Approved Document C2 of the Building Regulations requires that roofs should be designed and constructed so that their structural and thermal performance is not adversely affected by interstitial condensation. This requirement will be met if the roof is designed and constructed in accordance with Annex H of BS 5250 and BS EN ISO 13788.
To avoid excessive moisture transfer into roof voids, gaps and penetrations for pipes and electrical wiring should be filled and sealed, particularly in areas of high humidity such as kitchens and bathrooms and an effective draught seal should be provided to loft hatches to reduce the inflow of warm air and moisture. Vapour control layers can reduce the amount of vapour entering roof voids but cannot be relied on as an alternative to ventilation. A complete barrier to moisture is needed for this.
The Building Regulations do allow compliance to be demonstrated using products complying with UKAS approved Third Party certification for both product and installation. Some certificates for (LR) vapour permeable underlays allow the underlay to be used without additional roof ventilation, provided a number of conditions are met. Such requirements are restricted to domestic dwellings only and with a convection tight loft space. Given normal construction tolerances, it is very difficult to achieve a well-sealed ceiling and maintain a ‘convection-tight’ loft space.
Scottish Technical Handbook Section 3.15 requires that dwellings shall be so constructed as to protect the building and its users, so far as may be reasonably practicable, from harmful effects caused by surface and interstitial condensation. Both these requirements are deemed to be satisfied by following the guidance given in BS 5250.
BS 5534 also recommends that roof ventilation be provided in accordance with BS 5250.