Specifying air permeable pitched roof underlays with solar PV panels
Regulations like the Future Homes Standard, voluntary approaches such as Passivhaus Plus or Premium, and low energy retrofit are resulting in more pitched roofs featuring solar PV.
These roofs often include an air permeable underlay. Iain Fairnington, Technical Director at Proctor Group, looks at what to consider when combining the two and how guidance has changed.
As homes become better insulated and more airtight, they rely on membrane performance to help reduce condensation risk in loft spaces.
However, industry guidance around integrated( or in-line) solar PV panels has caused confusion over their compatibility with air permeable membranes.
How did industry guidance change?
In 2024, the NHBC updated its Technical Standards to state:
“ Where arrays of integrated solar roof panels are installed, forming the roof covering, then the roof covering should be treated as air impermeable and the whole roof ventilated accordingly, unless the panel manufacturer is able to demonstrate their system is air permeable.”
Ventilation above the underlay and below an impermeable roof covering is not new. Standards and guidance have long referenced impermeable coverings such as tightly jointed slates and tiles and metal sheets, with a ventilated batten and counter batten cavity commonly used to remove moisture beneath the roof covering.
What about ventilation of the loft space?
The permeability of a pitched roof underlay, installed over the rafters and below the external covering, may affect ventilation requirements.
An air permeable, low resistance( APLR) underlay permits air and moisture vapour to pass through the roof construction into the under-tile or ventilated batten space and then outside. It therefore usually requires no additional roof ventilation.
What research has Proctor Group undertaken?
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In the early 2000s, RoofCond software was developed as part of the‘ Partners in Innovation’ project carried out by BRE. It calculates heat and moisture transfer between an occupied house, loft, batten space and outside, and assesses condensate accumulation on the underlay and underside of the tiles.
To investigate the compatibility of solar PV and APLR underlays, Proctor Group engaged Chris Sanders of Glenfeulan Consulting to undertake modelling using RoofCond.
The software modelled a house with a high moisture load, unsealed ceiling, unventilated roof space and Proctor Air. Batten space ventilation and tile air permeability were varied to assess different roof build-ups.
What were the results?
In the worst-case scenario, with minimal batten space ventilation and 80 % of tiles sealed, a short-term winter peak of condensate build-up of 60g / m ² was observed.
The research concluded that this increased the short-term condensate peak, but not to a level expected to cause problems in the batten space.
The model represented replacing a typical air permeable tiling system with airtight PV panels over part of the roof. If an entire roof were covered with PV panels, additional batten space ventilation might be necessary.
Proctor Group subsequently asked the Fraunhofer Institute to validate the research findings. They agreed with the conclusions drawn by Glenfeulan Consulting.
Is the compatibility of APLR membranes and solar PV now accepted?
Proctor Group used the validated findings to develop a recommendation for installing inline PV panels on pitched roofs with Proctor Air.
The BBA also reviewed the research and updated Proctor Air’ s BBA certificate to reflect the recommendation. Proctor Air is the only APLR underlay to have this independent verification for use with in-line PV panels without additional ventilation in either the loft or batten space.
NHBC has not explicitly stated its support for the findings or recommendation and considers APLR membranes with in-line PV on a job-by-job basis. However, it has welcomed the research.
This provides a starting point for housebuilders and specifiers as PV use on pitched roofs increases.
Find out more: https:// proctorgroup. com / products / proctor-air