Purpose and standards
A vapour control layer (also known as a vapour barrier) is used to reduce the moisture vapour that passes through the external walls due to the difference in internal and external temperatures and pressures. Vapour control layers are a thin material that is fitted on internal side of the insulation between the plasterboard and studwork.
All vapour control layers must conform to I.S.EN1931:200 and EN12572:2001. All vapour control materials should come with an approval certificate or under EN13788 must be specified by condensation risk analysis. 500 gauge polythene sheeting with a vapour resistance of 250MNs/g is a common form of vapour control layer.
Installation
The vapour control layer must not be fitted until the studwork moisture content has dropped below 18%. The vapour control layer should be fixed to the timber studwork by austenitic stainless steel straps.
The vapour control layer should be lapped over studs to the following criteria:
Minimum 100 mm lap for horizontal
Minimum 150 mm for vertical
Laps are to be taped and sealed using pressure sensitive tape with a minimum width of 50 mm.
When opes are required in the vapour control layer for penetration of services it is critical that pressure sensitive tape is used to seal the ope after the services have been installed.
Protection post-installation
Similarly, the vapour control layer must be cut and dressed in door frames, heads, window sills etc. and extended a minimum of 75 mm into the ceiling to gain an effective seal.
Damage to the vapour control layer can allow penetration of moisture. It is wise to inspect the vapour control layer after installation. Small tears can be repaired. However, where large damage is inflicted it is recommended that the sheet be removed and replaced and the laps be resealed again.
