Our self build is really starting to come together. The roof is on and that means it’s time to turn our attention to insulating the outside walls.
Traditionally, when insulating the outside of a building, you’d have your block wall and cavity ties, with insulation in the void between the block wall and the cladding (or second skin of block) This process, however, wouldn’t prevent rain from getting in the void and degrading the insulative properties of the insulation.
We needed a different plan.
Fixed to the outside of our concrete block walls, and sitting on top of 150mm high performance polystyrene insulation, are numerous timber I-Joists from Steico.
These joists (made up of waste material from the timber industry) are lightweight, yet strong and dimensionally stable (which means they don’t warp or bend).
When we fix a highly breathable MDF panel to the front, the three elements (concrete wall, joists and MDF) create a completely enclosed volume which is then filled with carbon-activated polystyrene beads (which fit into every nook and crevice, leaving no gaps or voids to allow thermal bypass).
The joists themselves offer very little heat loss, and when the 240mm cavity is filled with the polystyrene beads, we get a U-value of around 0.13W/m²K.