Energy efficient foundations

Before considering popular technologies like solar cells and battery storage in the design of an energy efficient home, the very first step is to minimise energy use in the first place. Starting at the bottom the first place to look at is at the foundation and the slab.

Contrary to popular opinion the importance of floor insulation is overestimated (actually many energy efficient homes omit it altogether), and the real source of heat loss is at the perimeters. However, unless the perimeter of the building is insulated to a certain depth to act as a heat barrier, slab insulation is still required. The conventional approach is place insulation on top of the concrete slab, but this defies logic as you loss the benefit of all that lovely thermal mass, so why not put the insulation beneath the slab instead?

Perimeter foundation section

Placing insulation beneath the slab solves the problem of providing continuity of insulation to the perimeter, which is where the real problem solving is required in order to prevent thermal bridging. Thermal bridges, well known to every architect and building designer, wick out heat like electricity leaking across a short circuit, therefore reducing the opportunity for heat to flow from the perimeter to colder outside ground is paramount. But how to run insulation below a building in one continuous sheet? Well, not as hard as you might think using a high compressive strength insulation like foamglas

The idea of completely floating a building on insulation and totally eliminating thermal bridging is virtually unknown in UK, but has been popular in Scandinavia for sometime. Considering the simplicity of this detail and the huge advantage that it offers in reducing heat loss and preventing cold corners it is a shame that it isn’t adopted more widely.

Foamglas beneath foundations