Vapour barrier or breather membrane?

Bizzy Blue Design Ltd has reviewed the various terminologies to understand the correct usage and application of this often misunderstood technology.

Understanding the different applications for these products could mean the difference between a high performance building and a damp one that would be very difficult to put right without starting again.   We have investigated their applications and understood their respective performance values to become better educated towards their specification.

Definitions:

  • Vapour Barrier (or Vapour Control); withstands vapour pressure and minimises gaseous vapour penetration
  • Breather Membrane; water resistant and vapour breathable

Applications:

  • Vapour Barrier; applied internally to walls, roofs or exposed floors.
  • Breather Membrane; applied externally to exposed structures.

We understand that the principle of using a vapour barrier is to control and reduce the amount of gaseous water vapour within a building and prevent it travelling through an insulated external or exposed structure where risk of interstitial condensation may occur.   The vapour barrier is applied internally to minimise vapour penetration into the insulation layer.   A breather membrane is applied externally to act as a barrier to water from the outside whilst also allowing the structure behind to breathe away any water vapour within it.   Using both products together as part of a composite structure will act to minimise water vapour penetration from inside the building and through the insulated structure, while allowing any vapour to escape externally and resist water penetration from the outside.

A traditional masonry cavity wall would not require a breather membrane as the external skin would act as the breathable layer and would control the level of water penetration from the outside.   A breather membrane would, however, be applied to a timber framed external wall that then has a masonry outer skin as this would protect the timber from any water within the cavity.

This theory works perfectly and logically on any external element (wall, roof or exposed floor) in contact with external air.   However, exposed floors, such as a ground floor where there are no basements, need to be treated in a different manner.   Where the ground is the external substance you are dealing with ground water pressure and this should be resisted by a Damp Proof Membrane or water proof tanking system.   Depending on where the floor insulation is positioned, a breather membrane can then be applied over the insulation to allow any water vapour or moisture to evaporate into the heated space above.

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