Can Daylight Standards be met Without Overheating?

We are constantly faced with the need to balance solar gain in a dwelling to maximise internal daylight standards, whilst at the same time minimising overheating risk. Large glazed areas and glazing with high transmittance are required to achieve high levels of natural daylight. Reduced glazing areas, effective solar shading and darker glass are most desirable to mitigate against overheating.

The BRE Daylight Standards states that kitchens require an Average Daylight Factor (ADF) of 2.0%, Living Rooms an ADF of 1.5% and Bedrooms and ADF of 1.0%.

CIBSE Overheating Guidelines suggest that:

• For primarily naturally ventilated dwellings, main habitable rooms should be assessed against the adaptive comfort model CIBSE TM52 (2013).
• For dwellings that are NOT primarily naturally ventilated, main habitable rooms should not exceed 26⁰C for more than 3% of occupied hours assessed in line with CIBSE Guide-A (2015).

Achieving both is challenging but is possible through well-considered design. We recommend that:

• South-facing large windows are designed with horizontal shading (overhangs, balconies or ‘brise-soleil’) to protect facades from high sun-angles in the summer but allow for useful solar gain during the winter months when the sun angles are much lower;
• Windows facing east and west are subject to low sun angles all year round. They are better protected by adjustable vertical shading devices and deep reveals;
• External and internal shading devices should be adjustable to allow the occupant the freedom to restrict solar gains or allow for further daylight as they see fit;
• Deep plans layouts should be avoided in order to maximise daylight penetration into a space and natural ventilation without needing large windows;
• New developments should encourage planting of deciduous trees, which provide shading in summer months, but allow penetration of useful solar gains in winter;
• Solar transmittance of glazing remains as high as possible to allow for good daylight levels, as well as look clearer and be less costly. This should be possible if other effective forms of shading are effectively considered.