29. An insulation sandwich

Building our plinth requires some careful blockwork.

1st July 2014

It isn't straightforward to design and build a seriously low energy house. You have to pay attention to every detail. Every route for heat to escape the building has to be blocked, which is easy enough in the centre of a wall or floor but gets more complicated where they meet.

Today bricklayers Gill and Steve finished our plinth, the blockwork baseline on which the timber frame will sit. This is traditionally a fantastic 'cold bridge', a concrete heat highway lurking between the wall and floor. But, as you can see from Peter Smithdale's drawing, our plinth is a sandwich of low density concrete blocks and insulation. This means that any heat that does get through the insulation in the floor, wall and skirting still has to go a long way round to escape the building. There's no easy exit.

If all this is going to work properly, it has to be properly installed. If the sandwich has big gaps in it, this will provide a route for air to carry heat down to the slab and out. So rather than building the blockwork and sticking the insulation in afterwards, Gill built the first leaf, then pushed the insulation (Kingspan Styrozone H350R) up against, and then built the second leaf tight against it.

The shape of the building can now be seen. Well almost: the porch and conservatory at the centre of the building are outside the timber frame insulated envelope so will be built of brick straight off the slab. But our big ground floor room is clear enough. It's curious to step into this for the first time and imagine all that is to come within it. I have spent so much time imagining, specifying and building (elsewhere) the details of this interior space that this experience almost feels like recalling memories. But I'm not Merlin; the best is yet to come.


  1. Out of the ground and going up!!!!!!

  2. It's good to know that there are people who pay attention to things such as this. The key here is to make sure there are no "leaks." It's like the chain analogy: it's only as strong as its weakest link. I'm sure you're keeping track of this matter while they're putting it up. Thanks for sharing!

    Natalie Baldwin @ Envirotech Insulation