Introduction

In general, thermal bridges can occur at any junction between building elements or where the building structure changes. Reduced internal surface temperature and increased heat loss, compared with that of the unbridged structure, are the two primary effects associated with a thermal bridge where a building is being heated. Consequently there is an increased risk of condensation and mould growth at the thermal bridge and a significant additional heat loss through the thermal bridge. Thermal bridges fall into two categories:

(a) Repeating thermal bridges (such as timber joists, mortar joints, and mullions in curtain walling). The additional heat flow due to the presence of this type of thermal bridge is included in the determination of the U-value of the particular building element which contains these bridges (for further details see BR 443)

(b) Non-repeating thermal bridges (such as junctions of floor and roof with the external wall, and details around window and door openings) where the additional heat flow due to the presence of this type of thermal bridge is determined separately

This paper deals only with type (b). It gives guidance on limiting the risk of surface condensation and mould growth at these thermal bridges. It describes how to assess their thermal performance and how to incorporate the additional heat loss through such thermal bridges in calculations of the performance of the building fabric as a whole.