Natural daylight is something very important for human wellbeing and is often overlooked when designing buildings. As well as being good for us, natural daylighting also has a positive sustainable impact on buildings, requiring less artificial lighting use through the day.
When designing there are three main elements to consider regarding daylight. These are luminance, Illuminance and daylight factor.
Luminance is the measure of the amount of light output or reflected off a surface and is expressed in cd/m2. It is the measure of light used to evaluate visual comfort and glare in the interior.
The human eye and the camera respond only to luminance, not to illuminance (light incident on a surface). As a result the luminance setting provides photo realistic images as output.
The distribution of luminance is a measure of how lighting varies from point to point across a plane or surface. For good visibility some degree of uniformity across the task plane is desirable. Too high or too low contrasts may cause discomfort and tiredness. For offices ANSI/IESNA RP-1 is setting guidelines for maximum contrast ratios on surfaces.
Illuminance is a measure of the amount of light received on a surface. It is an absolute value typically measured in lux (lm/m2) that varies with the time of day and the weather conditions. Due to the variation of intensity it should be evaluated together with the Daylight factor.
Currently, illuminance is the measure of light used by most performance indicators to determine daylight availability in the interior. Thus, guidelines set recommended setpoints of minimum lux levels for occupants to perform certain tasks properly. When the recommended lux levels cannot be achieved by daylight alone, artificial lighting has to compensate for the lack of daylight.
The Daylight factor is a measure of internal illuminance relative to external unobstructed illuminance under standard CIE overcast sky conditions and is expressed as a percentage. It is a common measure, which permits determination of the availability of daylight in a room.
The Daylight factor is usually measured at a working plane where visual tasks are likely to take place, often 0.7- 0.8 m above the floor, corresponding to an office- or school desk. The higher DF the more daylight is available in the room. Daylight Factor <2% – Room looks gloomy, electric lighting needed most of the day. Daylight Factor 2-5% – Room has daylight appearance. Artificial lighting required on occasion. Daylight Factor >5% – Room has strong daylight appearance. Electric lighting rarely needed, but thermal issues may arise.
When designing the Infinite House types, we thoroughly tested and refined the daylight levels to the apartments. First, we tested existing house types and used these as a baseline to improve upon for our designs. Some of the key areas we focused on was to bring natural daylight into the hallways and to raise daylight levels by increasing window sizes and floor to ceiling heights.
Example section highlighting illuminance through our Infinite House prototypes.
We have focused on bringing as much natural daylight into the circulation spaces as possible. This included providing a skylight as well as a full height window at first floor. The resulting triple height space feels much more generous than is typical in standard housing and has very good natural lighting levels.
Example false colour section highlighting illuminance through our Infinite House prototype.
The daylight factor is also a very important factor in designing spaces. Aiming for a daylight factor of around 5% ensures that a space requires almost no artificial lighting throughout the day. The spaces feel much brighter and comfort/wellbeing will be improved. Bedrooms and bathrooms are often acceptable to have slightly lower daylight factors. We try to achieve results of 3% and above for these rooms to ensure that artificial lighting is only required on rare occasions.
Daylight Factor plan of a floor from the Loft House.
When running daylight studies it is important to model the real world context as accurately as possible. This allows you to factor in anomalies which may alter the results. For example, close neighbouring properties can significantly reduce the daylight levels to a room which would otherwise perform well.
The above image shows a daylight factor plan of a house showing the effect a neighbouring property can have on daylight factor within a room. The window to the bathroom on the left has a daylight factor of 1.3%. The bathroom on the right side of the plan has an identical window and similar room size, but a daylight factor of 0.1%. This is because the window is up against a neighbouring property, which significantly reduces the light levels.
When checking the daylight factor and impacts of neighbouring buildings, it is also important to check the no-sky line. The no-sky line is a divider between the part of the working plane from which a part of the sky can be seen directly and the part from which it can’t. It is given as a percentage indicating the area from which the sky can not be seen, compared to the total room area. It is important to be able to see the sky from a window and there are limiting percentages to how much of the floor area (at working plane level) should have a view of the sky.
The no-sky line for a floor on the Loft House
The example above shows that the Loft House achieves almost 100% of the working plane with a complete view of the sky. Just one room is slightly under with 93% visible. This is well within the targets and we have also orientated the room so that any tasks will be undertaken close to the window and not within the 7% of floor area.
Working with measurable daylight is a fundamental part of our practice in the studio and is considered from the outset. We review and test models of daylight to refine our designs and to ensure they are not only sustainable in terms of energy use, but also that we provide spaces that are full of natural daylight.