Pretlove, Stephen E. C.; (2000) Predicting relative humidity in UK dwellings. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Mould growth affects a significant proportion of dwellings in the UK and Europe. The house dust mite is also known to inhabit most dwellings and is one of the key factors affecting the health of the occupants. One of the key variables affecting mould growth and house dust mite populations is relative humidity. The relative humidity in a dwelling is dependent upon both the moisture levels and the temperature. The ability to assess the impact of different interventions on the relative humidity depends upon the ability to model both the internal temperature and the internal vapour pressure. This thesis develops, tests and assesses the impact of four combined moisture and thermal models which predict micro-environmental relative humidity. Two thermal models are tested, the BREDEM-8 monthly model, and the BREDEM-12 seasonal model. To each of these, two moisture models have been integrated including Loudon's steady-state moisture model and Jones' admittance moisture model. The BREDEM-8 Loudon model has been shown to be the most accurate model for predicting the airspace relative humidity in 36 dwellings during the heating season. The BREDEM-8 Loudon model has then undergone further development and testing and the applications of the model are investigated. A variable infiltration calculation has been implemented and tested within the BREDEM-8 Loudon model and the results show no improvement in the model prediction accuracy. Surface relative humidity calculations have also been incorporated for all dwelling surfaces, including cold bridges, and the significance of predicting surface conditions has been evaluated. The impact of fiael poverty is tested using simple versions of the BREDEM-8 Loudon model which have been adapted to account for situations where the expenditure available for fuel is limited and where the heating system is inadequately sized. Finally, a Mould Index has been developed which indicates the risk of mould growing on the coldest surfaces in a dwelling and various interventions in dwelling design and use are tested against this index and against the Affordable Warmth Index which defines the affordability of a particular dwelling. The results demonstrate a number of significant limitations in the current British Standard for condensation in buildings, BS 5250: 1989. It has been shown that the geographical and seasonal variations in internal relative humidity are significant, and that the highest relative humidity is unlikely to coincide with the coldest period of the year. It has also been shown that the modelling of surface conditions is critical in the assessment of mould growth in dwellings. Sensitivity studies carried out on the BREDEM-8 Loudon model have shown the most significant variables affecting the relative humidity predictions are the demand temperature, the heating pattern, the number of occupants, the ventilation rate and the level of insulation. The adequate sizing of the heating system and the ability of the occupants to afford to heat the dwelling to a comfortable temperature have been shown to be essential. It has also been shown that a change in the dwelling design or use may improve the affordability but may also lead to an increased risk from mould growth.

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