Often you can’t access the wall without removing weatherboard or plaster. However, the stud bays in my house are open – no noggings, few wires. Therefore I’ll try to pump in insulation via (1) top (2) bottom or (3) a hole in the plaster. The weatherboards form a water barrier (seal the gaps) but stills, there’s a risk of moisture. Best use a moisture-resistant insulation.
Weatherboard Wall External with Foam
Where the frame is exposed there are more options. Old Queenslanders have 70mm studs, which can be exposed during reno if removing the weatherboards.
We want an minimum R2.8, but R3.5 is better on exposed western walls. This can be made up of:
- 70mm batts: R1.6
- Wall normal airspace: R0.2 (R.0.6 if reflective)
- 40mm XPS : R1.0
- This gives a total of R2.8 which is just enough. A detail is below. Paint it white to reflect the sun and shade it. Using a breathable wrap and XPS polystrene . This construction is:
- vapour permeable
- rainscreen to allow drying of boards
- XPS stops thermal bridges. About $10/m2
Other alternatives would be:
- Optional: Add reflective layer : I don’t like these much: can be ineffective due to dirt and/or need a gap. But by adding over the battens to this construction it would add R0.4 for little effort and cost ($3/m2). Consider for walls in high sun. Use a ‘breather’ foil (i.e. with micro holes)
- Aircell: About $10/m2 and provide R0.2 (Aircell) plus 0.6 reflective, so similar to XPS 40mm. It would be easier to install and avoids the need for a building wrap. The airtightness and permeance are questionable. Same reflective problems.
- Add another 75mm wall of studs and increase batt insulation. Easier but no rainscreen.
Sub Floor Insulation
The subfloor can be insulated, but it’s a bit harder than walls or roof, since the insulation wants to fall out. There is a lower temperature difference compared to walls or roof (think 10°C instead of 40°C for a roof) so reflective insulation is less effective (note the ‘R Value’ is a useful simplification but hides this – details).
So foil or ‘aircell’ type products might be less effective compared to bulk insulation. Some manufacturer suggest rolling over the joists, and others cut between. A roll-over and screw-fix of particle floor would likely be okay and a fast way to be some insulation.
Bulk batts are difficult to install well and tend to get torn.
XPS polystrene foam works well. It’s about $10/m² for 40mm thick. You can get it in any size. A convenient size is 440 or 590 for 450/600 joists. It can then be push fitted and a foam gun is used to seal. Easy for timber but for C channel steel, this leaves a 10mm gap. Before the subfloor is sealed/weatherproof, be careful to not trap water – a small hole is cut and then sealed with foam on completion.