Low Water Vapour Resistance Underlays in Cold Pitched Roof Systems for Dwellings

Building Control Technical Guidance Leaflet No. 13

LOW WATER VAPOUR RESISTANCE UNDERLAYS IN ‘COLD’ PITCHED ROOF SYSTEMS FOR DWELLINGS

This leaflet is one of a series produced by the Hertfordshire Building Control Technical Forum

Commentary

A relatively recent introduction to the technology of pitched roof construction has been the development of LR or vapour permeable underlays for use in place of the traditional BS747 type IF bituminous felts. A type LR underlay is defined as an underlay with a vapour resistance of 0.25MNs/g or less. These high performance materials can provide a barrier against wind-driven rain and snow whilst allowing water vapour in the loft space to pass through to the air space beneath the external tiled finish where it can disperse to atmosphere. As with all building components, Building Regulations require that such materials be fit for their purpose. In order to assess their suitability it is usual to ensure that the manufacturer has obtained proper accreditation from an appropriate independent body such as the British Board of Agreement (BBA) or BRE Certification.

Roof Ventilation

The traditional method of preventing damage to roofs from interstitial condensation is to provide cross ventilation to the void between the insulation material and the underlay. Regulation C.2 and its associated approved document looks for accordance with BS 5250:2002 which allows for reduced levels of ventilation where LR underlays are used. The section below outlines these requirements. Whilst not according with the above documents, some products are certified by UKAS accredited bodies such as BRE and the BBA to be suitable for totally unvented systems. This is only on condition that the installation methods and limitations of use in the certificates are met. It is important to check that the relevant certification is still valid when specifying these materials.

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