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.

Using Magnesium Oxide Board

Unable to finish the ceilings, while we wait to try and exchange the plywood, we started on the magnesium oxide board internal walls. We chose to use the magnesium oxide board (MgO board) as it is breathable and therefore compatible with the hemp walls, but it is also fire proof, mould proof, rot proof. MgO board in a relatively new product in Australia, it is all made in China. Price wise it is cheaper than fibro but more expensive than plasterboard. I researched the Internet for fixing and cutting methods. It was recommended that the MgO board be cut with a jigsaw or a low speed circular saw. I have a circular saw but it is only a high speed one and I did not want to blunt the blade and did not use it. The other option is to score and snap the board but I was not confident that this would give a neat join with an 8mm board. So I used my jigsaw. I quickly blunted the first blade but did not take notice of how sharp the blade was in the first place, so I changed the blade. After two or three cuts the new blade was also blunt. I went to the local hardware store to get a fibre cement blade, as I had been using wood blades, only to find out the no such blade exists. I queried how people cut fibre cement sheets and was shown a tool that looked a bit like a pair of bolt cutters. These would never work on the 8mm MgO sheets. The alternative suggestion was an angle grinder with a masonry blade.

I tried cutting the MgO boards with the angle grinder. it worked well and was easy to get a nice straight line. It was dustier than the jigsaw, but the masonry blade was still going well after more than 20 cuts. The dust is supposed to be non toxic, but any dust is bad, so I made sure I used eye protection and a dust mask when cutting the boards. Cutting the boards was straightforward and not difficult. I set up a large flat area with a couple of movable spare OSB boards, reused from the formwork, so I could make a gap where I needed to cut and leave the remainder of the board supported. This worked well.

The MgO boards are fixed with screws and the boards can be laid vertically or horizontally, depending on the wall dimensions. Vertical joints should be on studs and fixed each 200mm on edges and 300mm centres on intermediate studs. However the instructions also state that the boards should not be fixed with in 60mm of corners, presumably so the corners do not crack off. Also the edge fixings need to be 12mm from the edge, presumable also to prevent cracking. With 40mm wide studs this allowed just enough room to join the boards on a stud. The boards were tapered at the edges so if you were painting them you could tape and fill the joins. As I am rendering the internal walls, so they look the same as the external walls, I am not doing this, but will use a render mesh.

Cut sheet of MgO board installed

Several sheets of MgO board put up

I went to a large chain hardware store looking for fibre cement screws, they have a different tip and cutting barbs under the countersunk head. The hardware store did not have any fibre cement screws so I bought a packet of galvanised timber screws that looked like they had some barbs under the countersunk head. The timber screws worked but were a pain to put in, and were slow, you had to push really hard to get the countersunk heads to go flush without shredding the Phillips head in the screws.

A week later I sourced a box of 1,000 30mm galvanised fibre cement screws with square heads from Nepean Boltmaster.  These were great, went in easy, no chance of shredding the thread. Fibre cement screws were recommended and they definitely make the job easier.

Timber screw in left, fibre cement screw on right

I was one bag of insulation short to finish the house, plus I had not bought insulation for the hallway. I bought the sheep's wool directly form Higgins Insulation and picked it up from their Villawood warehouse to save transport costs. When I complained about one bag of insulation being a batt short and one bag being misshapen, I was advised that their Sydney operation had closed down. Then when I called head office in Queensland I found out that although the Sydney operation had the same "Higgins" name it was separately owned, this meant that I could not do anything about my missing batt and if I wanted to finish the house in sheep's wool I would have to get it from Queensland. If I was going to get one extra bag of insulation, I decided that I might as well do the thinner raftered hallway in sheep's wool insulation as well.

Four not five batts

Misshapen batts, correct batt in the centre

I ordered three bags of R3.0 580 x 1200mm batts. Higgins in Queensland gave me a quote for transport and the first quote was higher than the cost of the insulation. They could see that was ridiculous, so made other enquiries about transport. The next quote was better and less than half the cost of the previous and was half as much again if I collected it from the transport company depot at Wetherill Park. So I collected it from Wetherill Park.

I had put some packs of insulation in the car before and thought I could fit three packs in, but the packs I had collected before were smaller and these were packs of 6 580 x 1200 batts.  I had the family with me as we stopped to pick up the insulation on the way down to Culburra. The lady at the transport company thought there was no way we could fit it all in and my family berated me for not bringing the trailer. So everything was emptied out of the car, my son's seat was moved to one side, and we fitted two packs of batts in the boot and one behind the passengers seat and the rest of our gear was stuffed in around the insulation. We were fortunate I could fit it all in, but lesson learnt when picking up insulation.

Once we had the insulation it was not a problem finishing the installation although one section where the front pavilion joins the main pavilion and all the water and gas pipes were was a little tricky and the insulation had to be slit and tucked around the pipes. The whole house in now insulated and it does make a noticeable difference to the temperature at night.

Work also continued on finding the right render colour. Our earlier render tests showed us that we would need to use some kind of cement colouring oxide to achieve the colour we were after. I went to South Coast Decorative Concrete Supplies hoping to get several different oxides to try out however they only sold the oxides in 9kg bags, which made it too expensive for me to get more than one bag. After much debate I chose Sundance from Concrete Colour Systems.

CCS Sundance oxide

I mixed up small batches of render using 1/20th of a bag of render mix and then divided this in half, making up a mix using 1%, 2% , 3% and 4% oxide. However I only had kitchen scales accurate to 1g to measure out the oxide and I was using such a small amount of oxide I was hard to get it accurate. I have since ordered a set of jewellery scales on E-bay that are accurate to 0.01g.

1% and 2% Sundance oxide when still wet next to the earlier render tests

I chose the Sundance colour as it had an orange tinge to it which I though would counteract the greenish tinge of the uncoloured render. However, the Sundance looked a bit more orange brown than we wanted. We are looking for a light warm yellow. So we bought a 500g jar of Yellow Builders brand oxide from the hardware store. That turned out way too yellow an still has a slight greenish tinge while wet. We then tried the 750g jar of Sandstone Builders brand oxide, although we have not yet seen it dry the test when wet looked too brown. The closest to the colour we wanted was the 2% Sundance so we did a further test of this on the hemp wall and on the MgO Board and will see how this looks when dry. The more accurate scales will allow us to do more accurate tests in the future.

Top row 4%, 3%, 1% and 2% Sundance all dry. Bottom row on MgO Board 2% Sundance, 2% and 1% Sandstone all wet

2% Yellow and 1% Sandstone both still wet in afternoon sunlight

2% Sandstone and 1% Yellow both still wet photographed with flash

Work has now stopped while we wait for the" blue marks on the plywood" situation. We hope to get building again soon.

Second hemp render test and Plywood Problems

Three weeks after our first render test we got to see the dry colour - grey. Not a successful test. Anticipating, from the colour when wet, that it may not be the sandstone colour we were looking for I sourced some dark yellow river sand from Thomas Landscaping at Lawson and they gave me a small plastic bag of it for free to test out if we liked it. So this became render test two.

Dry Sydney sand render test on left

Dry hemp lime render mix before sand added

 The Australian Hemp Masonry Company, who made the render mix, recommended using river sand but I first tried using Sydney sand (Beach sand) because local river sand in the Shoalhaven is a dark grey, not the colour we were after. I really liked the colour of the new dark yellow river sand, but when mixed with the render mix it got the grey/green look of the previous mix, albeit this time a bit yellower.

Yellow river sand render test showing colour of sand used

I am yet to see the second render test dried, but am ready to give up on the hope of being able to colour the render with sand alone and will now try some oxides. AHMC advise that the hemp wall need to be wetted before the application of the render, this was a bit tricky for our first render test, but for our second render test I used a garden pressure sprayer, of the type usually used for fertiliser. It worked well giving a fine spray of water that meant it was easy to control the area that got wet and how much water was applied.   

Pressure sprayer in front of render test

 Meanwhile work continued on the plywood ceiling. Despite one wall of the front bedroom not being square, we managed to do quite a good job on the ceiling.

Front bedroom ceiling finished

Further work was then stopped as the BB Hoop Pine ply had some horrible blue marks on it. Both on the back and face surface. We looked through the remainder of the pile and managed to find two sheets with which to finish the front bedroom. The rest of the sheets had blue marks that looked like mould, to use it on the ceiling would look like the roof had leaked and the ceiling was going mouldy, not what I paid good money for B grade ply for. 26 sheets had unacceptable blue marks on them.

Blue marks on ply

Stack of blue marked ply

A call was made to the supplier Huskisson Ply late on Friday, but the boss was not in, so no response. Several phone calls later, I finally caught the boss on the following Thursday. I e-mailed off pics of the blue stained ply and they will see what they can do. Between trying to get through to Huskisson Ply I also phoned the ply manufacturer Austral Ply to find out about the blue marks. It seems that the Australian Standard says that they are acceptable for a B grade ply - but who reads the standard! I thought by paying the extra for a B grade that the face would be good enough to use for the ceiling. None of the pics or info on the Austral Ply website mentioned blue marks. Apparently they can come about by the logs being too wet before they are made into ply and starting to go mouldy, while i was told that the ply production  process kills the mould - this does not help the fact that if used it would look like my ceiling was mouldy, not acceptable for a new house. Austral were very helpful and suggested I get the place I purchased it from to contact their supplier and see if they could exchange the problem sheets of ply.  

Meanwhile with no more usable ply sheets, we worked on the blocking that goes behind the express joints in the main room. Whether it was experience or working in a larger room but the work seemed to be getting easier. I have also accepted that nothing is ever perfectly straight in building and that you need to measure the exact location of where each piece goes and cut it accordingly. When I first started the blocking I had some problems skew nailing it with the framing nailer, particularly pushing the nailer in far enough to fire and getting the right angle. Now I can do this with ease, even using my right or left hand.

Blocking for express joints in ceiling

If necessary we can also move on to starting the internal stud walls since the Magnesium Oxide Board finally arrived. We are mostly working on site on weekends so originally booked in for the MgO board to be delivered on a weekday we would be on site. That day arrived and when it had not arrived and we had not heard about the delivery by mid afternoon I made some phone calls and found out that my instructions that the delivery had to be made on a particular date were not passed on to the transport company.

There were many subsequent phone calls as the delivery company could not deliver on weekends and could not deliver without us on sit  as we had to unload it, so because of other commitments we had that meant we could not be on site on a weekday, our delivery of MgO Boards arrived four weeks after we had originally booked for its delivery.

The MgO sheets are 8mm and did not look too heavy, but looks were deceiving, I was only able to carry one sheet at a time together with another person. Further because the truck was side loading it could not go down the driveway, otherwise there would have been nowhere for it to unload. We started carrying sheets down the driveway one by one. This would have taken a long time as we have a battle axe block. Things were sped along when the delivery boys started using a trolley loaded with four sheets at a time to wheel the sheets to the end of the driveway, from where Ben and one of the delivery boys were able to carry them two at a time into the house.

MgO Boards stacked in the house

Why did we use MgO boards - they are breathable, not damaged by water, dimensionally stable, do not harbour mould or mildew, are not eaten by termites, insects or vermin, are non toxic, and I had seen them used on Roger Bodley's hemp house in Tasmania, where they seemed to work well rendered as internal walls, which it the application I will be using them for. 

Cracks in the Concrete

Despite using 32MPa concrete, we have cracks. So far it looks like there are cracks in the larger sections of concrete approximately every metre. We can only see the edge of the slab and where there are door openings because we have covered the floor with plastic and boards to protect it while we build, our walls also cover most of the top edge of the slab. So far we cannot tell how far across the slab the cracks go, but the cracks go right through the outer edge of the slab. We were warned from several sources that it is in the nature of concrete that “it cracks”, but I hope that the cracks are not too obvious in the final finish of our polished slab.

Crack down the outer edge of the slab

Crack on the top of the slab

We have joined the owner builder sharing merry go round. On the take side, Tony and his owner builder mates have kindly lent us a couple of pieces of scaffolding. Now that we are building walls over 2.7m high the scaffolding is immensely helpful. With no noggins in the way, working on the scaffolding was almost as easy as working on the ground again, with the exception of having to climb up and lift buckets of hemp onto the scaffolding. On the give side, we have passed some excess OSB boards on to owner builder Nicole for her to use as form work for her hemp building project.

Bathroom window openings finished

The very useful scaffolding

 We now have the tricky task of working at head height under the roof, finishing off the top layers of the walls. This is difficult, particularly in the bathroom where the rafters are only 150mm deep. It will make us appreciate the 250mm rafters when we get back to working under them. However, once we realised that we could do the work by feel and did not need to look down into the form work it became easier.  The joy, after doing this work, was taking the form work down for the last time.

Finished to ceiling height under 150mm rafters

Hemping around the parapet wall where the lower front pavilion roof meets the vertical wall of the main pavilion was tricky, mostly because there was a a section that will be in the ceiling space of the front pavilion, but is the back of the wall of the main pavilion that we could not get any form work into and ended up stuffing the space with a piece of Formex left over from the concrete joint.

Parapet wall with black Formex filling gaps between the pieces of form work

Thoughts and organisation have moved on to the next stage of the process and I have ordered my plywood ceiling lining from local company Huskisson Ply – their price was good and delivery is only $20. After researching the performance, properties and price of various ceiling insulation I have chosen to pay a little extra and go with sheep’s wool insulation. My final choice was between the sheep’s wool and recycled polyester. Finding out how sheep’s wool insulation could deal with changes in moisture and that it was a natural product were the determining factors in deciding to go with the sheep’s wool. 

I found a good price through an on-line building supplier, but the insulation was coming down from Queensland and the cost of freight was almost as much as the cost of the insulation. So, I contacted the manufacturer, Higgins Insulation, whose website indicated that they sold the same insulation at an only slightly higher price than the on-line seller and the insulation could be picked up from their Sydney warehouse. It turns out that the Sydney warehouse did not have enough in the R-value and size I needed, so with the offer to get 3.5 for the same price as 3.0 I now have a mix of each and in 580mm wide and 430mm (which cut in half will make 580).

I will use the 3.0 in the main pavilion as it has higher ceilings and the best ventilation, to make use of cooling breezes in summer, and the most amount of north facing glass, so I hope it will warm up well in winter. The rest of the house will have 3.5 except the hallway which has 90mm rafters. I am yet to work out the best form of insulation for the hallway but suspect it may have to be some sort of expanded polystyrene, in order to get the same R-value in a thinner material. 

I will be using Magnesium Oxide (MgO) board on my internal cavity walls and then rendering this with the same 10mm of lime based render as the inside of the external walls, to give the walls a consistent finish. The MgO Board will be coming down from Quantum X in Queensland as a distributor in Sydney from another company could only supply 10mm or 12mm boards. MgO Board is not manufactured in Australia, but in my view is a greener option than fibre cement or plasterboard and I will render it with the same lime based render I am using on the external walls so you will not be able to tell the difference between the external walls and the cavity walls.