BS 5250 is the code of practice for control of condensation in buildings. It provides building designers with guidance on safeguarding the health of occupants by considering likely sources of moisture, avoiding the build up of excessive moisture, and preventing mould growth and condensation.
Due to the natural movement of air within a building, some moisture vapour is likely to reach the roof space - especially where insulation is laid along the horizontal ceiling. The question then is how to specify the roof to deal with that.
Generally speaking, the specification of ‘breathable’ underlays and pitched roof ventilation in combination is not what it could be. The extent to which the ceiling below is made airtight, and therefore how much moisture can reach the roof space, should also influence the choice of underlay and the provision of ventilation.
Among other things, BS 5250 gives guidance on general design principles for buildings, the role of air and vapour control layers, and the vapour permeability of common materials. Annex H deals specifically with design principles for roofs, including pitched roofs, and should be used to check a proposed roof design against good practice.
It also specifies minimum levels of roof space ventilation. Designers should confirm with the manufacturer of their chosen ‘breathable’ membrane that it provides the required air and vapour permeability, and be confident that the required level of ventilation has been designed for and can be achieved on site.