Part O Compliance: Mitigating Overheating with Skylight Placement

As UK homes become more energy efficient and airtight, the unintended consequence of overheating has become increasingly common. Warmer summers, combined with highly insulated building envelopes, mean that internal temperatures can easily exceed safe comfort levels. To address this, the UK government introduced Part O of the Building Regulations, placing a legal requirement on designers to manage overheating risk. 

Effective part o overheating mitigation is now a fundamental part of residential design. While windows and shading often receive the most attention, skylight placement plays an equally critical role. Skylights Roof Lanterns can dramatically improve daylight and occupant wellbeing, but if poorly designed or poorly placed they can also be a major source of unwanted solar heat gain. 

Understanding Part O and Its Design Implications 

Part O applies to new residential buildings, care homes, and certain change-of-use projects. Its core objective is to limit excessive solar gains and ensure that buildings can remove excess heat when it does occur. Compliance can be demonstrated either through the simplified method or by dynamic thermal modelling using TM59. 

What makes skylights particularly important under Part O is their exposure. Positioned horizontally or at shallow pitches, roof glazing is subject to more intense solar radiation than vertical windows. As a result, skylights can disproportionately influence internal temperatures, especially in top-floor spaces such as loft conversions and apartments. 

For designers, this means skylights must be considered early, not added later as a purely aesthetic feature. 

Skylight Placement and Its Impact on Overheating 

Size and Location Matter 

One of the most common causes of Part O failure is excessive roof glazing. Large skylights may deliver impressive daylight levels, but they also introduce high solar gains during summer months. A more effective approach is to carefully size skylights in proportion to the room and, where possible, distribute daylight using multiple smaller units rather than a single large opening. 

Equally important is where the skylight is located within the plan. Skylights positioned above circulation areas or stairwells can provide useful daylight without significantly increasing overheating risk in occupied spaces such as bedrooms. 

Skylight Orientation South Facing: Managing Risk 

Orientation has a direct influence on solar exposure. Skylight orientation south facing presents the greatest challenge under Part O because it receives the highest levels of solar radiation during the hottest part of the day. South-facing rooflights such as the pyramid skylights offered by Skylights Roof Lanterns can be beautiful design features, but they require careful mitigation. This typically involves high-performance glazing, reduced opening sizes, or supplementary shading strategies. In contrast, north-facing skylights provide more consistent daylight throughout the year with minimal solar heat gain, making them easier to justify from a compliance perspective. 

East- and west-facing skylights sit somewhere in between. West-facing rooflights can be particularly problematic because late-afternoon solar gains often coincide with peak external temperatures and reduced opportunities for heat purge. 

Glazing and Shading: Solar Control Glass vs Blinds 

Why Glazing Choice Is Critical 

Under Part O, not all overheating measures are treated equally. Passive, permanent solutions are generally preferred over those that rely on occupant behaviour. This is where the discussion of solar control glass vs blinds becomes particularly relevant. 

Solar control glass reduces the amount of solar heat entering the building by lowering the g-value of the glazing. It achieves this while still allowing high levels of visible light, making it ideal for roof glazing where daylight is a primary goal. 

Blinds, on the other hand, are reactive rather than preventative. By the time internal blinds are closed, much of the solar heat has already entered the space. They also rely on occupants using them correctly and consistently, which Part O assessors may not always accept as a reliable mitigation measure. 

Comparative Performance Table 

The table below highlights how different skylight mitigation strategies perform in relation to Part O requirements: 

This comparison clearly shows why solar control glass and careful sizing are often the most robust options for part of overheating mitigation. 

Ventilation as a Complementary Strategy 

The Role of Heat Removal 

Part O does not only focus on limiting heat gains; it also requires that excess heat can be effectively removed. Skylights can contribute positively here when they are openable and integrated into a broader ventilation strategy. 

High-level openings are particularly effective at promoting stack ventilation, allowing warm air to escape naturally. When combined with lower-level windows, this creates a pressure difference that encourages airflow through the dwelling. 

Summer Bypass Ventilation Explained 

In homes with mechanical ventilation systems, summer bypass ventilation is a key feature for Part O compliance. In winter, MVHR systems recover heat from extracted air to improve energy efficiency. In summer, this function must be disabled so that warm indoor air is not used to heat incoming fresh air. 

When skylights are used alongside MVHR systems with summer bypass, designers can demonstrate that the building has both passive and mechanical means of reducing internal temperatures during warm periods. 

Integrating Skylights into a Part O Strategy 

Successful Part O compliance depends on seeing skylights as part of a whole-building approach rather than an isolated component. Orientation, glazing performance, shading, and ventilation must all work together. 

In more complex or highly glazed designs, dynamic thermal modelling can provide additional flexibility. TM59 assessments allow designers to offset higher solar gains from skylights with increased ventilation rates, thermal mass, or improved glazing performance elsewhere in the building. 

Crucially, these decisions should be made early. Retrofitting overheating solutions late in the design process often leads to compromised aesthetics or increased costs. 

Common Causes of Part O Failure 

Many overheating issues arise not from skylights themselves, but from how they are specified and integrated into the wider building design. A frequent problem is the over-reliance on internal blinds, which are often treated as a primary mitigation measure despite only addressing heat after it has already entered the space. Excessive south-facing roof glazing without appropriate solar control measures can also lead to rapid temperature increases, particularly in top-floor rooms. In addition, the absence of effective purge ventilation either through openable rooflights or coordinated window strategies limits a building’s ability to release trapped heat during warmer periods. 

Further complications can occur when mechanical ventilation with heat recovery (MVHR) systems are specified without summer bypass functionality, causing unwanted heat retention at precisely the time cooling is needed. Material selection also plays a significant role in overall thermal performance, and a lack of understanding around rooflight construction and glazing options can result in poor specification choices. Designers and homeowners can benefit from deeper insight into these performance considerations, as outlined in this detailed guide to rooflight materials and performance, which highlights how informed decisions can support comfort and compliance. 

Avoiding these issues requires clear and early coordination between architects, building services engineers, and Part O assessors. By aligning product selection, ventilation strategy, and thermal modelling from the outset, project teams can reduce the risk of non-compliance and deliver homes that are both regulation-ready and comfortable for occupants throughout the year. 

Looking Ahead: Future-Proofing Skylight Design 

As climate change continues to push summer temperatures higher, overheating regulations are likely to become more stringent. Designing skylights that only just meet current Part O thresholds may leave buildings vulnerable to future discomfort or retrofit requirements. 

Future-proof skylight design is likely to rely on better glazing technology, smarter controls, and an increased emphasis on passive cooling measures. Designing beyond minimum compliance today is an investment in long-term performance and occupant wellbeing. 

Conclusion:  

Skylights can enhance daylight, wellbeing, and architectural quality, but only when designed with overheating risk in mind. Effective part o overheating mitigation depends on understanding how skylight placement, orientation, glazing, and ventilation interact. 

By carefully managing skylight orientation south facing, making informed choices in the solar control glass vs blinds debate, and integrating summer bypass ventilation, designers can achieve Part O compliance without sacrificing design intent. 

Ultimately, the most successful projects treat skylights not as a risk to be managed, but as an opportunity to deliver comfortable, resilient, and future-ready homes. 

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