Two possible solutions exist, at least in theory. First, prevent the moisture-laden air entering the roof space or, second ventilate the moisture-laden air before it can condense and cause any harm. Examining the first option, this at first seems the most simple method, and in theory this can be achieved by placing a vapour barrier of polythene or similar vapour proof sheeting immediately above the ceiling finish, yet beneath the insulation. This would contain the moisture-laden air within the building below. In practice, however, there are numerous small holes through this vapour barrier in the form of electrical services, hot and cold water services and soil and vent pipes. There is also the problem of effectively sealing joints between the sheets of polythene used. The major drawback to this option in the writer's opinion is that a vapour barrier immediately above the ceiling finish effectively traps some of the vapour, and can lead to unsightly mould growth which can be extremely difficult to eradicate once it has appeared. It is virtually impossible to contain moisture within rooms such as bathrooms, shower rooms and kitchens and therefore this first option of a vapour barrier is not a practical one.
We are therefore left with the option of ventilating the roof void. The British Standard 5268: Part 3 no longer sets out minimum requirements as this is considered the responsibility of the 'building' designer as distinct from the roof structure designer. BS 5250 gives guidance on prevention of condensation in roofs. NHBC cover the subject in clause 7.2/D10, Ventilation. The Building Regulations themselves in approved document F clause f2, Condensation, also require the designer to take account of the possibility of condensation within the roof space. The TRA technical bulletin gives details of how this ventilation should be provided in conjunction with trussed rafter roofs.
Satisfactory ventilation will be provided by designing a minimum gap of 25 mm along at least two opposite sides of a roof where the pitch does not exceed 15°, or 10 mm for roof pitches above 15°. Furthermore, when a monopitch roof is being considered or a duopitch roof in excess of 20°, or 10 m span, consideration should be given to providing a further 5 mm of continuous ventilation at the ridge. Care must also be taken to identify condensation traps such as those below dormer and roof-window cills, providing some 5 mm of ventilation at those points. Do not forget to introduce ventilation above the dormer roof or roof window head, remembering also that the dormer roof itself should be provided with ventilation. There are many ways of effectively providing this ventilation, mostly via the soffit in the form of slots or holes covered with an insect-proof gauze. Having introduced the airflow into the soffit void, care must be taken to prevent insulation blocking the space between the rafters, thus preventing this flow of air into and through the roof space. In addition, therefore, some method of controlling the insulation must be introduced.
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