Ventilation, Ventilation, Ventilation
Ventilation is one area of roofing where there still seems to be some common myths circulating in the industry, so much so that both the NHBC and British Standards Institution have recently sought to clarify the situation, particularly surrounding the use of vapour permeable underlays.
Correct roof ventilation is vital because cold and damp conditions can cause condensation to form on both the underside of the roof tiles and slates as well as the roof underlay, causing droplets of moisture to form in the batten void and roof space. This can cause damage to timber battens, rafters, fixings and insulation, not to mention the house owner’s cherished belongings left in the loft.
Condensation is caused by warm moisture-laden air from within the building being moved, partly by diffusion through the ceiling, but mainly by convection due to the stack effect of air movement which drives air through any gaps and holes in the ceiling and around the pipes and loft hatch into the colder loft and roof voids, where it condenses on cold surfaces such as underlay, tiles and metal fixings.
Obviously correct ventilation is the best way to avoid the problems associated with condensation but there is a common roofing myth that if you use a vapour permeable underlay, then you don’t need to worry about ventilation.
Actually that simply isn’t the case and this has become such an issue in new properties that the NHBC recently issued guidance that from the beginning of this year, all pitched roofs with insulation at ceiling level should always be ventilated to the outside air to minimise the risk of condensation. The NHBC guidance notes will take effect immediately and apply to all new houses, even those at foundation stage, and takes precedence over third party certification which may not currently have this requirement.
To try and clear up any confusion in this area, the ‘Code of practice for the control of condensation in buildings’ (BS 5250), is currently being revised and is expected to be published shortly. The revisions aim to clarify the recommendations concerning roof ventilation and the guidance to be adopted when using vapour permeable (LR) or impermeable (HR) underlays in both cold and warm pitched roofs. Current recommendation would therefore still be to adhere to BS 5250, as this is still in force.
When selling both vapour permeable and impermeable underlays, merchants need to be aware that BS 5250 recommends ventilating roof spaces and voids as an established and proven method of reducing the risk of harmful condensation in roofs. Using LR underlays without ventilation (sealed roof) remains strictly under the control of third party certification (e.g. BBA) and is outside the scope of BS 5250. If customers are omitting ventilation then they require special construction details to ensure that ceilings and penetrations through them can be classified as 'well sealed' in accordance with the recommendations of BS 9250 and other strict requirements of the LR underlay manufacturer’s Third Party certification, however even the best sealed buildings could benefit from the extra ventilation and should always consider extra system elements.
Ten tips to prevent condensation
Marley Eternit has put together its top ten tips to prevent harmful condensation in pitched roofs and meet current regulations that merchants can give to customers:
• Do I need to ventilate? If in doubt, the answer is yes. We would always recommend ventilating
• Identify what type of underlay you are using and what its limitations and requirements are - is it vapour permeable (LR) or impermeable (HR)? Has it got Third Party certification? If so check the conditions of use – if not then check if it complies with either BS 5534 or BS EN 13859-1
• Check the design details of the roof junctions and ceiling specification, including penetrations such as light fittings and its airtightness - if you’re not sure they provide the required airtightness then assume the roof void needs ventilation in accordance with BS 5250
• Thermal insulation needs to be continuous and thermal bridging should be avoided at roof / wall junctions and around any openings and service penetrations
• If it’s a new building using an LR underlay, provide ridge or high level ventilation equivalent to a continuous opening of 5mm to take account of any additional moisture created during the drying out of the building fabric
• Check the insulation specification meets the requirements to satisfy Part L of the Building Regulations
• Ensure ventilation products are correctly installed (use protective trays or seal around pipes where the underlay is cut)
• Last but not least – remember this guiding principle – ‘Insulate tight and ventilate right’.
• If in doubt please contact our technical support team for further support.