External walls partially filled with insulation, and the potential to "top-up" the residual cavity.

This review found that the connecting voids in partially filled cavity walls leads to considerable variation in thermal performance. Whilst photographic records found considerable evidence of gaps in the insulation resulting from poor site practice and installation, research also shows that relatively small breaks between insulation sheets or gaps between the wall and insulation result in a thermal bypass. As the gaps and connecting voids increase air circulation, convection currents and pressure induced exchanges reduce the effectiveness of the thermal barrier. Where effective installation is possible, the topping up of partially filled cavity walls with insulation shows potential to improve the thermal performance of the wall. In the cases reviewed, the installation of blown mineral wool fill reduced variation in heat flow and increased thermal performance. By filling the voids with insulation the passage of air and thermal bypasses were restricted.