Assessing the Impact on Heat Loss and Mould Growth of Thermal Bridges Resulting from Internal Wall Insulation

Over the coming decades a significant number of existing solid wall dwellings, which represent a large proportion of the current UK housing stock, will have to be retrofitted to reduce fuel poverty and help the Government reach its carbon targets by 2050 (DECC, 2012). One of the most likely retrofit measure is the installation of internal wall insulation, which although it would certainly improve the thermal quality and energy performance of those dwellings, it could create, if wrongly designed and installed, thermal bridges that will increase heat loss and the likelihood of moisture related problems such as mould growth.

This paper examines the severity and impact of thermal bridges on both the risk of mould growth on internal wall surfaces and the heat loss through the junctions of an internally insulated solid wall, as a function of the window position in the wall and the wall insulation thermal conductivity and thickness. The analysis focused on an uninsulated lintel detail. The severity of the thermal bridge was assessed using the surface temperature factor and the temperature difference ratio criteria. The risk of mould growth was analysed considering the mould growth criteria established in the Approved Document F 2010, of the Building regulations for England and Wales (HMSO, 2010).

It was found that the lack of insulation in the lintel has a significant effect on the severity of the thermal bridge and on mould growth risk. The heat loss and risk of mould growth increased with the window position shifting towards the outside surface of the wall;
nevertheless the severity of the thermal bridge was found ‘unacceptable’ in all cases.