Vapour-Permeable Underlay (Breather Membrane)

A vapour-permeable (VP) underlay, often called a breather membrane, is a roof underlay designed to stop liquid water and wind-blown snow while allowing water vapour from inside the building to diffuse outward. Such underlays are standard in pitched roofs in the United Kingdom, Ireland and much of Europe, and similar products are used in Australia and New Zealand.

Background

VP underlays have been used in Europe since the 1970s. In the UK the British Board of Agrément first certified them in 1982 for timber-frame sheathing and ventilated cold roofs.

LR and HR underlays

UK guidance divides underlays by vapour resistance:

• Low-resistance (LR): vapour-permeable products that let moisture through to the batten space.
• High-resistance (HR): traditional vapour-impermeable sheets; the loft beneath must be ventilated to BS 5250.

The two can be hard to tell apart by eye, so third-party certification should be checked. Some VP underlays are also "air-open", allowing air movement, while others are air-tight.

Batten space ventilation

When an LR underlay is used under an air-impermeable covering, UK warranty guidance calls for the batten space to be ventilated, for example with openings equivalent to a 25 mm slot at the eaves and a 5 mm slot at the ridge, created with counter-battens. BS 5534 Annex L describes a test for whether tiles or slates are air-permeable.

Installation

In a typical UK cold roof the underlay is draped over the rafters and held by battens. BS 5534 refers to a nominal 10 mm drape so water can run beneath the battens to the eaves. Because VP membranes do not self-seal around nails as bitumen felts do, poor drape can let trapped water leak through batten fixings. Counter-battens fixed over the underlay create a drainage path and reduce this risk. BS 5534 also sets out a method for checking the underlay's resistance to wind uplift.