Knauf Insulation is offering a 15-year guarantee on Knauf Breatheline, its vapour permeable breather membrane and roofing underlay. Formed by bonding two layers of spunbonded polypropylene to a micro-porous film, Knauf Breatheline offers benefits to designers, builders and owners: the need for ventilation is reduced, installation is easy and cold lofts are cleaner. Knauf Breatheline is a versatile product, representing a universal solution suitable for warm and cold pitched roofs, profiled metal roofs and timber-framed walls.

Weatherproofing a building not only keeps water out, it can also keep water in, trapping the 12 to 14 litres of water moisture created each day by the users of an ‘average house’. Unless a method of allowing it to escape is provided, it represents a threat to the structure, its contents and, of course, its occupants. Historically, the challenge has been resolved by using ventilation to physically remove moisture laden air – but an altogether better solution is to let the structure ‘breathe’.

Breather membranes were introduced into the UK construction industry about 30 years ago and, since then, there’s been widespread acceptance of the concept and a major shift towards their use. The technology used to make the ‘hi-tech’ materials is the same as that used to make the breathable, lightweight protective clothing we sometimes rely on to provide personal protection.

Knauf Breatheline is a non-woven material made by ‘melding’ a micro-porous polypropylene film between two layers of spunbonded polypropylene. The film provides an impervious but breathable layer that acts as a barrier to water in liquid form but allows water vapour to pass through its structure. The spunbonded outer layers provide the material’s strength and cloth-like feel which make it pleasant to handle and easy to use. In addition, it’s light in weight, has excellent tear characteristics, ‘drapes’ easily and is unaffected by cold weather. Knauf Breatheline is classified as a breather membrane as it offers little resistance to the passage of water vapour: 0.24 MNs/g against the defining limit of 0.60 MNs/g set by BS5250:2002 section 3.2 – it’s been independently tested and holds three BBA Certificates together with their IAB equivalents.

Permeable membranes are commonly used as a roofing underlay, a use that both demonstrates their benefits and highlights their success. Traditionally, ‘lo-tech’, bitumen underlay – Type 1F Felt, for example – has been used for this purpose but, as this creates a physical barrier, some form of ventilation is necessary to allow moisture to escape. In a cold roof this usually means providing ventilation at the eaves but a warm roof is more complicated. An unobstructed, ventilated air space has to be created between the insulation and the underlay; awkward to achieve successfully and needing deeper rafters. In either case, the underlay becomes brittle with age, gets torn by wind action and needs to be replaced sooner rather than later. A vapour permeable underlay performs exactly the same role except that, by allowing water vapour to escape, no specific provision for ventilation is necessary. This means that condensation is more effectively controlled and roof construction is quicker, less expensive and the loft space cleaner.

A brochure giving more information about Knauf Breatheline, its use and installation is available via Knauf Insulation’s website at www.knaufinsulation.co.uk or by calling 08700 668 660. Please quote KR 227 04.

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