Richard Williams of Velux explains why windows are fundamental to the balancing act architects need to perform in order to design the right building fabric.
Windows and doors are not simply part of a building’s envelope; they shape how spaces perform, contribute to fabric efficiency, and help deliver low-carbon, healthy environments for occupants. Designing openings within the building fabric has always required architects to balance aesthetics, structure, and performance. Today, that balance has become increasingly complex as regulation, sustainability targets, and occupant expectations converge. Doors and windows play a critical role in defining thermal performance, daylighting, ventilation, and wellbeing, making them integral rather than peripheral to design intent.
The conversation around doors and windows is no longer limited to material selection or frame detail. These components are now part of a broader system in which energy efficiency and carbon reduction are paramount. With Building Regulations tightening, the performance of glazing and frames contributes directly to whole building targets for heat loss, overheating mitigation, and operational energy use. In practice, this means architects must treat the design of openings not in isolation, but as interdependent with insulation strategies, airtightness measures, and services integration.
Access to natural light remains one of the most powerful benefits that well considered glazing can deliver. Research consistently highlights the role of daylight in improving occupant health, productivity, and mood. For designers, window and rooflight positioning determines how light is distributed within a space, whether the aim is to minimise artificial lighting demand or to create a specific spatial character. The proportions and placement of openings can transform small areas into generous, uplifting environments while maintaining energy performance.
Studies show that roof windows can deliver up to two times more daylight than facade windows of a comparable size, because of the direct angle of overhead light. This is especially relevant for deep plan or single-storey spaces where wall glazing alone may not achieve sufficient penetration. Flat roof windows extend this opportunity further, and can offer up to three times more daylight as they introduce it into areas where conventional windows cannot be used at all. The proportions and placement of these openings can transform small areas into generous, uplifting environments while still supporting energy performance goals.
One example of this approach is the use of roof windows, which are frequently specified to introduce daylight deep into plan forms or retrofit projects. By opening up previously underlit spaces, such solutions can significantly improve spatial quality while supporting low-energy lighting strategies.
Ventilation & indoor comfort
In parallel, windows and doors are vital in supporting natural ventilation strategies. Cross ventilation, stack effect, and purge ventilation depend on the thoughtful arrangement of openings within a floor plan. Mechanical systems continue to play a role, but low-energy buildings increasingly look to natural or hybrid strategies to reduce reliance on active cooling. Operable windows, rooflights, and glazed doors provide adaptable solutions for seasonal shifts, enabling designers to respond to both occupant comfort and carbon targets.
Roof windows
Although roof windows may sometimes be perceived as ‘standard’ items, in practice, specification decisions can have significant implications for both performance and long-term maintenance. Architects need to weigh options carefully to ensure they suit the intended use and context of each space.
Operation type also has an impact on usability. Centre-pivot windows, for example, are practical where accessibility is straightforward, but top hung or dual function models may be preferable where clear outward views or higher installation heights are priorities. Where manual reach is limited, electric or solar-powered roof windows support options for remote control or integration into home automation.
The number and arrangement of roof windows are another design variable. A single unit can provide valuable daylight and ventilation, but multiple windows in combination can transform interior spaces.
Responding to the unintended consequences of Part O
Part O’s simplified calculation method has led to unintended consequences by limiting maximum glazing areas and reducing daylight opportunities. For architects, this creates a tension between compliance and design quality. The key is to prevent overheating without compromising daylight or spatial character.
To resolve this, approaches such as external shading systems stack-effect ventilation, and passive design that balances glazing ratios, shading, and ventilation are emerging as practical ways to comply while keeping interiors bright and welcoming.
Richard Williams is senior architectural development manager at Velux