Ventilation IN-ROOF SOLAR ROOFING: SHINING A LIGHT ON NHBC’S NEW GUIDANCE Matt Neary, from leading roofing products manufacturer Glidevale Protect, takes a closer look at the new NHBC guidance and the choice of roofing underlay and ventilation solutions available to help lower condensation risk when using integrated, in-roof solar PV panels. At the end of 2023 the National House Building Council (NHBC) published new standards for <strong>2024</strong> which included guidance on the use of integrated in-roof solar PV panels, how they are classified and what associated ventilation strategies are therefore required, in alignment with requirements set down in BS 5250. The standards apply with immediate effect to every new home registered with the NHBC where the foundations are started on or after 1st January <strong>2024</strong>. So, what do contractors need to know? Air permeable or air impermeable? That is the question! It’s important to have clarity on whether roofs with solar panels are classed as being air permeable or air impermeable, and the new NHBC guidance aims to address this head on. The new NHBC Standards contain revisions to clause 7.2.15 ‘Ventilation, vapour control and insulation’, stipulating that where arrays of integrated in-roof solar PV panels are used, the whole roof covering should be treated as air impermeable unless the solar panel manufacturer is able to demonstrate their system is air permeable. As a result, roofs with integrated PV panels will now be classed as air impermeable as standard, even if the roof tiles themselves are air permeable. So a small number of PV panels would render the whole roof impermeable and should be treated as such, unless it can be proven otherwise. And as an air impermeable roof covering, there is a higher risk of interstitial condensation on the underside of Left: Matt Neary. Above: Protect A1 Solar underlay. the underlay, the external covering and behind the solar panel arrays if the correct ventilation strategy is not used. What does this mean in terms of roofing underlays? When a low vapour resistance (LR) membrane or vapour permeable ‘breather’ membrane underlay is used on a warm or cold roof in conjunction with integrated, in-roof solar PV panels, Glidevale Protect’s recommended best practice is to install counter battens. Using 25mm deep counter battens with a minimum free ventilation area of not less than 25,000mm²/m at eaves or low level, and 5,000mm²/m continuous air gap each side of the ridge or at high level is advised. On a cold roof, ventilation is also required under the membrane at 7,000mm²/m at eaves level with a normal ceiling, or 3,000mm²/m at eaves with a well sealed ceiling. Alternatively, it is also accepted to ventilate with 5,000 mm²/m at ridge instead of 3,000 mm²/m at eaves if there is a well sealed ceiling. Without the use of counter battens with an LR underlay, the risk of interstitial condensation forming on the underside of the solar panels increases, as the water vapour will pass through the membrane but cannot escape into the atmosphere. One alternative option when using an LR underlay on a cold roof however, as detailed in BS 5250, is not to counter batten but ventilate underneath the underlay as if it was an HR underlay, thereby requiring eaves level ventilation and additional ridge ventilation, the latter only required subject to the roof’s pitch, span and if classified as a mono pitch. An alternative solution to using an LR underlay where in-roof solar is being installed would be to use a high vapour resistance (HR) or impermeable non-breather underlay which is designed to prevent moisture from transferring through the underlay and reaching the exterior layers above the membrane, thereby not requiring the use of counter battens. Ventilation would still be required underneath the underlay on both cold and warm roofs, with cold roofs requiring 10,000mm²/m at eaves level and 5,000mm²/m at ridge, the ridge ventilation only being required when the roof pitch is over 35 degrees, a span over 10m or a mono pitch. Warm roofs require additional airflow with 25,000mm²/m required at eaves or low level and 5,000mm²/m at ridge or high level. The key takeaways When it comes to installing integrated PV panels as part of the roofing structure, the appropriate ventilation strategy needs to be factored in from the outset. To discuss your individual project requirements and how the updates to the NHBC Standards will affect new roofing specifications using in-roof solar PV, please contact Glidevale Protect at technical@glidevaleprotect.com or 0161 905 5700. Contact Glidevale Protect www.glidevaleprotect.com 22 TC FEBRUARY <strong>2024</strong>
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