Further studies of passive ventilation systems - assessment of design and performance criteria.

Increases in building air tightness for purposes of energy saving have, unfortunately, also led to a significant increase in the number of instances of condensation damage, particularly in domestic properties. The cost effective control of condensation is a large problem in the United Kingdom, especially for local authorities with large housing stocks. The use of ducted passive ventilation systems, relying upon stack and wind effects to provide extraction , has several advantages, one of which is that the occupants of dwellings fitted with such systems need little , if any, knowledge of the principles involved, or instructions in its use, to derive maximum benefit . This paper descirbes two programs of research carried out on two houses fitted with passive systems: the first house is a highly airtight , timber framed structure , whilst the second is a significantly leakier council owned property of traditional construction. The effect of passive ducts upon the ventilation rate in each dwelling is measured and related to internal / external temperature difference , windspeed and direction . The measured ventilation rates are used to calculate likely rates of moisture extraction during occupation, and the resulting effects upon condensation risk are assessed in the light of the predicted minimum ventilation rates neccessary in order to avoid condensation. Theoretical calculations of the expected flow rates through passive systems are presented, and are shown to be in broad agreement with measured values. Finally, design considerations of importance when specifying passive systems are discussed.