PART O · Overheating
How can large windows avoid overheating
Large windows can provide daylight and views, but they also influence summer heat gains. In a new residential building, the Part O assessment needs to account for the design and the way excess heat can be removed. Window size alone does not tell the whole story.
Why orientation matters
Different elevations receive sunlight at different times and angles. A repeated window design may therefore behave differently around the same building.
Ask the designer to assess the actual orientation rather than describing the glazing generally as energy efficient. Winter heat-loss performance and summer solar behaviour are separate characteristics.
Can shading help
Appropriate shading may reduce unwanted gains. Its effectiveness depends on position, geometry and the approach used for the assessment.
Include intended shading in the design and procurement documents. If it is omitted during construction, the assessment may no longer describe the completed building.
Will bigger opening windows solve it
Only if the opening arrangement is usable and fits the chosen compliance route. Noise, security and protection against falls may limit the assumptions a designer can make.
For example, a window that occupants cannot reasonably leave open at night should not be treated as an unrestricted night-time cooling route.
What should the calculation use
Use the actual glazing specification, opening arrangement and shading proposal. Review changes to window sizes, glass or room layouts before manufacture.
Where the simplified approach is unsuitable, a competent assessment using the appropriate modelling route may be needed. A supplier's product leaflet is not a whole-building overheating assessment.
What should the owner receive
Keep the assessment and clear instructions for the intended operation of windows, shading and ventilation. Good results depend on a realistic design that occupants can understand and use.
Coordinate those choices with fire safety, accessibility and energy requirements before settling the final specification.