Roofing COVERING ALL THE BASES
As an effective method of addressing condensation in roofs that needs no ventilation or tapes, the Roofshield air and vapour permeable membrane from the A. Proctor Group presents a compelling case
Vapour permeable membranes have been gaining favour in recent years for preventing condensation in cold pitched roof applications, providing a combination of water resistance and a high degree of breathability, alleviating the need for traditional eaves and ridge ventilation. This ability of the A. Proctor Group’s vapour permeable membrane product Roofshield was recognised by the NHBC in 2011 following an amendment to their Technical Standards resulting from complaints of condensation occurring in some non-ventilated roofs. This change required ridge ventilation to be installed in all cold roof constructions where a vapour permeable underlay was used. The Partners in Innovation (Pil) study, carried out in 2004, showed that a vapour and air permeable underlay, Roofshield, could be used to prevent condensation without the need for additional ventilation.
The potential cost savings on a construction project of being able to install an effective membrane without the need for ventilation, despite the high quality nature of the Roofshield membrane itself, have been shown in a 2014 report undertaken by property consultants Hardies. More recently, the case for Roofshield has been further bolstered with amendments to official guidance on roof construction due to its ability to resist wind uplift without requiring tape, as is detailed below.
BACKGROUND
Roofshield was developed 20 years ago in response to problems caused by the double-whammy of the UK’s fondness for cold pitched roof construction, and increasing thermal performance requirements which led to place additional insulation above the ceiling joists leading to an increased risk of condensation in the roof space. With vapour condensing on the cold surfaces in the roof, particularly in winter months, a variety of efforts were made across the industry to develop membranes which would allow the vapour to escape. These breathable membranes were designed to be installed over the rafters as the roof underlay as an alternative to traditional 1F felt.
The majority of those membranes were vapour permeable but air tight, rather like a Gore-Tex jacket. While water resistant, offering additional benefits during construction similarly to Roofshield, they did not completely prevent condensation within the roof space, meaning some low and/or high level ventilation had to be introduced in order to allow air to circulate and fix this problem.
Therefore two types of technological solutions have been presented to specifiers; vapour permeable but air tight solutions, based on film laminated polypropylene technology which has been favoured by several manufacturers, or the vapour and air permeable version alleviating the need for ventilation, such as Roofshield. A debate continues to run in the industry around whether roofs fitted with the air tight membranes will still require ventilating due to their being vapour permeable, although testing undertaken within the 2004 Partners in Innovation study confirmed this was the case. The continued discussion has however led to a recommendation for a vapour control layer to be considered at ceiling level to reduce the moisture load into the roof space when film based laminates are used as the underlay.
Iain Fairnington, Technical Director of the A. Proctor Group, 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. The 2004 study was conducted on a cross- industry basis with Glasgow Caledonian University because there was
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