External wall – below DPC

External wall – below DPC

Construction techniques for traditional building methods are well-established. For projects pushing at the boundaries of low-energy construction and sustainable building, however, new solutions must be found using existing materials. The junction between the ground floor of a building and its external walls is an excellent example.

The external walls extend below ground level to be supported on the building’s foundations. Where the wall is insulated externally or in the cavity, masonry acts as a thermal bridge between the ground floor insulation and the wall insulation – it is impossible to achieve satisfactory continuity of the thermal envelope from one element to the other.

Specialists in low-energy construction methods, such as the Passivhaus and EnerPHit standards, have developed floor/wall junction details featuring greater levels of insulation, to minimise this thermal bridging effect.

The biggest issue is that the area is below damp proof course (DPC) level, so it is potentially a hostile environment for most common external wall insulation materials that are not tolerant of damp or wet areas. Furthermore, when installed below ground level, the insulation material may also be subject to loads associated with soil pressure or ground heave.

Extruded polystyrene (XPS) insulation boards not only offer superior load-bearing performance, but are unique in being tolerant of damp or wet environments. Their thermal performance is not impacted, meaning the junction detail will consistently perform as designed.

Polyfoam XPS Ltd is a leading manufacturer of closed cell, extruded polystyrene insulation. The Polyfoam range is lightweight, strong, moisture resistant and easy to cut and shape, providing thermal performance and strength for the lifetime of the building. Recommended Polyfoam XPS products for external walls below DPC are: