Wrapping Up
Architectural designer and TV presenter Charlie Luxton explains his solution to insulating the external walls of his Cotswold self-build
T hings are continuing to move at a good pace, and having got the roof on and the walls up, it’s now time to turn to insulating the exterior walls. We’ve opted for a slightly unusual method — but it’s one that we’ve developed over a number of years on a few other projects, and I think that it works well. This house sits on a polystyrene tray that’s about 300mm thick, and that’s so that no part of the structure touches the ground. The job we’ve got is to extend that insulation in a continuous layer up, around and onto the roof, so that the whole building is completely wrapped. To achieve this, the first job is to lay 150mm of a higher performance polystyrene down on top of the tray. We then use timber Steico I-joists on top and fix them to the blockwork walls. The reason these I-joists are clever is that they’re made up predominantly of waste material from the timber industry: the central section is actually wood pulp that’s bonded together under pressure to form a board, and the two end sections are skimmed down bits of wood that have been glued back together to make a strong unit. What’s amazing about it is that it’s light – two people can pick up a 13m beam; if it was solid timber it would be incredibly heavy – and it’s also dimensionally stable, meaning it doesn’t warp or bend. Because of the way that it’s designed, very little heat is lost down this connecting piece of wood, which means you’re getting really small amounts of heat loss through that insulation layer. Yes, it may seem like a weird system, taking a concrete blockwork wall and building a sort of timber frame over the top of it then cladding the outside with stonework, but what it allows us to do is create a completely enclosed volume. It will be boarded up by highly breathable, formaldehyde-free MDF, creating a void that is then packed with polystyrene bead, blown under very high pressure, so that all the nooks and crevices are completely filled. Finally, we will cover the MDF with a waterproofing membrane before adding the stone. The reason we go through all that effort is because by blowing the insulation we get a really good fill – there’s no gaps or voids to allow thermal bypass – and because it’s completely wrapped up and out of the weather, the stonework can be built over a number of months. Traditionally you would have a block wall, cavity ties, stick a load of insulation in and then build up a stone wall on the outside (or have a second skin of block and then the stone). And that means the whole time that you’re building up, rain is coming in and degrading the insulative properties of the material in the void. So this system allows us to create a void, fill it and keep it completely dry. Using our method means that with a 240mm cavity, filled with Ecobead Platinum polystyrene insulation, we’re looking at a U value of about 0.13W/m2k — which we think is pretty good!
Next month: Building up the stone cladding
“It may seem like a weird system, taking a block wall and building a sort of timber frame over the top of it”