Detailed information on methods to control harmful condensation is given in British Standard BS 5250: 'Code of practice for control of condensation in buildings' Section 8.4 'Roofs'.
Approved Document C 'Site preparation and resistance to moisture contains information relating to 'Roofs (resistance to surface condensation)' and 'Roofs (resistance to surface condensation)' and 'Roofs (resistance to surface condensation and mould growth)' in part 6.
The relevant document is Approved Document Part F1 'Means of ventilation'
Designers should consider the position of terminals to ventilation systems when designing the roof. Approved document F1 states that there shall be adequate ventilation provided for people in the building. It does not apply to a building or space within a building into which people do not normally go, or which is used solely for storage, or a garage used solely in connection with a single dwelling.
Condensation in roof spaces has become more of a problem with the increase in highly insulated buildings. Moreover, changes in life style have led to higher levels of water vapour in modern buildings. This water vapour naturally ascends to the roof space, where it condenses on contact with cooler surfaces. Further condensation is likely to be caused by climatic conditions, and may eventually result in timber rot, metal corrosion and damage to insulation and fittings.
Approved Document C states: '6.10 A roof will meet the requirements if it is designed and constructed in accordance with clause 8.4 of BS 5250 and BS EN ISO 13788.'
Detailed information on methods to control harmful condensation is given in British Standard BS 5250: 'Code of practice for control and condensation in buildings' Section 8.4 'Roofs'. Prevention of condensation in roof voids is best achieved by the provision of natural air ventilation
BS 5250 states that the designer should take account of the following moisture sources in buildings:
BS 5250 emphasis the importance of well-sealed ceilings as a means to curb the transfer of moisture into a roof space by means of moisture laden air. This means:
Air leakage through gaps in a ceiling transfers more heat and moisture into the roof by convection that passes through the ceiling materials by diffusion.
Sealing the ceiling is therefore an essential requirement when considering the design of the roof envelope.
Key design issues to consider are as follows: