Condensation risk: comparison of steady-state and transient methods

Accurate assessment of both surface and interstitial condensation risk is important not only to reduce the damaging effect of moisture within the structure of buildings, but also to provide a healthy environment free from mould growth. The current British Standard (BS EN ISO 13788: 2002) contains an assessment procedure based on the assumption of a steady-state heat flow through the building envelope, neglecting the transient nature of the problem. This paper compares and evaluates numerical results of the condensation risk calculation under both steady-state and transient conditions using the existing numerical codes. Significant differences are apparent between the predictions of the simple (steady-state) and complex (transient) methods for all construction details modelled. Practical application: The current British Standard (BS EN ISO 13788: 2002) gives calculation methods for determining the internal surface temperature of a building component or building element below which mould growth is likely, given the internal temperature and relative humidity-the method can also be used to assess the risk of other surface condensation problems. The calculation methods in the Standard are steady-state. The paper concludes that for cases where the steady-state method predicts that surface RH values will remain below the key value of 80%, a transient method can predict surface RH values to be above 80% for several hours. The practical implications of this work then are that transient calculation methods may be necessary under certain circumstances. This is particularly relevant given that the issue of a time period of a few hours is now more pertinent as it relates to a new transient performance standard in the new draft Approved Document F (Building Regulations-England and Wales).