Vapour Barrier / Damp Proof Membrane

Vapour Barrier / DPM – Glossary

A vapour barrier or damp proof membrane [DPM] is a very important item to use in association with engineered wood flooring and with solid wood floorboards; particularly when flooring is being laid over concrete. In such a situation the concrete must already be of a relative humidity appropriate to the average ambient RH that will be encountered in the building long term and in turn, and in line with the RH of the flooring to be laid.

A vapour barrier / DPM often takes the form of a thick layer of polythene such as Visqueen that serves as a barrier to movement of moisture between the floor and the subfloor. Edges of the vapour barrier must be overlapped generously over each other and taped along the joint so as to stop moisture movement through the joints. Additionally the vapour barrier should come up above the level of all edges of the floor and sit behind the skirting.

Where a floor is being nailed to plywood, over a concrete floor, it is advisable to lay the vapour barrier directly on to the concrete then lay the plywood on top of the vapour barrier then a foam underlay over the ply and finally the wooden floor boards then nailed through the foam underlay into the plywood subfloor.

Important note – in situations where inadequate insulation is fitted beneath the membrane there is a risk of condensation settling on top of the membrane, leading to damp conditions beneath the wood [or other] flooring. Expert advice should be sought and dew point / temperature / humidity levels measured and recorded prior to installation beginning.

See also the Liquid DPM section of this glossary.