Tuesday 6 January 2015

Rendering Magnesium Oxide Board

The laundry and second bathroom had been left unrendered as these rooms had tiled floors and did not need to be done for the polished concrete floor to be finished. Further delay was caused as I had not finished the mosaic tiling for the bathroom. With Christmas coming up and more people staying in the house, it was time to finish the second bathroom.

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
I tried three different mixes applied using two different methods. The top row from left to right are (1) standard AHMC hemp render mix, (2) half AHMC hemp render mix half lime/sand mix and (3) straight lime sand. The lime/sand mix replaced the same volume of AHMC render with straight builders lime. The bottom row uses the same mixes but with the addition of a coat of Bondcrete on the MgO board left to go tacky. The Bondcrete did not work at all and in fact it was harder to get the render to adhere to this mix than to the untreated MgO board.

Dry render tests on MgO board

(1) standard AHMC hemp render mix

(2) half AHMC hemp render mix, half sand/lime

(3) sand/lime
 I had a bit of mix (2) and did some extra sections at either end of my test MgO board. I have some disclaimers to this test as firstly I am not a good renderer, secondly the test was carried out on a hot day and thirdly the number (3) mix had too much lime to sand and should have had more sand, the replacement of AHMC mix with straight lime should have been done by weight rather than volume as the AHMC mix had hemp fibres in to which are very light but bulky.

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 bathroom also had one hemp wall. The amended render went on this wall really well. When John started rendering this wall I wondered why he finished so quickly, it was because even with the amended mix it was still much easier to to render a hemp wall than a MgO wall.

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
The only disappointing news was that we were one bag of render short and one wall in the laundry is still unrendered. This will be fixed up in  the new year. While disappointing it was better to leave this wall than try and do a too thin coat of render and have it crack or not work well.

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