Maintenance & Refurbishment


Roofing & Cladding Feature


charter for social housing residents, launched by the Government, commits to reviewing the current Decent Homes Standard to support the decarbonisation of social homes. This means, as homes are designed with a fabric first approach and therefore increased airtightness, it is vital to balance this with the correct ventilation strategy, protecting against the excessive build-up of moisture and pollutants. This includes the roof, where it is key to have an appropriate combination of underlay and ventilation provision, particularly if the loft is a liveable space.


MEMBRANE CATEGORIES So, with the undoubted importance of ensuring ventilation is provided into dwellings, what different categories of membrane are available, and how does this impact on the ventilation requirements for each, whether cold or warm roof? Starting with vapour permeable or low resistance (type LR) products, two


types of underlays are available in today’s market. Firstly, the traditional LR product is airtight but allows water vapour to disperse into the atmosphere. For a cold roof solution on a dwelling sized roof, BS 5250 recommends the combination of a vapour permeable, airtight underlay with high-level ventilation of 5000mm2


/m through tile or ridge vents, whether a well-sealed or


normal ceiling is in place (for both). Alternatively, where there is a well-sealed ceiling, a ventilation provision of only 3000mm2


/m at low level is required - Glidevale in-line tile ventilator


The need to ventilate within housing has never been more important. With the onset of climate change and the ongoing commitment by the UK Government to bring all greenhouse gas emissions to Net Zero by 2050, energy efficiency is top of the agenda with Building Regulations set to become more onerous by lowering U-value notional targets


however, for re-roofing projects where a normal ceiling is in place that is not likely to be airtight, ventilation at low level should be increased to 7000mm²/m. In warm roof construction, where the loft area has been designed for a habitable space and thus the insulation is laid at rafter level, the use of an airtight and vapour permeable underlay will not require any additional ventilation, subject to a well-sealed ceiling and an air and vapour control layer (AVCL) being installed on the warm side of the insulation. If there is doubt about the ability to provide an effectively sealed ceiling and an AVCL at specification stage, further ventilation should be planned into the design and the chosen manufacturer should provide suitable guidance. The second type of LR underlay is both air and vapour permeable, allowing


ventilation to be delivered into the roofspace due to its special meltblown technology designed within the membrane core. This ensures airflow through the material and allows vapour to escape, helping control the risk of condensation. For these product types, specifications should be made for underlays which are also hydrophobically treated in order to repel water to offer added protection. While an air-permeable underlay may involve higher initial material costs, this is offset by no additional requirement to ventilate, saving time and labour on installation. Whether a cold or warm roof, air and vapour permeable membranes can be used without additional ventilation. This is accepted by the NHBC in their Technical Requirements R3 Document related to cold roof construction. The only exceptions are where the underlay is installed on closed jointed sarking board or if an airtight roof covering is used, such as metal tiles or fibre cement slates. In this case, it would be necessary to ventilate the batten space with 25mm counter battens. The chosen manufacturer should always recommend a well-sealed ceiling and separate AVCL is used, where achievable. The third category of pitched roofing underlays is vapour impermeable or


high resistance (type HR) membranes which are also airtight, where the diffusion of water vapour is prevented from travelling through the material. Designed to overcome the disadvantages of traditional type 1F felts, type HR membranes are generally the most cost-effective when viewed in isolation of just the membrane, however this must be balanced against the requirement for additional high and low-level ventilation for both a cold and warm roof. This extra ventilation provision is imperative to avoid the risk of interstitial condensation forming on the underside of the underlay. As an example, for dwelling sized cold roofs with pitches over 15 degrees, a minimum of 10,000mm2


/m of ventilation is required at low level, in addition to 5,000mm2


at high level. The chosen manufacturer should provide relevant ventilation guidance on a project specific basis. With the undoubted synergy between roofing underlay choice and


ventilation, it is vital that a manufacturer is selected who can advise on the full solution, ensuring the roof is future proofed.


Protect A1 T3 Plus vapour impermeable (type HR) roofing underlay 34 | HMM February/March 2021 | www.housingmmonline.co.uk John Mellor is senior product manager at Glidevale Protect.


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