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.