I had been unhappy with the finish of the AHMC render mix on the MgO board walls and the renderers had found it very hard to get the render to stick to the MgO walls. I raised my render on MgO issues with some other hemp builders, sought their assistance and received several suggestions. I had also seen some straight sand lime renders used on strawbale houses and with all this I undertook my own rendering experiments.
|Freshly done render tests on MgO Board
|Dry render tests on MgO board
|(1) standard AHMC hemp render mix
|(2) half AHMC hemp render mix, half sand/lime
Mix number (1), the standard AHMC mix, was still difficult to adhere to the MgO Board and as on the MgO board the mix can not press into the wall like it can with a hemp wall, the hemp fibres came to the surface and stuck out too much for my liking. Mix number (2), the half AHMC half sand/lime mix was easier to spread and adhere to the MgO Board, it also produced a smoother surface. Mix number (3) the sand/lime, mix cracked badly when applied at the same thickness as the other mixes and was hard to apply in a single 8-10mm coat. I hoped to be able to get away with using a single coat to save labour and hence cost.
John, my first renderer, came around and looked at the tests. He agreed that the second mix looked best but we agreed to use a little less lime in the sand/lime mix. The mix we used was two bags of 22.35kg of river sand (this had been measured out earlier for the render and we had some measured out bags left over), one 9kg bag of AHMC hemp render mix, half a 9 litre bucket of builders lime (so about 4 1/2 litres) and 160g of yellow oxide (this was double the amount used for a single AHMC mix, since in effect we were making a double mix).
The new double size mix of render was mixed up in our pan mixer (mortar mixer) that we had used to make the wall mixes. Previously we had used a standard cement mixer for the render as out pan mixer was out on loan to another hemp builder. The pan mixer was a bit slower than a cement mixer but did a good job mixing the render and was much less dusty.
The amended render mix adhered to the MgO board much better and easier to move around on the wall. The MgO was still not the greatest substrate to render on but it was now much more achievable. A smoother more consistent finish was also able to be achieved. The only issue was that the walls were a little yellower than expected. I really should have tested the actual mix we used first rather than just relying on my test. I assumed it would be ok as my test used a proportional amount of oxide, but as we now used less lime the colour became more intense. The good thing is that the family liked the more intense colour and it is a nice bright sunny yellow in these small south facing rooms.
|Amended render on MgO Board
|Rendering the laundry
The last walls in the bathroom were rendered between Christmas and New Year. It was great to be able to hang the bathroom mirror. When we were building the walls we put an extra nogging in the frame right where the mirror had to be hung and thankfully we remembered where it had to go and the screws hit the nogging.
|Bathroom mirror on wall
|Freshly rendered bathroom wall