Condensation and moisture transport in cold roofs: effects of roof underlay

Journal article, 2009

Their form of construction makes pitched roofs with insulation on a horizontal ceiling, cold pitched roofs, intrinsically vulnerable to condensation. This study reports the results derived from using the simulation package HAM-Tools to investigate the risk of condensation in cold pitched roofs fitted with a vapour permeable underlay (VPU) of known characteristics. In order to visualize the effect of the VPUs on moisture transfer, several scenarios were modelled, and compared with the results from a conventional bituminous felt with high resistance (200MNs/g, Sd = 40m). The results obtained from the VPU showed that ventilation is essential in the roof to reduce condensation. However, a sensitivity analysis proved that reducing the overall tightness of the ceiling and using lower resistance VPUs would help in controlling condensation formation in the roof. To a large extent, the proposed characteristic performance of the VPU as predicted by manufacturers and some researchers may only be realistic if gaps in the ceiling are sealed completely during construction, which may be practically difficult given current construction practice.