Roof space ventilation is necessary to evacuate water vapour to avoid condensation and to conserve the wooden roof supports. It has been affected by 1. increased insulation, 2. snow screens fitted under the roof, 3. increased humidity due to
Evidence of the importance of air infiltration in moisture control in building structures has been steadily accumulating. A general model of moisture behaviour in structures has been built up including for the effects of cavity air leakage, for the hygroscopic behaviour of timber, for the effects of condensation and various geometric factors.
Examines the causes of condensation problems and ways of reducing or eliminating them. Deals with diffusion through the building envelope, mechanical dehumidification and ventilation. Considers condensation inside cavities and roof spaces.
Describes the types of damage that condensation can cause in a domestic pitched roof with insulation, and outlines the design options available to reduce the risk of condensation. Most of the water vapour comes from within the house, both by movement of air through gaps in the ceiling and by diffusion through the plasterboard. The type of damage depends on the structure of the roof. If there is a non-absorbent lining, water can condense on it and subsequently run or drip on to the timbers and ceilings. Absorbent linings are then wetted and may rot.
Shows that efficient moisture removal in a ventilated roof via ventilation openings is a function of roof length. Beyond a certain critical length all moisture diffusing in from below will be deposited in the roof. The critical roof length will be greater the more vapour tight the section beneath the roof space is. If a vapour barrier is necessary, as in an unventilated roof, it maybe that an unventilated roof is preferable in the first place.
Describes the extent of the problem of condensation in roof spaces of well-insulated dwellings, discusses the mechanisms resulting in condensation, and evaluates possible solutions. Factors considered in condensation occurrence include air movement to the roof space, and roof space ventilation rates. Control measures discussed include direct extract ventilation to the kitchen and bathroom to control water vapour, and the provision of adequate roof ventilation according to British Standard BS 5250.
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