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
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
|Four not five batts
|Misshapen batts, correct batt in the centre
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