Breather membranes offer specifiers and designers the opportunity to look at pitched roof construction in a new way.

The function of a breather membrane is to allow internally produced water vapour to permeate through its special layered composition to the external environment, whilst inhibiting the ingress of wind driven rain and snow.

According to BS 5250: 2002, a membrane is considered breathable (or vapour permeable) when it has a water vapour resistance of less than 0.6 MNs/g. These membranes are generally applied vertically as part of a walling system.

Under the same British Standard, a breather membrane used within a pitched roof is referred to as a vapour permeable underlay. Such membranes can only be used where they have a water vapour resistance figure of less than 0.25 MNs/g.

The Icopal range of breather membranes includes:

• Monarperm 700 - suitable for use within pitched roofs
• Monarflex SP 400 - suitable for vertical applications usually as part of a wall or cladding system