What is Average Daylight Factor (ADF)?
As mentioned above, all roads to compliance using daylight modelling reach a single word: Daylight Factor. But what is Daylight Factor?
NCC describes the Average Daylight Factor (ADF) as “the ratio of the illumination level within a room provided by daylight to the level of daylight outside the building during overcast conditions” (NCC 2019 Vol 1.0 Schedule 3 Definitions).
Average Daylight Factor (ADF) depends on:
- Area of glazing: the greater the area of the glazings serving a room, the higher the ADF.
- Orientation of glazing: the more angled the window (e.g. roof lights) the more daylight could be let inside that room. This is simply due to a wider view range of skylight to the sky.
- Sill height: higher sill heights, especially in rooms with higher depths guarantee higher levels of daylight.
- Depth of room: deeper rooms (where the distance between windows and distant walls is great), are subjects of lesser ADF.
- Outside obstacles: outdoor buildings and other features are considered as obstacles to the clear sky view from the windows. Window location, area and sill height can be set to minimize the obscured view.
- Reflectivity of fabrics: brighter floors, ceilings and internal walls reflect more light and increase ADF. The typical light reflectance of some materials are:
- Visible Light Transmittance (VLT) of glazing: ADF mainly revolves around the visible spectrum of light. Indoor daylight level contribution of a clear window is more than that of the tinted window. Typical light transmittance values of different windows are shown below:
VLT is the portion of visible light that is transmitted through the window, whereas SHGC is the total solar radiation (including Infrared) that is transmitted through the window. In most cases, SHGC directly correlates with VLT so the higher the SHGC the higher the VLT. However, this is not the case in many modern Low-E technology windows.
What is overcast sky condition?
Evaluation of Average Daylight Factor (ADF) is done assuming an overcast sky condition. The luminance of the standard overcast sky reduces looking from vertical to the horizon (Zenith angle below) but not with horizontal orientation and location. In other words, luminance is consistent across azimuth angles while gradually reducing across the zenith angle. For ease of understanding, an overcast sky condition could be deemed similar to a cloudy sky where it is brighter near the zenith compared to locations near the horizon. Simply put, the orientation of windows doesn’t change ADF for two identical rooms with identical window areas, whereas window angle does.
How Average Daylight Factor (ADF) is measured?
We can measure ADF in three ways:
- Experimental: one can calculate ADF using a typical Lux meter by simply measuring outdoor illuminance and divide it by the average illuminance inside a room. Measurement of indoor illuminance levels must be done by averaging out the measured data in various bright and dark locations. The more the measuring locations, the more accurate the average illuminance of the room.
The experimental method is costly and has no use in the pre-construction stage.
- Theoretical: there are a set of formulas that estimate the ADF inside a room. For instance, NCC Vol 1.0 prescribes using the formula below to calculate ADF from each window:
W = Net area of the light-transmitting area of the window (m2);
A = Total area of the internal wall, floor and ceiling surfaces (m2);
T = Diffuse light transmittance of the window;
θ = Visible sky angle in degrees, measured in a vertical plane normal to and from the centre of the window, and
R = Area-weighted average reflectance of area A.
We use the theoretical method as an estimate, and this method is not 100% accurate. Also, evaluation of ADF using this method is not possible for complex building geometries with many openings and various and non-uniform outdoor obstacles.
- Modelling: the best method of ADF evaluation is modelling using certified software. This method is used widely in the design stage of any type of building to accurately address daylight levels.
At Energy Compliance, we undertake daylight modelling for residential and commercial buildings using accredited commercial tools. We aim for accuracy of assessment and superiority of solutions. Contact us for undertaking daylighting compliance and solution.