Daylighting; An Art or Science?

People enjoy well daylit buildings. People are inspired by beautiful architecture - by spaces that please the eye and lift the spirit. Not only that, but surveys and studies repeatedly show that people are more satisfied and more productive when they occupy spaces that are naturally lit. And despite the bad press that the UK weather regularly receives, there is abundant natural light available thoughout the year. So how do we ensure that the buildings we design achieve these laudable benefits? How do we get this abundant natural light from outside our buildings into them? The joy of a well daylit space is part art, part science, and is not an easy challenge to rise to.

Our challenge is made still more difficult by a design standard that can result in the exact opposite of our intentions: the daylight factor. This simple ratio of outside to inside light levels is a great place to start as a quick rule-of-thumb, but quickly gets out of its depth in real building design in the real world, with real weather. The daylight factor uses, in effect, a dull overcast sky - most typically seen in winter in the UK, and a highly conservative and static measure of light. As a result, buildings designed to this standard tend to be overglazed, and worse, require users to operate blinds whenever the sun comes out, to avoid discomfort glare. Designers are left using intuition and experience to shape and design building form and facade, without any way of knowing in advance whether they have struck the correct balance and achieved the well daylit space they desire. Considering the capital investment that new buildings represent, and the ongoing investment that their occupants present, it is suprising that daylight design is so often based on guesswork. When contrasted on the information available to the designer in the area of energy, or ventilation and overheating, through advanced modelling, the lack of information about daylight is all the more surprising.

Climate Based Daylight Modelling (CBDM) is increasingly accepted as the answer to these challenges. Like the now commonplace dynamic thermal modelling of buildings, CBDM uses real climate data local to the intended site of the building to simulate light levels throughout the year under real sky conditions. CBDM includes an eventual user operating blinds when required, and crucially includes bright and sunny skies as well as overcast skies. As a result, this modelling method allows building designers to better understand their design options, and to create built environments that have the joy of good natural light.

Results from CBDM can be surprising: a building with external solar shading can actually have more usable light than one without, as the shading can reduce glare and redirect sunlight usefully. Usable daylight levels can actually be higher towards the rear of a room, if levels of glazing at the facade are high and result in excessive illumination at the building perimeter. And daylight-specific design features such as light shelves, clerestorey windows and deep window reveals can be justified and their benefits clearly demonstrated at the design stage.

Daylight is a joy-giving natural resource that pleases people and makes them more satisfied and productive. Lets start designing buildings for best real-life daylight.

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