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HEATING, VENTILATION & SERVICES
those transformed from offi ce buildings with sealed facades and extensive glazing, architectural legacies that can work against thermal comfort.
A 2023 survey on PDR housing and health published by UCL showed that only 63% of respondents were able to keep comfortably cool during hot summer weather conditions.
The risk is growing
The result is a proliferation of homes that are
technically compliant, yet thermally dysfunctional
This year has been the driest January to June for England since 1976. Spring 2025 is the UK’s warmest and sunniest on record, with June the second warmest for the UK since records began in 1884. The situation is worsening; the state of the UK Climate Report 2024 shows the UK is warming at a rate of approximately 0.25°C per decade.
These temperature extremes have serious implications. There were 2,985 heat-related deaths in 2022, the year that saw the highest recorded temperature in England, according to the UK Health Security Agency. This fi gure is the highest since recording began. Beyond the health risks, overheating affects mental wellbeing and economic productivity.
According to the Chartered Institution of Building Services Engineers (CIBSE), a predominantly mechanically ventilated home overheats when internal temperatures exceed 26°C for more than 3% of annual occupied hours.
No reliance on natural ventilation The core challenge with PDR projects is that they often repurpose buildings designed for a completely different occupancy. Former offi ces were not built to house people 24/7. They typically feature large glazed facades, minimal external shading, and restricted natural ventilation. In densely built up urban heat islands, where many of these buildings are located, opening windows (if they can be opened that is) may not be an option due to noise, air pollution, or security risks. These environmental constraints effectively render passive ventilation strategies unworkable.
Part O prescribes maximum glazed areas on facades (depending on orientation), encourages the use of solar shading and glass with lower g-values to limit solar gains, whilst setting minimum openable window areas so that excess heat can be passively ventilated away. But in the case
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of PDR schemes, this requirement is not mandatory, leaving developers to comply only with the bare minimum ventilation requirements which are stipulated by Building Regulations Part F. The result is a proliferation of homes that are technically compliant, yet thermally dysfunctional.
A viable solution
Manufacturers work extensively with sustainability engineers carrying out dynamic thermal modelling simulations to assess overheating risk in new build developments. Elevated mechanical ventilation rates and Mechanical Ventilation with Heat Recovery (MVHR) with hybrid cooling must be trialled before a designer can consider resorting to air conditioning, and this approach really should be considered in PDR conversions, even if it’s not a requirement. MVHR systems offer a practical and effective approach to meeting Part F and can also assist in improving thermal comfort. MVHR not only preconditions air and fi lters out pollutants but also provides summer bypass functions, which can be used in combination with elevated airfl ows to provide free cooling when external temperatures are favourable. These systems can help maintain acceptable indoor temperatures year round and are still regarded as passive means in Approved Document O.
Where window openings are limited and
insuffi cient to naturally ventilate excess heat, standard MVHR systems may not suffi ce. Hybrid cooling units working in tandem with MVHR could assist in mitigating overheating, activating when indoor temperatures exceed a set threshold (typically 23°C). The potential effectiveness of such a system should be assessed through dynamic thermal modelling.
A call for reform
The urgent need to reform policy around overheating is becoming widely recognised. The House of Commons Environmental Audit Committee recommended expanding Building Regulations Part O to include refurbishments and material change of use. The built environment community must confront the reality that current practices in PDR conversions are insuffi cient for the demands of a warming climate.
Andrew Nash is residential divisional manager at Nuaire
ADF OCTOBER 2025
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