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APRIL 06 - BS5250 AMENDMENTS: CLARIFYING THE ISSUES

The last six years have seen some heated debate regarding the need for roofspace ventilation in both warm (insulation at rafter line) and cold (insulation at joist level) roofs when vapour permeable (Type LR) underlays are used. In 2001 the debate prompted the start of the Partners in Innovation (PiI) research project entitled "Thermal and moisture performance of pitched roofs", funded by the Dti and industry including the NHBC and HFRC, which concluded in April 2005. One of the key outputs of the project was to provide amendments to BS5250: 2002 control of condensation in buildings. These amendments have been incorporated into Amendment 1 2005 of BS5250 which was published just before the New Year.

BS5250 is now the main means of compliance with regulatory requirements following the replacement of Approved Document F2 Condensation in Roofs by Approved Document C2 Resistance to Moisture: 2000 (2004 edition), In Scotland, Northern Ireland and Eire, BS5250 has always been the means of compliance in their respective Building Regulations. Approived Document Part C in England and Wales does not make any specific recommendations to control condensation in roofs but refers to BS5250 as a means of compliance with the Regulations.

Leading roofing supplier Glidevale Ltd has taken steps to help clarify the issues in easy to understand language with the publication of two "white papers".

The white papers "The new roof ventilation provisions explained" and "well sealed ceilings explained" address the specific means of compliance with Amendment 1 in England and Wales (Scotland and Northern Ireland having their own Regulations).

COLD ROOFS

In essence, in cold roof construction, ventilation must be provided, but the degree of ventilation required varies dependant on the following:

• whether or not the ceiling is "well sealed2- a term now clearly defined in the Standard. It warns that achieving such a well sealed ceiling in re-roofing applications may not be possible as the roofer would not normally address such issues

• the underlay used, be it impermeable (Type HR) or vapour permeable (Type LR)

• the size and pitch of the roof

BS5250 states that the Standard does not consider the situation where it is proposed to provide no ventilation and if no ventilation is proposed, then reference must eb made to the conditions of use attached to Technical Approvals.

The only Technical Approval body who issue certificates suggesting no ventilation is the BBA. Some of the key conditions of use of such certificates include:

• the use of such systems in dwellings only (any other type of roof is not covered by the certificates)

• the use of convection tight loft traps (not currently commercially available)

• a ceiling air leakage rate of not more than 10mm 2 /m 2 of ceiling (this represents an extremely well sealed ceiling some 7.5 times more onerous than the target set in the new BS5250 for a well sealed ceiling)

WARM ROOFS

The only situation where ventilation is not required is in warm roof construction, when using a vapour permeable (Type LR) underlay. Such constructions require a well sealed ceiling and a separate vapour control layer with sealed laps. If there is any doubt about the ability to provide and maintain an effective sealed vapour control layer, especially in room in the roof constructions, then ventilation should still be provided.

ROOF COVERINGS

In all roofs when vapour permeable (Type LR) underlays are used, they contribute to the control of condensation by allowing vapour to escape through the material by diffusion. It is important that this water vapour can escape to atmosphere from the batten space before condensation occurs. BS5250 defines by means of a test, roof coverings that are sufficiently or insufficiently air open. Traditional concrete and clay tiles should be sufficiently air open but advice should be sought from the manufacturer/ supplier.

Neil Rideout, Glidevale Technical Director, observes, "Our aim with the white papers is simplify the main points of Amendment 1, and clarify new elements, such as well sealed ceilings, so the industry understands it and can ensure its specification and build will comply."

Download 'Well Sealed Ceilings BS5250 White Paper' pdf >

Download 'Condensation Control BS5250 White Paper' pdf >

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