Not All Roofing Underlays Are Created Equal

For years the debate on how to achieve a truly ventilation free cold pitched roof has continued across the construction industry. Now, reputable roofing contractors and leading housing developers such as bellway, bovis homes and crest nicholson, backed by the nhbc, and independent industry research, have come to the realisation that some underlays perform at an exceptional level, providing a failsafe option, without the need for additional ventilation.
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Improved thermal and moisture performance of pitched roofs The A. Proctor Group developed Roofshield 20 years ago in response to problems caused by the requirements of the UK’s demand for cold-pitched roof construction, and the drive towards ever increasing thermal performance requirements. Today, trusted by architects, developersand contractors, the Roofshield brand is synonymous with the highest quality, providing a pitched roof underlay, which is both air and vapour permeable.

The combination of cold-pitched roof construction, and increasing thermal performance requirements has fuelled the tendency to place additional insulation above the ceiling joists, leading to an increased risk of condensation in the roof space. In an effort to combat this problem, the industry developed a series of breathable membranes, which were designed to be installed over the rafters as roof underlay, and allow the vapour to escape. The majority of these membranes were vapour permeable, but air tight, rather like a Gore-Tex jacket. While water resistant, they did not completely prevent condensation within the roof space, meaning additional ventilation had to be introduced in order to allow air to circulate.

Not All Roofing Underlays Are the Same

Generally, two types of technological solutions have been presented to specifiers; 1.Vapour permeable, but airtight solutions, based on film laminated polypropylene technology, or 2. A vapour and air permeable version, such as Roofshield, alleviating the need for additional ventilation.

Iain Fairnington, Technical Director of the A.Proctor Group, explains, “Roofing underlays come in all manner of different colours, but it’s what you can’t see that does most of the work – the middle layer. Whilst the colour of the top surface or underside is useful for identification, it is irrelevant in terms of performance. This middle layer can be likened to the same robustness as normal kitchen cling film, and needs the protection of the sandwich construction to make this suitable and fit for purpose in a pitched roof. Taking a microscopic view of the middle layer clarifies the difference between an airtight and air permeable membrane. Fig.1 shows the Roofshield membrane, Fig.2 shows a typical airtight roofing underlay. The difference between air permeability and air tightness is fundamental to the products performance and use.

In a traditional UK cold pitched roof construction, the large cold void above the horizontal insulation requires the quick release of vapour laden air that is reaching its dew point and potentially causing damaging condensation; Roofshield is certified by the BBA and accepted by the NHBC for use without ventilation or a VCL in even these most demanding circumstances. This is particularly advantageous in refurbishment projects where the installation of the VCL can lead to owners or tenants having to move out if a non-ventilated roof strategy is adopted.”

Independent Research and Nhbc Guidance

Iain Fairnington, explains the building physics around why air tight membranes’ limited vapour permeability means that condensation can still occur: “If you have a big cold roof space, and you have a sudden drop in temperature, you want to have air movement. People assumed that because they were installing a vapour permeable membrane you didn’t need to ventilate your roof, but in certain circumstances moisture levels were too high or temperatures too cold to allow the vapour to permeate without condensing.”

By contrast, Roofshield has a far higher degree of vapour permeability, as well as air permeability, so will still perform in conditions in which air tight alternatives will not. Between 2001-2004 independent research was carried out, conducted on a cross-industry basis with Glasgow Caledonian University. The Partners in Innovation study was necessary, because there was a need for consensus on whether it was still necessary to ventilate buildings where vapour permeable membranes were installed.” says Fairnington.

In fact, the study contained the crucial finding that when a roof was unventilated, and used an air and vapour permeable underlay, such as Roofshield, this would further reduce and inhibit the formation of condensation on the underlay. This contributed to the NHBC making a statement in their Technical Extra bulletin Issue 6 that independently certified air and vapour permeable underlays, such as Roofshield, could be used without additional ridge ventilation in cold roofs.

As Roofshield is accepted by the NHBC without the requirement of ventilation or a VCL, even in cold-pitched roof constructions, the choice of underlay becomes simple. Another independent research report undertaken in 2014 by property consultants Hardies shows that the use of Roofshield without ventilation or a VCL is the most economical choice. When looking for the best vapour permeability, combined with the added benefit of air permeability – Roofshield is the only membrane that meets both these criteria. The evidence is clear for architects, house builders and contractors looking for a failsafe roof that is cost effective, and delivers the highest performance, Roofshield is in a class of its own.

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