Vapour-balanced 'breathing' construction

A vapour-balanced roof construction allows moisture to permeate, removing the need for ventilation of the roof construction. The vapour resistance on the inside of the construction should be five times that on the outside. The impervious roofing felt is replaced by 'breather felt'.The 50mm ventilation gap, the soffit and ridge ventilators as well as the polythene vapour barrier are all omitted.

This simplifies the construction and leaves more space for insulation. However, as the existing roofing felt must be replaced by the vapour-permeable type, all the existing slates (or tiles) and tiling battens must be removed and replaced.