Simulating the Energy Benefits and Reduction in Condensation Formation that is obtained from Houses with Cold Pitched Roofs

Paper in proceeding, 2008

Ventilation often accounts for a significant portion of the heating or cooling load of a building and also affects the moisture levels of buildings. Arguably in the UK, houses with cold pitched roofs, with insulation on a horizontal ceiling, are the most vulnerable to the formation of condensation in the roof. This study is a numerical investigation that compares the occurrence and the risk of condensation formation in a cold pitched roof fitted with two underlay (a vapour permeable underlay (VPU) and a bituminous felt). The energy benefits that would be obtained from reducing the ventilation gaps in the eaves and improving the airtightness of the ceiling are investigated. In order to visualize these effects, several cases and scenarios have been modelled with a HAM software package. The results showed that, the VPU with a very low vapour resistance, Sd< 0.02 is most effective in reducing condensation formation when the roof is sealed. It was observed that reducing the thickness of insulation on the ceiling would enhance energy benefits as well as reducing condensation formation risks. 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 off completely when houses are being built, which may be practically difficult given current construction practice. Substantial energy gains were predicted when the roof was completely sealed off. Ideally, reducing the overall tightness of the building envelope to smaller air change rates (<5 ach at 50 Pa), would reduce the airflow through the ceiling, and would help in controlling condensation formation in the roof as well as reduce the energy loss through the roof and hence of the building.