A.Proctor’s history of cold pitched roof construction for houses in the UK has led to a variety of solutions for insulating roof spaces either on the rafters or at ceiling joist level in order to achieve greater energy efficiency. However the resulting temperature differential has led to issues of condensation on insulation occurring in the roof space particularly in colder months. The original solution to this was to introduce some low and/or high level natural ventilation in order to allow air to circulate in the roof space.
Recently more technological solutions have been developed to address this issue by installing a breathable membrane over the rafters as the roof underlay, which will allow water vapour to escape, however not all membranes are the same. Many manufacturers have gone down the vapour permeable but airtight route, due to the increased drive to create airtight building solutions. Based on film laminated polypropylene technology, these solutions generally only offer moderate vapour permeability and a debate has been running for some time in the industry over whether roofs they are installed in will still suffer from condensation and require ventilating. This has led to a recommendation for a VCL to be used at ceiling level to reduce the moisture load into the roof space when the film based laminates are used as the underlay.
As Iain Fairnington, Technical Director of the A. Proctor Group explains, building physics places limitations on the effectiveness of air tight membranes in alleviating condensation in roofs due to their limited vapour permeability:
“If you have a big cold roof space, and you have a sudden drop in temperature, you want to have air movement, which is what ventilation provides. 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 developed by the A. Proctor Group is a membrane that has a far higher degree of vapour permeability, in fact the highest available, so will still perform in conditions in which air tight alternatives will not. It’s credentials were endorsed following a cross-industry Partners In Innovation (PII) study undertaken by Glasgow Caledonian University, which “dispelled a lot of theories around ventilation” says Iain Fairnington. In fact the study, which included the NHBC, found that air permeable as well as vapour permeable membranes such as Roofshield would further reduce and inhibit the formation of condensation on the underlay when the roof is unventilated.
This led to the NHBC making an official recommendation that Roofshield could be used without ventilation in roofs, which has seen a swelling of interest in the product among the UK’s housebuilding industry recently, also aided by its hydrophobic and UV resistant qualities.
Meeting latest standards
The 2015 version of the BS5534 Code of Practice for Slating and Tiling is causing a stir in the industry currently, because in addressing wind uplift issues it makes requirements on certain membrane products to be taped at laps. However for the A. Proctor Group it has the benefit of further driving specification of heavier roof membranes like Roofshield. The company has undertaken full third party testing to establish that it is fully compliant with the standard, requiring no tape, and is providing specifications to interested parties on that basis.
Before the PII study there had been a lot of resistance to change in the industry, with some unsure as to the case for membranes as a solution to the problem of condensation in roofs in the context of a lack of clarity on whether ventilation was necessary. However it has shown that as an accepted water resistant air and vapour permeable solution which needs no roof ventilation and meets the latest standards, specifiers can be assured that Roofshield offers the simple answers.