Changes to BS 5534:2014 and NHBC technical requirements use of ‘air-open’ (or ‘air-permeable’) underlays, such as Klober Permo® air have enabled the reduction of roof ventilation issues.
It should be stressed that you use an air-open underlay if no supporting high-level ventilation is provided. The only problem this poses for roofers is that claims are still being made that some vapour permeable underlays (VPUs) meet the water vapour transmission and air permeability criteria set for air-open products.
Of more than a hundred VPUs now available, there are only two which provide this higher level of ‘breathability’ and vapour transmission. As a result, a product which offers a level of performance at the top end of the vapour permeability scale will still fall a long way short of being air-open.
A BBA certificate will identify very clearly that a product is air-open. The best advice is to ask the manufacturer to confirm if the water vapour resistance is no more than 0.1 MNs/g (MegaNewtons seconds per gram) and use that as a simple benchmark of performance. For roofs that need the highest standard of ventilation performance, Klober Permo® air, for example, has a water vapour resistance which has been independently tested and shown to be as low as 0.03 MNs/g. This is essential during the critical drying out period on any new build.
Look out for BBA certificates for both cold and warm non-ventilated pitched roofs and for information on projects on which the product has been used. These should include complex roof designs and high profile buildings where traditional tile or slate vents were not acceptable. You may also want to consider asking for a technical specification to be provided as an additional safeguard or proof of suitability. Manufacturers such as Klober also provide a specific condensation risk analysis service as an added means of support for their product.
The bottom line is never to assume that just because a product is vapour permeable that it is also classed as ‘air-open’. You could say it’s an underlaying truth!