With modest glazing designed for daylight rather than maximum solar gain, it is often possible to achieve sufficient summer shading of south glazing via the existing roof overhang or window reveals.

To fully shade south glazing at summer solstice we need a 60° shading angle in the UK. Low sun in autumn and spring may still cause some overheating but we can open the window for cool air. On hot days in summer the outside air will be warmer than indoors so throwing open the large glass doors doesn’t really help.

For hotter climates or where the glazing has been overdone, it is necessary to include external moveable shading. However, blinds and shutters are expensive, require maintenance and obscure the daylight and view that the window was installed to provide.

Internal blinds have much less impact on overheating as the heat is already in the room but have a slight benefit in that they can control glare and provide privacy.

Either inward or outward opening windows are possible but inward opening windows allow external shutters or blinds and fly screens. Note that is it very difficult to effectively shade rooflights, so large skylights can pose an overheating risk. 

Good practice guidance

• Maximum south glass area about 25% of internal wall area

• Total glazing about 15-20% of TFA

• Limit skylights to about 10% of floor area for that room

• Glazing below about 850mm from the floor does little to improve daylight

• Window heads close to ceiling maximise daylight penetration (but leave room for curtain rails, binds, and ventilation opening for tilting windows)

• 2% daylight factor is a good target; higher than 4% might indicate overheating risk for that room

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