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.

Windows Installed

A mini tornado lashed parts of the south coast the previous weekend and upon our arrival at Culburra we were greeted with a big Eucalypt branch on our back gate and garage.

Fallen branch

We had arranged for our windows to be delivered Friday, with an estimated time for arrival of 1.30pm. It was raining Friday morning and a 7am call from Rylock gave us the opportunity to cancel the window delivery. I took a gamble that the rain would stop and had the windows delivered.

We spent the morning covering up the polished concrete floor. While we had been away from the site, Henry and Adrian from the Dayman Group came in and gave the concrete floor a further grind and applied a water based acrylic finish. The finish was more reflective than we expected, so it is a good thing we only chose the satin finish. We did not want to damage the finished floor and so covered it with black plastic and some sheets of OSB to protect it while we did the internal linings and rendering.

Polished concrete floor

Close up of the aggregate in the floor

I was expecting a call advising that the windows were leaving Sydney as I had arranged for Rodney, my window installer to be on site at 1.30pm to unload the windows. I had to delay them about 1 hour after I called up Rylock at 11.45 to see where the windows were only to find out they were to leave Sydney in about 1/2 an hour. Then with news that the truck left Sydney at 1pm. I arranged for Rodney to be on site at 3.30pm, as the trip from Sydney is about 2 1/2 hours. Rodney arrived on time, but no windows. We waited, made phone calls and waited some more and at 5.15pm the windows turned up. Rodney, Joe and Tom worked until 6.15pm on Friday night unloading the windows. The window installers plan to start installing the windows on Friday afternoon was not to be.

Unloading the windows

Sliding door stacked in the house ready for installation

Rod and Joe started early Saturday morning and by late on Saturday all the windows and sliding doors were in. The integral hardwood timber reveal in the Rylock windows made it easy to install the windows in the hemp walls, as they could be screwed to the timber frame in the centre of the walls through the reveal. The installers used plastic shims in different millimetre thicknesses to level the windows. The doors were a little trickier as they were set down in the concrete which was not always perfectly level. The doors ended up having the tracks a few millimetres above the concrete, but this will work out well as it is not big enough to trip over but enough to stop the dust and dirt falling straight into the sliding door tracks.

Sliding doors fill the big empty holes

Windows make the building look more like a house

The windows are a mid grey colour called Windspray, however in the photos the windows still have protective black tape on them from the factory. The windows have about a 20mm gap all around, which was required to be left to install them. We will probably fill this before we finish the walls with 10mm of lime render. The window installers recommended creating a small groove between the render and the windows and filling with Sikaflex to prevent cracking at the joint as the Aluminium windows will expand and contract with changes in temperature.  They also recommended filling the small gap on the inside between the polished concrete slab and the sliding doors with black silicone sealant so that it does not show up against the floor.

Upper windows match sliding doors

Diagonal windows in hallway

Double horizontal windows in the bathroom

While the windows were being installed we started on the ceiling. The ceiling will be made of plywood with the joints between the sheets expressed with a 9mm shadow line. The long end of the sheets goes across the joists and the joints are all on the joists, but in the opposite direction I had to add timber noggin behind each of the joints. I offset the joints and painted the timber behind them black using Resent VOC free paint. This gave a nice matt black finish. We then started installing the sheep's wool insulation.

Rafters and noggins behind joints painted black

Insulation being installed

 

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.