Breathable Membrane
/ ˈbrɛðəbəl ˈmɛmbreɪn /
Also known as: vapour-permeable membrane, breather membrane, vapour-open underlay, LR underlay, HR underlay
Definition
A breathable membrane is a sheet material used as a roofing underlay or wall sheathing membrane that is simultaneously water-resistant (blocks liquid water) and vapour-permeable (allows water vapour to pass outward). Positioned on the cold outer face of insulation in warm pitched roofs and timber-frame walls, it allows moisture vapour that penetrates past the interior vapour control layer to escape rather than accumulating in the insulation. Characterised by its Sd value (equivalent air thickness, in metres) - breathable membranes have Sd below 0.25m; traditional non-breathable sarking felt has Sd above 200m. Low-resistance (LR) breathable membranes can be used without a ventilated eaves gap; high-resistance (HR) breathable membranes still require ventilation below the tiles. Not to be confused with a vapour control layer (VCL), which goes on the warm interior side of insulation.
In practice
A common mistake on roof insulation upgrades is installing rigid insulation boards between the rafters with traditional non-breathable bitumen felt on the outside. The felt traps any vapour that enters the rafter void, causing interstitial condensation at the felt-insulation interface. Rafter insulation upgrades should always use a breathable underlay - either retaining and supplementing the existing underlay if it is already breathable, or replacing it with a breathable membrane if it is not.
For timber-frame walls, the breather membrane is typically fixed to the OSB sheathing before the external cladding or brick outer leaf is constructed. Laps must be taped with proprietary tape to maintain continuity at joints, and penetrations for pipes, vents, and fixings must be sealed to prevent liquid water ingress at these points while the vapour-open characteristic of the field area is maintained. Breather membranes are UV-sensitive - they must not be left exposed to sunlight for extended periods before the cladding is applied (typically limited to 3 months, check manufacturer data).