Meir Roofing and Insulation Supplies

BS 5250:2002

Amendment 1 2005 recomends the combined use of vapour permeable (LR) roof underlay and high level ventilation to combat the risk of harmful roofspace condasation in cold pitched roofs.

After the prolonged cold spells over the last winter this solution has now been adopted by the NHBC who require high level ventilation in all cold pitched roofs whether they incorporate a vapour permeable (LR) underlay or not. It is one of the amendments to the 2011 NHBC Standards, see below, and will take precedence over 3rd party assessments (eg. BBA certificates) which may state that ventilation is not required in this type of roof.
Many products have been marketed on the basis that the roof ‘breathes’ through the underlay and therefore eaves ventilation is not required.
To work properly, any moisture vapour entering the roof space from the rooms below needs to be able to pass to the outside around the gaps in the tiles. If the amount of moisture vapour exceeds that which can safely pass through the
underlay, then problems can occur – usually condensation forming on the underside of the underlay and dripping onto the ceiling insulation.
Research by BRE has shown that condensation is likely to occur early in the life of the home when it is drying out.
The amount of warm moist air passing into the roof space can overwhelm the underlay.

The prolonged cold, frosty and snowy weather of last winter caused some horrendous condensation problems in roof spaces. That’s the feedback we received from a number of sources including builders and homeowners.
Generally, the problems appeared to have been in homes less than two years old and most were in their first heating season. Construction moisture was still drying out and, because warm air rises, the moisture vapour migrated to
the roof space. The snow and frost on the roof meant that the migration of the vapour to the outside was not able to occur. In the conditions experienced last winter, vapour permeable underlays used in unvented roofs did not seem able to cope with the situation and condensation occurred on the inside of the underlay.
BS 5250 ‘Code of practice for the control of condensation in buildings’ does provide guidance on such situations and recommends high level ventilation at or close to the ridge.
The BS 5250 requirement is for a ventilation gap equivalent to a 5mm slot for the length of the ridge. The BS also provides guidance on how to calculate the ventilation for hipped and other roofs where the ridge length is not representative of the plan. This requirement is now incorporated in the 2011 Standards in clauses 7.2- D11 and S11.

It should be noted that when NHBC introduces a standard for a particular purpose, it will take precedence over third party assessments (eg. BBA certificates) which may not require high level ventilation.

If you have been using vapour permeable roof underlays in unventilated roofs, from January 2011, NHBC will be looking for high level ventilation.