With the Future Homes Standard expected to make solar PV a common feature of new homes from 2028, and the Warm Homes Plan driving investment in low-carbon technologies across the existing housing stock, rooftop solar is set to play an increasingly important role in the delivery of high-performance homes, both in newbuild and retrofit contexts.
One area attracting particular attention is the specification of roof membranes beneath in-roof solar PV systems. While integrated solar products have evolved rapidly, guidance relating to moisture management beneath these systems has not kept pace. As a result, questions remain around ventilation strategies, condensation risk, long-term durability and the suitability of different membrane types within solar-integrated roof constructions.
Traditional pitched roofs are designed around well-established principles of airflow and vapour movement. Current British Standards and guidance assume that roof coverings allow a degree of air circulation and moisture transfer, informing ventilation requirements and condensation risk assessments. Integrated solar systems can alter those assumptions. Depending on the product design and extent of roof coverage, solar modules may create substantial areas that behave as impermeable roof coverings. This raises questions about how moisture moves through the roof build-up and whether established ventilation strategies remain appropriate.
The NHBC sought to address part of this through revisions to its Standards that came into effect in January 2024. These classify any roof containing integrated in-roof solar PV as an impermeable roof covering, regardless of how much of the roof area is occupied by the array. However, beyond this classification, there is no dedicated British Standard test methodology or agreed industry wide approach, and no universally accepted guidance specifically examining membrane performance beneath integrated solar systems.
THREE APPROACHES
Current discussions tend to focus on three broad membrane strategies, each based on different assumptions regarding moisture movement beneath integrated solar arrays. The first approach specifies an impermeable membrane. By preventing moisture transfer from the loft space into the batten cavity, this removes the risk of moisture accumulation within that part of the roof build-up. But it places greater reliance on effective low- and high-level ventilation to ensure moisture is removed from the roof space itself. As with any ventilation-led strategy, detailing becomes particularly critical to long-term performance.
The second approach uses vapour permeable membranes. These are the most widely specified underlays in pitched roofing and designed to allow moisture vapour to pass through the membrane while maintaining weather resistance. The question is whether moisture entering the batten cavity can continue to dissipate effectively over time when an impermeable solar array is positioned above.
The third option is the use of air permeable membranes. These allow both air and moisture vapour to move through the roof construction and can reduce dependence on additional roof-space ventilation measures, depending on overall roof design. However, they raise similar questions regarding moisture accumulation beneath solar modules and are often used in roof designs that incorporate fewer supplementary ventilation provisions. Each approach can be supported through technical reasoning. Yet without standardised testing or long-term field data, it remains difficult to determine which assumptions most accurately reflect real-world performance.
Recognising the need for real-world data, wienerberger partnered with Liverpool John Moores University through its Renatus programme to investigate the performance of different membrane types beneath integrated solar installations. Rather than just relying on lab modelling, the project captures performance under realistic operating conditions. Three full-scale roof structures were constructed, each incorporating wienerberger’s in-roof solar system alongside one of the membrane types currently at the centre of industry discussion. To replicate the conditions found within occupied homes, the test environment simulates the moisture loads associated with daily living, including heating cycles, hot water usage and normal household humidity.
A network of sensors installed in the loft space and batten cavity monitors temperature, humidity, moisture content, dew point and heat index. This allows researchers to observe how conditions evolve over time and identify any meaningful differences between membrane types. Following 12 months of uninterrupted monitoring, analysis is now underway. Early findings confirm that the batten cavity beneath integrated solar panels operates at higher temperatures than equivalent areas beneath traditional tiled roof coverings. The project is helping to quantify this difference under real service conditions for the first time.
If research demonstrates that moisture risk is not significantly increased when permeable membranes are used beneath integrated solar arrays, it could help simplify specification decisions and reduce design-stage uncertainty. If the findings point in the opposite direction, they will provide a clear technical foundation for future guidance and specification approaches.
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