Updated NHBC standards - Guidance on ventilation requirements for integrated solar panels

In January 2024, the National Housebuilding Council (NHBC) updated its 7.2.15 standard, covering ventilation, vapour control and insulation in pitched roofs. This update aimed to provide improved clarity about how roof-integrated solar panels, such as the Marley SolarTile®, should be ventilated. The updated document sets out clear guidelines for achieving compliance with NHBC standards, as well as the British Standard, BS 5250:2021, the code of practice for the management of moisture in buildings.

Why has the NHBC updated its guidelines for integrated solar installations?

As architects and housebuilders look to create more sustainable homes, and homeowners look for ways to cut their energy costs, more and more new homes are being fitted with solar panels – and roof-integrated solar panels in particular – to help power the properties, cut carbon footprints and reduce reliance on the energy companies. The uplift to the Building Regulations Part L: Conservation of Fuel and Power, which went into effect in 2022, along with the Future Homes and Buildings Standard, which comes into effect in 2025, both place greater emphasis on energy efficiency.

Meanwhile, BS 5250 aims to prevent excess condensation in roofs – a problem that is particularly an issue for new-build homes that contain excess moisture, and which are highly insulated. Without proper ventilation, a variety of problems can develop, ranging from poor indoor air quality to mildew, mould and structural damage.

As roof-integrated solar panels, like the Marley SolarTile®, have become increasingly popular, thanks to their ease of installation, excellent aesthetics and reliable energy output, the question of how to ensure they are adequately ventilated has become more pressing. The NHBC’s 7.2.15 standard aims to address the issues of ventilation and condensation, helping to ensure that all new homes meet the requirements of Part L, the Future Homes and Buildings Standard and BS 5250.

What does the NHBC say about ventilating around in-roof solar panels?

The NHBC’s updates to 7.2.15 address the concerns about ventilation around roof-integrated solar panels, stating: “Where arrays of integrated solar roof panels are installed forming the roof covering, then whole roof covering should be treated as air impermeable unless the panel manufacturer can demonstrate their system is air permeable.”

In other words, if an in-roof solar panel is installed, the whole roof should be considered impermeable and other roof ventilation options, such as ridge ventilation, eaves ventilation and tile ventilation, should be used to ensure sufficient airflow and effective moisture control.

The NHBC also notes that some manufacturers of integrated solar panels may recommend even more space than its 7.2.15 standard requires, further increasing the amount of ventilation and cooling needed for the solar array.

How to achieve the NHBC’s ventilation requirements with roof-integrated solar panels

Since the new NHBC guidelines classify all roof-integrated solar systems as air-impermeable roof coverings, ventilation must be calculated as though no air flows through the roof face. Therefore, roof ventilation strategies must be implemented to provide airflow through other parts of the roof.

The right ventilation strategy will depend on whether the roof in question is a warm roof or cold roof, and whether a non-breathable underlay (also known as high-resistance or HR) or a vapour permeable underlay (also known as low-resistance or LR) is used. Here is a breakdown of the requirements for typical roof build-ups:

Cold roofs with non-breathable underlay

• Eaves/low-level ventilation: 10,000mm²/m under the underlay
• Ridge/high-level ventilation: 5,000mm²/m at the ridge (if the roof pitch is greater than 35°, the span is more than 10m or if it’s a mono or lean-to roof)

Cold roofs with vapour-permeable underlay

• Eaves/low-level ventilation: 7,000mm²/m under the underlay for a normal ceiling, or 3,000mm²/m for well-sealed ceilings
• 5,000mm²/m of high-level ridge ventilation can be used as an alternative to eaves/low-level ventilation if the ceiling is well-sealed
• An LR underlay must include batten space ventilation; otherwise, it should be treated as an HR underlay
• An additional 5,000mm²/m ridge/high-level ventilation if the roof is above 35° roof pitch, the span exceeds 10m or is mono pitch or lean-to

Warm roofs and hybrid roofs with non-permeable underlay

• Eaves/low-level ventilation: 25,000mm²/m beneath the underlay
• Ridge/high-level ventilation: 5,000mm²/m beneath the underlay
• An AVCL on the warm side of the insulation

Hybrid roofs with vapour-permeable underlay

• Eaves/low-level ventilation: 7,000mm²/m beneath the underlay and 25mm deep counter battens with 25,000mm²/m eaves ventilation above the underlay
• Ridge/high-level ventilation: 5,000mm²/m beneath the underlay and 5,000mm²/m ridge/high-level ventilation above the underlay
• An AVCL on the warm side of the insulation

This chart also sets out the requirements for ventilation in a roof with integrated solar panels.

Achieve NHBC standards with in-roof solar panels from Marley

Marley’s complete roof system includes everything necessary to create high-quality and well-ventilated roofs with integrated solar panels.

Our product range includes:

• Marley SolarTile®: This roof-integrated solar tile replaces standard roofing tiles, providing a weatherproof roof covering and a sustainable source of electricity, along with sleek aesthetics and excellent wind and fire performance.

• Underlays: We offer both vapour-permeable and non-permeable underlays, which include integrated tape for quick and efficient sealing laps.

• Marley ventilated dry ridge system: Specifically designed for quick and easy installation, this system provides 5mm continuous ventilation along the ridge and is suitable for pitches up to 55°.

• Marley eaves vent system: Available in formats to provide 10mm or 25mm continuous ventilation along the eaves, this system allows clip fixing of all tiles at the eaves. It also compresses insulation to ensure airflow, prevents ponding on the underlay and keeps out vermin and wildlife.

• Tile vents: These are available in a wide range of colours and designs to seamlessly blend in with roof tiles while providing extra ventilation to the roof space.

All Marley roofing products are designed to work together seamlessly, providing architects, specifiers and contractors with a single supplier for the entire roof build-up. We also offer a 15-year roof system warranty for additional peace of mind.

For a full breakdown of the NHBC updates relating to integrated solar panels, please download our NHBC update 2024 Data Sheet. Or, for further technical support with your next project, contact the Marley team.