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.

  • Philip Angus

    Great solution. I was pondering the same concept, but there is a huge leap from concept to identifying the products (and then building it!). Charlie, are you able to say which MDF panel you propose to use and is it weatherproof, or is there another layer?

  • Owen Short

    Charlie – Interesting – if expensive and very thick overall – but where’s your dewpoint in that wall construction, please? Thankyou. O.

  • William Speirs

    Totally wasteful – you could increase U-values – single trade. This is retrograde – are you toadying to every single manufacturer. You’ll never have affordable homes like this.

  • George McMillan

    I was wondering how you keep the whole project dry especially in a wet November. What precautions do you suggest short of covering the building with scaffolding and sheeting including the roof?

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