As the UK experiences more frequent and intense heatwaves, regular overheating in homes has become a significant concern for building services engineers.
The publication of Approved Document O and increasing adoption of CIBSE’s TM59 assessments have focused attention on design strategies that can reduce solar gains and maintain thermal comfort.
While external shading is widely recognised as the most effective means of controlling solar radiation, internal shading remains one of the most commonly deployed solutions in new and existing homes.
Internal blinds, curtains and shutters can play a valuable role in reducing overheating risk, but their effectiveness depends heavily on the type of shading, occupant behaviour and the overall building design.
Within TM59 and Approved Document O assessments, solar gains through glazing are a critical factor influencing overheating risk. A notable aspect of Approved Document O is that internal blinds are not recognised as an acceptable solar-control measure.
Instead, the document focuses on measures that prevent solar gains from entering the dwelling, such as external shutters, overhangs or appropriately sized glazing, and other permanent measures that won’t be removed or replaced by occupants.
Once solar radiation has passed through the glazing, a significant proportion of the associated heat has already entered the building. Internal blinds may intercept sunlight and reduce glare, but they do not stop solar energy crossing the building envelope. The absorbed heat is subsequently released into the room, increasing indoor temperature and, then, cooling demand.
Part O also reflects concerns about reliability. Internal blinds depend on occupant behaviour, with their effectiveness varying according to choice of blinds and whether residents choose to deploy them appropriately.
Internal shading benefits are achieved as part of a wider overheating strategy
Regulatory compliance, however, requires measures that can be expected to perform consistently throughout the life of the building. External shading systems and fixed design features provide a more predictable outcome than occupant-operated blinds or curtains.
This does not mean internal shading has no value. Dynamic thermal modelling based on TM59 methodology can demonstrate measurable reductions in overheating risk from reflective blinds, curtains and shutters.
However, the regulatory hierarchy established by Part O prioritises passive measures that exclude solar gains before they enter the dwelling. In this sense, the exclusion of internal blinds is less a judgement that they are ineffective and more a recognition that they are less robust, and less reliable, than external solar-control strategies.
Climate projections suggest that overheating risk will continue to increase across much of the UK during the coming decades. Existing housing stock, much of which was not designed with summer comfort in mind, will be particularly vulnerable.
Internal shading represents one of the most practical and cost-effective interventions available for many homes. Installation is generally straightforward, disruption is limited and costs are modest compared with more extensive retrofit measures.
Nevertheless, internal shading should not be viewed as a standalone solution. The greatest benefits are achieved when this forms part of a wider overheating strategy that includes appropriate glazing design, natural ventilation, thermal mass, night-time cooling and, where feasible, external solar control.
As overheating becomes a mainstream design consideration, engineers and regulations will need to evaluate a broader range of mitigation measures. Internal shading cannot match the performance of well-designed external shading systems, but it remains an important tool, particularly for retrofit projects and situations in which external interventions are impractical.
The challenge for designers and policy-makers therefore is to understand both the technical limitations and the behavioural realities associated with internal shading. By integrating shading strategies with occupant needs and whole-building performance objectives, it is possible to deliver homes that remain comfortable, resilient and energy efficient in a warming climate.
In the transition towards climate-resilient housing, internal shading may not be the first line of defence, but it is likely to remain one of the most widely used.
About the author
Dr Anastasia Mylona is technical director at CIBSE