Tuesday 14 August 2012

First Full Day of Hemping

With bad weather predicted we headed off for our first full weekend of hemp building. Friday night we finished off the formwork for the following day and got the ingredients ready for the first three mixes the following day. With two adults and a three and a half year old we had no idea how much we would get done in one day. Saturday morning we started mixing and tamping. Our three year old helped out measuring out the hemp and the sand and had a little play with the tampers, and he loved rolling in the pile of sand. Unfortunately the lime in the mix meant that we did not let him touch the mix and kept him away from the mixer when using it.

We became more proficient at mixing, discovering that it was best to put the hemp in first, lifting the grate off the mixer to stopping the fine bits of hemp getting stuck to the grate. We then slowly poured in one bucket of water. Pouring ti in slowly meant that less leaked out the bottom and any water that did was caught in a bucket and addded back to the mix. The binder was best put in again when the mixer was stopped with the grate opened. We found that if the top of the bag was placed into the mixer and the binder poured out, this kept the binder dust down. Putting in half the bag of binder mixing it a little bit then putting in the rest of the binder seemed to mix it quickly and evenly. Once the binder was in we then slowly poured in another half a bucket of water. The sand went in last, again when the mixer was stopped and the grate open. We resolved the problems we had previously with the mix being pushed arounf the mixer withour mixing, by somewhat unsafely putting a piece of wood through the grate on the mixer to push around the top of the mix and keep it mixing up.

The damp proof course made the first two or three layers of hemp difficult to tamp down. Firstly where there was a step down in the concrete the hemp mix had to be poured down into this small section taking care not to push the top of the damp proof course out toward the outside edge of the hemp wall. This first section was tamped  then layers added on each side of the damp proof course until the top was reached. Up to the top of the damp proof course the hemp on each side of the damp proof course had to be tamped separately, making it a much more fiddly job. Once this part was done we could tamp the full width of the wall, building it up in 5cm layers. We used several different tampers, from a wide one to a narrow one to a block of wood for some small sections and sometime our hamds for around the spacers. Part way through the day we had to make up some tampers with shorter handles to get under the noggings and window sills. As we were not sure how much we would get done in a day we unfortunately had to stop part way along a wall, we finished the section off mid wall with a steep diagonal.

We used flexible tub buckets to move the hemp from the mixer to the walls. Each mix confortably filled four 36 litre flexible buckets. The buckets could be shaped to make pouring into the formwork easier, dependoing on wheather it was a wide or narrow section. Plastic scoops made from old juice bottles also made putting the hemp into small spaces easier. Hemp walling definitely requires gloves. At first I used fabric gloves where the palm and fingers are dipped in rubber, there are comfortable and great for protecting your hangs while doing formwork but do not offer enough protection for hemping, as the do not come high enough up the wrist and the lime gets in through the fabric on the back of the glove. The problem with full rubber gloves is that your hands sweat in them as you work. The solution was to wear a pair of cotton gloves under a pair of double strength rubber washing up gloves. This kept our hands clean and dry and we will just wash the cotton gloves between each days use, or it it is a hot day we can change the cotton gloves half way throught the day. 

By lunchtime we had done 9 mixes and finished the first part of our wall up to about 55cm high. We had also hemped around our firts electrical outlet box and conduit. So, after lunch we continued on after our diagonal stop and finished up to a natural stop where the internal bathroom wall intersects with the outside wall. This took another 9 mixes.  All up we think it was a successful day - 18 mixes in a day for two adults and a three year old. This was 10m of wall 550mm high and 20mm thick. We have also coined a new word "hemping" - a verb meaning the mixing and tamping of hemp walls.

Sunday we removed the formwork and moved it up. In some sections we were not able to move it up the full height of the next course as we had noggings in the way that we will have to hemp underneath. we hope that by putting the formwork just at or under the noggings we will still be able to pour and tamp the hemp. For the window sill, on the inside we formed up to the height of the sillon the inside and about 35mm below the sill on the outside. We will slope the hemp on the outside of the sill down a bit to allow water to run off. 

The formwork was easier and quicker to move up than it was to make in the first place, as when the formwork was first placed each piece was cut and fitted so that it could be easily atached to the studs. When joining pieces of formwork together we often attached the two pieces to the same stud so that the formwork was held top and bottom at each end with a further spacer and screw in the centre to stop the formwork board bowing. Where there werewater pipes and electrical cabling protruding through the wall we just made holes in the formwork at the appropriate places.

The results of our first full day of hemp walling were good. The part where the two section of wall joined togther was hardle noticable and our tamping was quite consistent. The only isse we had was a small section of the bottom of a wall falling out when the spacer was removed. This was probably for two reasons, the section under the sspacer was not tamped enough and the damp proof course had buckled in this section creating a gap when the formwork was removed. No problems, we will just patch this section next time.


  1. Thanks again for your very detailed blog!

    It will be interesting to see how long the 'hemping' takes and whether it speeds up as you gain more experience.

    Have you made an estimate of how the building will cost per m2? I am curious how it compares with conventional methods.

  2. Cost per cubic metre for wall materials was $333 and with 200mm thick walls this equals $66.60/ m2. This does not take into account the volume of the timber frame and does not include the cost of the render. Cost will vary depending on where you build as transport is a significant part of the cost.