Air carries water vapour, the amount increasing proportionally
with the air temperature. As the water vapour increases so does
the pressure and this causes the vapour to migrate from warmer
to cooler parts of a building. As the air temperature reduces, so
does its ability to hold water and this manifests as condensation
on cold surfaces. Insulation between living areas and roof spaces
increases the temperature differential and potential for
condensation in the roof void.
Condensation can be prevented by either of the following:
- Providing a vapour control layer on the warm side of any
insulation. - Removing the damp air by ventilating the colder area.
The most convenient form of vapour layer is vapour check
plasterboard which has a moisture resistant lining bonded to the
back of the board. A typical patented product is a foil or
metallised polyester backed plasterboard in 9„5 and 12„5 mm
standard thicknesses. This is most suitable where there are rooms
in roofs and for cold deck flat roofs. Ventilation is appropriate to
larger roof spaces.
Ventilation of Roof Spaces---1