Fin-type cold bridges: Heat loss and surface temperature

Abstract

This paper describes theoretical modelling of fin-type cold bridges carried out as part of a programme of research into the effect of cold bridges on heat loss and condensation in buildings. One-dimensional fin theory is used to show that in many buildings, fins on the exterior of the envelope do not increase the local rate of heat loss and, therefore, do not lead to additional problems of cold bridging. The ratio of heat loss from a building envelope with fins to that from one without is found to decrease with increasing external heat transfer coefficient. A common cold bridge (a floor slab penetrating an external cavity wall) is analysed in detail by the finite element method. The effect of extending the floor slab to make an external fin (e.g. as a balcony) was found to have an insignificant effect on both heat loss and surface condensation risk. One-dimensional fin analysis is developed to produce a simple thermal model of externally extended cold bridges of utility in estimating minimum internal surface temperature.