Ventilating Domestic Pitched Roofs
In the last few decades there have been several significant developments designed to reduce the energy consumption of new and modernised buildings. These changes have had important effects on the behaviour of water vapour in buildings and the formation of condensation, particularly within the roof space.
The roof space in new and modernised buildings is now significantly colder than the living area due to the installation of thermal insulation at joist level, and the wider use of central heating and cavity wall insulation in the dwelling area. Water vapour generation in dwellings has also increased as a result of the greater use of showers washing machines and tumble driers.
Natural ventilation in the dwelling area which in the past significantly removed water vapour at source, has been reduced by the installation of double glazing, draught proofing systems and the elimination of fireplaces and chimneys.
Condensation Problems in Roofs
A typical house occupied by five people can generate more than 14 kg (3 gallons) of water vapour per day and more than 10 percent of this water vapour can reach the roof space, primarily through the loft hatch, cracks in the ceiling, joints of plaster board and around service pipe and wire access holes. The water vapour reaches dew point in the roof space and condensation is deposited on the surfaces of the various building elements. The consequent dampness in the roof space from condensed water vapour causes damage to the fabric of the building, damages ceilings and household goods stored in the roof space. The thermal resistance of the loft insulation material, too, will be significantly reduced if it becomes damp from condensation.
Three main methods could be considered for controlling roof space condensation:
- Increase the roof space temperature (warm roof);
- Install a vapour barrier at the ceiling
- Ventilate the roof space.
A warmer roof can be achieved by locating the thermal insulation at rafter level instead of at joist level. However unless the roof space is lived in, this is not a cost-effective solution to the problem because of the increased energy required to heat the roof space. Also a larger quantity of self-supporting and mechanically fixed thermal insulation material is required.
A continuous vapour barrier at ceiling level is unlikely to be effective due to unavoidable vapour leaks which would occur through loft hatches, and service entry points, etc. Even if a vapour-tight barrier is achieved an increased condensation problem would still exist within dwelling areas.
In practice, the most efficient and effective method of controlling roof space condensation is by providing adequate natural air ventilation roof space.
Eaves-to-eaves ventilation through fortuitous or purposely-provided gaps at the eaves was commonly used in the past and is recommended in codes of practice. However there is evidence to suggest that the eaves-to eaves method of roof space ventilation is often inadequate and that an eaves-to-ridge airflow is more officiant.
Eaves-to-ridge natural air ventilation enables ventilation to be achieved in all wind directions and conventional ventilation still takes place when there is no wind. More efficient air mixing occurs at all levels in the roof space to ensure a uniform removal of the water vapour.