Culburra Hemp House: Progress Made on Hemp Lime Rendering

We bought our pre mixed bags of render at the same time we bought the binder for the house. The lime render came with just a list ingredients to add and instructions to wet down the wall before applying the render. I had never used the AHMC render before I bought a pallet load of it for my house. I had done a brief bit of hemp lime rendering with Steve Allin at his workshop in St Ives in January 2012, but they used a different binder, very large hemp fibre and put on a layer almost 1 inch thick. This process was so different to that done at the St Ives workshop that it was of no use when it came to the hemp lime render we were using. My experiments to get the right colour and sand only gave me experience in doing 300mm square patches and even then the render sometimes went on really well and other times was really difficult.

Thankfully we had John our renderer to help us out, so these issues of getting the right mix and consistency were placed in his experienced hands. The first test wall John did, inside the wardrobe just in case things did not go well, was slow going but worked out fine. The next wall was faster but still quite slow. He quickly learnt that the hemp lime render does not spread easily across the wall, whether it is the hemp fines or some other reason, it is best to apply the render as close as possible to the location you want it and as close as possible to the finished thickness, then just use the float to smooth and even it out. The steel float gives a nice smooth finish, but the pieces of hemp and some of the large pieces of river sand ensure that the render maintains some texture.

The order of mixing using a standard cement mixer was ¼ bucket of water, ¼ of the 22.35kg bucket of river sand, ½ a bag of AHMC render mix, 80g of yellow oxide, the rest of the water and the rest of the sand and the rest of render mix. Mixing was a bit dusty but the holding of an empty bag over the mouth of the mixer after the render mix was added kept a little bit of the dust down. Later we changed this to using a bit less water and then adding a spray of water from the hose after the render mix went in and this kept the dust down.

The first part of the third wall went well then a slightly stiffer mix slowed John down substantially. He was not able to achieve the same finish as the first half of the wall, so he had to damp down the stiffer render, use a sponge float to work this water into the render then steel float it all again. Lesson learnt that there needs to be consistency in the amount of water in the mixes and the slightly sloppier mixes go on better. Achieving consistency in the wetness of the mixes is not as simple as using the same amount of water, as some buckets of river sand contain more or less water this needs to be adjusted for. So, like cooking it is about getting the look and feel of the mix right.

Front pavilion one day after rendering


Left wall fully dry and right wall 12 days after rendering

3 weeks after rendering

Wall freshly rendered

12 days after rendering

3 weeks after rendering

The weekend John worked on the third part of wall, my mate Tony was on site and having been to two hemp building workshops himself, he was keen to give hemp rendering a go. Tony and I got to work on one of the small MgO Board walls on the inside of the wardrobe. I used the stiffer mix and it was very slow going, in the confined space of the upper wardrobe it was awkward to trowel the render on, so I rubbed handfuls on the wall pushing them on until they were the right thickness then giving it a trowelled finish. Given that John was at that time rectifying the effect of the stiffer mix Tony experimented with using a much wetter mix. The much wetter mix went on faster and could be put on with a trowel, however in parts it had difficult adhering to the wall. Going back several hours later to fix a patch where the wetter render had slumped a bit, a whole patch fell out. It appears that you need the “just right” mix, not too sloppy and not too stiff.

The skinny wall done and patched by the non professionals

The following day John only had a couple of hours to spare and finished one of the walls with lots of windows. I used the “just right” render left over from John to patch the hole in the render on the side of the wardrobe. I had sloped the edges of the patch that had fallen out so there was less of a line where the join would be. Wetting down the edges of the patch and working the new render into the old, by making circles on the wall with my rubber gloved hand, the patch was filled and while it was not prefect it was acceptable. John had also left a small join next to one of the doors and when he finished it the following day had to feather in the new mix.

With still more render mix left over I worked on the upper inside section of the wardrobe. A ridiculous section to render as no none will ever see it, but a good place to practise. I was painfully slow at rendering and it was very hard work on my arms, but the finished render was acceptable and was sticking to the MgO Board walls. Given the snail like speed of my progress I am glad to have outsourced the rendering.

We have applied self adhesive render mesh diagonally across the corners of the windows and along the joints between the hemp walls and the MgO walls as well as along the joins in the MgO Boards. I am using a 5mm mesh that comes in 200mm wide rolls from Foamex. The render mesh is to try to restrain any movement within and between walls to prevent cracking. Despite using the render mesh we did get some hairline cracks at the corners of the first window we did. Later window corners had either no cracks or only very small cracks.

Only a very fine crack at the corner of the window

No crack on the other corner of the window

We used 200mm x 300mm strips of render mesh across the corners of the windows, however had some adhesion problems and these sections of render had a tendency to pull away from the wall while they were still wet. These patches were identified and fixed while the render was still wet. The cause of the problem was hypothesised to be the hemp fibres in the render preventing the render from pushing through the 5mm grid in the render mesh and coming into contact with the wall. For further corners we tried halving the width of the render mesh to 100mm x 300mm across the corners of the windows and doors. This resolved the adhesion problem and when fully dry after about 4 weeks we will see if there is any cracking. The adhesion problem was not such a big issue at the corners where 200mm wide render mesh was run along the joints between the hemp and MgO walls, but here only 100mm wide on each wall was being rendered at each time.

I had originally thought that we would need render mesh to cover the whole of the MgO Boards so that the render would adhere to it so I bought from Foamex (manufacturers of polystyrene walling and insulation systems) two rolls of 200mm wide mesh and two rolls of 1200mm wide mesh. I had believed that both widths were self adhesive, I checked the small rolls when i purchased them but the larger rolls were wrapped up and I just assumed they were the same. But, when I got them to site I found that the 1200mm rolls were not self adhesive. Going back and checking my original quote I found that the mistake was mine and the 200mm rolls were listed as self adhesive and the 1200mm were not. I confessed to my mistake and e-mailed Foamex asking if I could return or exchange the 1200mm rolls. As they were unopened and unused this was not a problem. I went back to their warehouse where I had originally picked up the mesh from and had no problems at all returning the 1200mm rolls, getting three more 200mm rolls and getting a refund for the difference. The fabulous service I received from Foamex could not be in more contrast to the problems I have had with the plywood.

While I was off site John continued rendering, as rendering is his second job it had to fit in around work and family commitments. John did three walls of western bedroom in the back pavilion and a section of the external wall, before he was struck down by illness and forced to take a break. It was interesting to watch the process of the internal walls drying. For the first three days after they are done it is hard to see much visual evidence of drying, although you can feel the render firming up. After about a week the walls look unpleasantly patchy as different parts of the wall dry unevenly. Two weeks later the wall looks much more even and after three or four weeks the render is all dry and stops changing.

Back pavilion a few days after rendering

About a week after rendering

Right wall done later than left

Rendering our first external wall was good and bad. Without the ceiling and floor detail to worry about the rendering went quickly and we chose to have a slightly rougher texture on the outside. Problems arose when a chunk of hemp wall next to the window fell out. I had taken an angle grinder with a masonry blade on it to the walls and taken off the sharp corners around the corners and around the windows, so that I could get a nice curve around the window frames. However, I believe that this cutting off of the corner may have cracked a weak section and we will not have to try and patch the section with render. Next time we will try rendering around the window without cutting the corners off the hemp walls.