Vapour permeable membranes allow moisture from the rooms below the roof to pass into the roofspace and up out through the membrane without having to ventilate the roofspace using conventional vents. It is important to get rid of this moisture otherwise it condenses on cold surfaces in the roof causing water damage. Vapour permeable membranes theoretically save energy, as less vents mean less heat loss, and another advantage is that roofing contractors don't have the job of installing them.
The debate centres on whether it is necessary to ventilate the batten space, which is the gap between the membrane and the tiles or slates. The moisture passes through the membrane but still has to exit through gaps in the outer roof covering; the crux of the debate is whether or not enough natural gaps exist in the outer covering to allow the moisture to dissipate. According to Glidevale, DuPont Tyvek has been claiming its membranes were suitable for use in all non-ventilated cold-pitched roofs without the need to ventilate the batten space between the membrane and the tiles or slates.
Glidevale had its complaint upheld on the grounds that DuPont said its product met the requirements of British Standard BS5250:2002 Control of Condensation in Buildings and had BBA approval for all types of pitched roof. The ASA agreed that this was not the case, as BS BS5250 recommends ventilating the batten space, and the BBA approval only covered dwellings.
Glidevale is a member of a trade body called Surevent. Its members include the National Federation of Roofing Contractors, the Concrete Tile Manufacturers Association, the Clay Tile Roof Council and the Fibre Cement Manufacturers Association. Surevent is running a campaign urging roofing contractors to take the following precautions when installing a vapour permeable roofing membrane, and says its guidance is based on Part L and J of the Building Regulations, construction research group BRE's Good Building Guides 37 & 51 and BS 5250.
Surevent says the first step is to ensure the membrane has a moisture resistance of less than 0.25 MNs/g. Penetrations such as pipes and wiring into the roof need to be sealed and if there is a kitchen or bathroom below the roof a vapour control layer should be installed to prevent moisture getting into the roof. After installing the membrane counter battens should be fixed prior to fitting the tile battens to maintain a gap of at least 50 mm between the outer roof covering and the membrane. This gap should be ventilated because Surevent says gaps between the tiles or slates cannot be relied on for the life of the roof.
Dupont Tyvek says research indicates that there is always sufficient gaps between tiles or slates to ventilate the roofspace, so fitting vents in the batten space is unnecessary.
Glidevale has published two papers giving guidance on whether pitched roofs should be ventilated. The two papers Condensation Control in Cold Pitched Roofs and Condensation Control in Warm Pitched Roofs are available from Glidevale at www.glidevale.com.
A BRE-led Partners in Innovation project is investigating the performance of various cold pitched roof constructions. More advice is likely to be issued by bodies such as the NHBC once the project reaches its conclusions.
Bill sets out stall for tougher regulationsUpdated standards covering waste, energy, water and procurement of construction materials could be implemented in Building Regulations if a new bill is passed in parliament. The Sustainable and Secure Buildings Bill will give the government powers to include new energy and security measures in the Building Regulations. The bill, introduced by Liberal Democrat energy spokesperson Andrew Stunell to promote greener and safer buildings was given an unopposed second reading in the House of Commons in January. If Stunell succeeds with his private members’ bill it will give the government the power to apply the recommendations of the Sustainable Buildings Task Group, which was set up at the Better Buildings Summit in November 2003. The group is looking at areas where new and refurbished buildings could achieve higher standards of sustainable performance. The recommendations should be made in April and will cover energy, water, waste reduction and procurement of timber and other construction materials. The task group is not looking at security issues. Standards for each group could then be incorporated into the Building Regulations. Currently, under the Building Act 1984, the government is only ~ able to deal with the conservation of fuel and power. Security standards can only be enforced through council planning requirements. The WWF supports the new bill, which it says will do the following:
- Improve the crime resistance and security of buildings – at present there are no statutory requirements to comply with police advice;
- Require that in certain circumstances, large-scale repair and renovation work should comply with the same standards and crime resistance as equivalent new building work;
- Bring into the scope of Building Regulations certain types of buildings that are currently exempt such as schools and operational buildings owned by public utilities;
- Place a duty on the government to report to parliament at intervals on progress made in making the building stock more sustainable and crime resistant.