Water vapour is always present in varying amounts in the atmosphere, and when it comes into contact with a cold surface the vapour may condense into liquid.
Condensation is liable to occur on the internal surfaces of the walls and roof of a building if the temperature and humidity of the air inside the building is appreciably higher than the outside atmosphere, particularly if the walls and roof have low thermal insulation values.
In practice such condensation is frequently more severe under the roof than on the walls, partly because roofs often cool rapidly at night by radiation to a clear sky and partly because, in the past, roofs have usually had lower thermal insulation values than walls.
When designing a roof the problem of condensation must always be borne in mind. Any provision required to control condensation should be determined as recommended in BS 6229 but with the calculation method modified to conform with BS 5250:2002 – Code of Practice for control of condensation in buildings.
In the case of a roof incorporating mastic asphalt, a suitable thermal insulation may be included within the system. This layer must have sufficient insulation value for its underside to remain above the temperature at which condensation can start, even on the coldest nights. The provision of insulation alone, however, may not be sufficient to prevent condensation. If the insulation is permeable to water vapour, the vapour will pass upwards through it and condense on the underside to the waterproof membrane.
To prevent this happening a vapour control layer in the form of a vapour check or full vapour barrier should be provided on the underside of the insulating layer. A vapour check may consist of a single layer of roofing felt, whilst a vapour barrier can be formed of two layers of roofing felt or a proprietary metal lined vapour barrier. It is essential that all side and end laps are fully sealed.
Consideration should be given to a 13mm thick single coat of asphalt roofing on an underlay of glass fibre tissue which would serve as a vapour barrier. (Fig 1.2)