Roofing
Previously an underlay would be defined as type HR (high water vapour
resistance) – an underlay with a vapour resistance greater than 0.25 MNs/g and type LR (low water vapour resistance) – underlay with a vapour resistance less than 0.25 MNs/g. Te updated standard now also defines a type LR and air permeable underlay as “underlay that has a water vapour resistance not more than 0.25 MNs/g combined with an air permeability of not less than 20 m3/m2 h at 50 Pa which allows for the transfer of both water vapour and air.” Iain Fairnington, explains what this means: “Selecting a pitched roof
underlay which is both air and vapour permeable will ensure the highest performance and protection for the building fabric.”
NHBC TECHNICAL GUIDANCE In 2011 NHBC confirmed that it would be adopting guidance as outlined in BS5250 ‘Code of practice for the control of condensation in buildings’, requiring a ventilation gap to be installed at high-level, equivalent to a 5 mm continuous slot at or near the ridge. Te basis for this was that vapour permeable roof underlays permit the movement of vapour through the membrane, but generally do not permit the passage of air. Since 2012 NHBC issued technical guidance, which acknowledged that there are some vapour permeable roof underlays that permit both vapour and air to pass through them. Where an underlay can be shown to provide suitable ventilation, i.e., at least the equivalent of a continuous 5 mm high-level slot, NHBC will accept that underlay without the need to provide any further ventilation. In line with the guidelines issued by the NHBC, independently certified air
and vapour permeable underlays can be used without additional ridge ventilation in cold roofs. Iain Fairnington, explains the benefits of using an air and vapour permeable
membrane: “Te use of a high quality air and vapour permeable membrane will lead to savings in labour and material costs, due to no VCL required, and its high-performance air permeability means that the roof space has similar air changes to that of a roof using traditional eaves/ridge ventilation.”
Iain Fairnington is the technical director of the A. Proctor Group 44 | HMM September 2018 |
www.housingmmonline.co.uk
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