Provide vapour and moisture barriers to prevent condensation. Vapour barriers include polythene sheet, reflective foil, foil backed plasterboard and well maintained water resistant painted surfaces. Water resistant insulation such as polystyrene can also act as a vapour barrier. Tape or glue all joints in vapour barriers to keep out moisture and ensure that there are no tears or pinholes or damage in the membrane.Use vapour barriers to protect from condensation:
In cold climates place the vapour barrier on the inside of the insulation (directly above the ceiling lining and next to the internal wall lining).
In warm climates place the vapour barrier on the outside of the insulation. In hot humid climates it is ideal to have bulk insulation with a double sided vapour barrier. The top vapour barrier can be installed on site.
Note: The above is a very wide rule of thumb approach and a general design guide, for more critical design each application must be considered on its own merits and may require professional advice input.
Use perforated reflective foil in walls and under mezzanine floors when building with porous materials. The perforations prevent water droplets from penetrating but allow vapour through so that the insulation can dry if it does somehow get wet. This prevents rotting behind weatherboards or under timber floors, for example.