Brise Soleil Design for Glare Control
As well as reducing the amount of heat that enters a building, the design of the external shading can also control glare.
Fin Angle and Cut-Off Point
For a higher fin angle (relative to the plane of the glazing) the cut-off point for direct sun moves down, i.e. it becomes lower. In practice for external shading the angle of the fin is set between 30° and 45°. As the sun rises and sets it moves in a low arc above the horizon and hence for most periods of the day the lower angle of the fin will prevent sunlight from passing through the glazing.
Horizontal Versus Vertical Fins
Horizontal fins are often used on South facing walls, where the sun is at a high angle and behaves in a predictable manner. East and West facing walls, however, have the sun at a low angle for a large part of the day, and thus horizontal fins on these walls will offer very little protection against glare.
Fin Spacing and Diffuse Sky Glare
By increasing the space between fins, the amount of material used can be reduced. However, this also means that more diffuse sky light will enter the room. On overcast days, this can also cause glare. Therefore, a tighter spacing of fins (fin-to-gap ratio of about 1:1) is recommended for both direct sunlight and diffuse sky light.
Surface Finish Matters
Any reflective surface such as a brise soleil can bounce light back into a room, potentially creating more problems than it solves, especially secondary glare. The finish of any shading system should therefore be as ‘ matt’ as possible. Ideally in a mid-grey anodised finish.
Check Daylight Factor Targets
Daylight factor also needs to be checked, and here too cross-referencing to the daylighting design guidance with the glare design using the proposed fin geometry will indicate if the proposed interior daylight factors are within acceptable limits.
So, remember, careful detailing at the specification stage is better than having to try and put right poor facade design for glare control after the event.
