OK – so I’ve played around with drawings and taken some pics and measurements. My wife keeps asking me why I keep standing at looking at the structure rather than working!
To follow through the “mass on the outside” approach, I took Glenn’s diagram and made some modifications.
- Mass outside - components
- Mass outside - from above
The
first modification is to move the outer frame outside the
mullion column we’ve been focusing on rather than butting up against it – the reason for that becomes clear when I include the column at the end of the wall – the frame needs to butt up against the end wall column to allow the cladding to sit in front of the column in the corner. I’d lost track of this in focusing on just the isolation around the mullion column.
The
second modification is to put part of the mass on the outside of the frame (1 layer of hardie board – 8.5mm and one layer of 9mm ply) but the rest is in the gaps between the framing (2 x 13mm drywall/gyprock) – I’ve seen this done on a couple of other builds. The reason for only part of the mass sitting outside the frame is to keep the cladding inside the edge of the roof line.
The inner wall is per Glenn’s diagram – noting that I’ve included a layer of mdf/ply/OSB to replace one layer of drywall.
Unless I’ve misinterpreted something – it all hangs together EXCEPT for the weak point at the base.
- Weak point
The damp course and weatherproofing has not been designed or implemented to the strict standards for isolation because I thought my isolation would begin from inside the outer frame.
You can see in this pic below that there is air space between the base plate and the DPC – the 10mm packer sizes it at this point - there are other packers but they are in the shadow under the timber.
- Gap between DSP and Base plate
- DSP fixing inside FYI
This is due to variability in the slab. This means that for the majority of the perimeter the only surface between the outside world and the inside of the outer wall is 0.3mm of aluminium – the DPC. (What I was going to do in my original design was to lay down some extra base plates inside the current one with a DPC and plenty of caulk underneath. The outer leaf was to sit on this. The bottom 90 x 45 would be planed to get close to the slab profile so that the gap underneath stayed manageable for the caulking/sealant underneath and the top would be level.)
So – how to solve the weak point?Each of these options relies on my calculations that the surface density of treated pine when measured through the 90mm dimension is equal to or greater that my 3 layered wall.
Option 1. Remove all the nails holding the DPC to the base plate and lift the base plate off. Remove the packers. Cut a piece of 90mm treated pine so that it sits under the base plate (call it the fill), caulk heavily the contact between the DPC and the fill, then drop the fill and base plate back on to the dynabolts and re-tighten. Then re-attach the DPC with nails on the inside. I then have continuous mass from slab to the leaves - albeit with the DPC and zincalume sill as part of that.
Option 2. Use the solution I was going to use in my original design (image below) - lay down some extra base plates inside the existing base plate with a DPC underneath and plenty of caulk.
- Option 2
The bottom 90 x 45 would be planed to get close to the slab profile so that the gap stayed manageable for the caulking/sealant underneath and the top would be level. The question is whether this placement of mass has continuity with the rest of the mass - I'd have thought it does through the base plate of the frame.
Option 3. Move to a hybrid structure depicted below where I bring the outer wall
inside the frame and have it sit on the 2 layers of 90 x45 as described in Option 2 above.
- Hybrid - option #3
This gets me a continuous mass; brings me to the same position in the room as Glenn’s design; and gives me an air gap of 200mm – my target. I have included a shot of the cladding rather than try to do it in sketchup - while it could be considered a third leaf is has very low mass (.42mm thick) and air flows freely behind it
QUESTION: - which is the best option - or is there something better?Thanks in advance – I know this post is long and I’m chewing up people’s time here but it’s appreciated – I also live in hope that the exchanges might be of value to others
Andrew