The need for structural integrity, and the need to allow light and access into a building leads inevitably to the use of different materials with different thermal properties. Good design of each of these details to minimise the thermal loss is crucial both to keep heating demand down and to avoid cold spots where condensation might form. Thermal bypass is the wicking of heat from a building by air movement around and through its fabric. A breather membrane layer on the cold side of the building’s fabric will help avoid some of the more common causes.