Good energy performance starts from the ground up. Tim Doherty reveals what you need to know to get this part of your project right

This illustration from Viking House shows its Passive House Budget Build package, which includes a highly-insulated foundation designed to achieve a U-value of 0.10 W/m2K

When we think of foundations, most of us probably imagine straightforward strip or trench fill versions – where the concrete and blockwork footings are positioned only beneath the external and internal loadbearing walls. But if you’re thinking about your insulation strategy, you also need to factor in the ground floor type you’re using.

For instance, groundbearing slabs must have their own foundation of suitably compacted hardcore and sand blindings. Meanwhile, suspended floors – whether timber or concrete – are directly supported by the house’s external walls and foundations. So everything becomes part of the same thermal element.

Performance standards

Nearly all houses built before 1980 have cold ground floors. No thought was given to heat loss through the supporting soil or, worse still, active draughts are present through gaps in suspended structures.

Obviously, standards have changed considerably since then – but so have our expectations. The trend now is for wooden, stone or porcelain floors set atop underfloor heating, so we can have a snug feeling underfoot; and that automatically means we need better insulation. Putting this in place can be a challenge for renovators, but there are plenty of options for people selfbuilding from scratch.

The Building Regulations set out target U-values for heat loss through foundations. The most common solutions typically sit at around 0.14 to 0.18 W/m2K, but the backstop is 0.25 W/m2K. There are also minimum standards for airtightness (10 air changes per hour), which the majority of self-builders will look to improve on. Overall, this demonstrates our increasing intolerance for draughty house foundations that can undermine improvements in the insulation of the wall and roof fabric. So getting the thermal detailing of floor and foundation right is now considered to have a huge role to play in the building’s overall performance.