BS 5250 Ventilation Guidance

Standards and Regulations

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 moistre 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 Doucment Part F1 'Means of ventilation'

Designers should consider the position of terminals to ventilation systemswhen designingthe 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 grage used solely in connection with a single dwelling.

Control of condensation

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:

  • Water incorporaated during the construction process (including precipitation);
  • Precipitation after construction;
  • Water vapour arising from the occupants and their activities;
  • Temporary condensation occuring when cold weather conditions are followed by warm, humid weather

Sealed ceilings

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:

  • The avoidance of gaps and holes in a ceiling;
  • The sitting of access doors or hatches in to the roofspace away from moisture producing areas such as bathrooms or kitchens;
  • That hatch covers must be effectively sealed;
  • High levels of workmanship

Airtightness of ceilings

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:

  • Avoid construction gaps
  • Avoid roof access doors or hatches in rooms that produce excessive moisture
  • Use a sealed loft hatch and frame to BS EN 13141-1
  • Seal all services and rooflights
  • Use recessed light fittings rated IP60 to IP65 to BS EN 60529
  • Seal the head of cavity walls to prevent transfer of warm moist air into the loft

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