemp. We have been using half a bale of hemp until it became light enough to lift on top of the other bales of hemp, this is so we could clear more floor space and gain access to more of the external walls so that we can form them up.

Layers of hemp - differing heights of layers due to location of noggins and windows

Layers of hemp finished over different weekends

Sunday was a glorious sunny spring day and this made the work of moving the form work up more pleasant, as did working with a tall helper. As the form work was moved higher up it became necessary to have two people to move it up. In one section the form work has been moved up to where it will join the eaves lining, we will probably have to do this section in small rises, with the form work lower on the inside.

Form work getting higher

Form work moved up, one section on outside to eves height

Materials Choice

In February 2011 I saw an article in Owner Builder Magazine about hemp construction, but dismissed it as too new and untested. At that time I wanted an exposed post and beam timber frame and was looking at natural infill materials. I researched Light Straw Clay, but had concerns about the straw rotting before the walls dried and the time required for the walls to dry. Hemp lime masonry and light stray clay use different materials but in a similar way, combining a fibre together with a binding substance.

I began to research hemp lime masonry construction and found that while it is relatively new in Australia it has been used in the UK for over 10 years and in France for 20 years. First stop I read “Build a House of Hemp”. I found UK book “Hemp Lime Construction” by Rachel Bevan and Tom Woolley a great resource, scientifically looking at the pros and cons and data in relation to hemp lime building. Also useful was an early study in the UK on the Haverhill Project done by the Suffolk building society, comparing the construction and performance of a hemp masonry building as compared to standard UK masonry construction.  "The Green Self Build Book" a UK book by Jon Broome also has a section on the hemp masonry renovation and extension carried out by architect Ralph Carpenter.

If I was going to build with hemp masonry the exposed timber frame had to go, and this was probably good from a maintenance point of view. One of the benefits of hemp lime construction is that it completely covers the timber frame protecting it from termites and rot, the hemp lime being termite proof and the lime and breathability of the walls assisting in the rot resistance.

Weighing up all the factors hemp lime masonry came up as my first choice for walling, it combines insulating and thermal mass properties, it reduces gaps by being poured monolithically in place, reduces the double handling of mud bricks, it is lighter and would be easier to work with than mudbrick or rammed earth, the lime sets as it mineralises avoiding the problems of clay rewetting, it has a comparatively low embodied energy and primarily uses natural materials.