Daylight Modelling for ESD
There are several measurements to address daylight levels in a building: Illuminance, Luminance, Daylight autonomy, Useful Daylight illuminance (UDI) and Average Daylight Factor (ADF). Most of the regulations focus on the Average Daylight Factor as a basis for daylighting assessments and daylight modelling.
Daylighting requirements for buildings
National Construction Code (NCC) has set some mandatory Performance Requirements for some building classes, pushing them to achieve a high quality of daylight levels. Also, governing councils are mandated as well to address daylighting in their Environmental Sustainability Development (ESD) requirements.
NCC Daylighting provisions
National Construction Code (NCC) 2019 Vol 1.0, Part F4, FP4.1 requires:
“Sufficient openings must be provided and distributed in a building, appropriate to the function or use of that part of the building so that natural light, when available, provides an average daylight factor of not less than 2%.”
We verify compliance with the above for these Building Classes:
- Habitable rooms of Class 1 buildings (NCC 2019 Vol 2.0, Part 2.4, P2.4.4)
- Habitable rooms of Class 2 buildings and Class 4 part of buildings; and
- Bedrooms and dormitories of Class 3 buildings; and
- Rooms used for sleeping in Class 9a and 9c buildings; and
- General-purpose classrooms in primary and secondary school and playrooms or the like for the use of children in an early childhood centre in Class 9b buildings
Local Councils Daylighting provisions
Local councils also mandate developers to address best practice for daylighting in their ESD report. The government has formatted the BESS tool to award points to applications that satisfy the minimum daylighting level in their proposed development.
Living areas – Points are awarded where at least 80% of the total number of living areas achieve an Average Daylight Factor (ADF) daylight factor greater than 1% to 90% of the floor area of each living area, including kitchens. Additional points are awarded where 100% of dwellings comply. BESS in-built tool or daylight modelling software are used to show this compliance.
Bedrooms – Points are awarded where at least 80% of the total number of bedrooms achieve a daylight factor greater than 0.5% to 90% of the floor area in each room. Additional points are awarded where 100% of dwellings comply. Points are also awarded where at least 90% of bedrooms have an external window.
Winter daylight – Points are awarded where at least 70% of dwellings receive at least 3 hours of direct sunlight in all living areas between 9 am and 3 pm in mid-winter. This compliance can only be demonstrated with daylight modelling using accredited software.
Points are awarded where it is demonstrated by daylight modelling software that over 30% of the nominated area achieves a daylight factor of at least 2%. Additional points are awarded where it is shown that over 60% or over 90% of the floor area achieves 2% daylight factor. This compliance can only be demonstrated with daylight modelling using accredited software.
5 factors affecting Average Daylight Factor
NCC describes 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 the ease of understanding, 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.
What is the importance of daylighting in building designs?
We have concealed ourselves in our comfortable homes, separated from natural aspects of the world. Perhaps a little flower pot, or perhaps a little daylight, would give us a sense of time and space and connection to our surrounding environment.
Daylight is an important parameter for the wellbeing of the occupants in any type of building. Daylighting is associated with improved mood, enhanced morale, less fatigue and reduced eye strain (Robbins, 1986).
Introduction of fair amounts of daylight in buildings has some significant benefits:
- It promotes the wellbeing and maintains our health as it sets our body clock to the level of awareness we need throughout the day, and a sense of connection to our natural world.
- Well-day lit spaces make people more productive and more alert. Various studies show that the work performance is elevated where a fair amount of daylight is introduced to the office buildings.
- Introduction of natural daylight through glazing is a game-changer in the overall cost of dwellings and commercial buildings. Preference of a clear and deep view generates financial benefits for the builders.
- With acceptable levels of daylighting, comes the decrease in using artificial lighting and consequently, cooling demand to offset the heat generated by lighting fixtures.
There are also some restrictions to the levels of daylighting in the buildings. A good building design must address acceptable levels of daylight into the rooms, while preventing from the defects of too much daylighting:
- Minimum daylighting maintains visual benefits. However, too much daylight may cause glare and reduce the performance efficiency of workers.
- Too much daylighting in an office environment adds to the reflectance from surfaces and screens. To work properly, workers prefer the use of artificial ambient and task lighting which is costly and adds to the cooling demand.
- Too much daylighting is directly related to the ratio of glazing area to the floor area. Glazing is one of the major costs in construction development, therefore, non-optimized daylighting through glazings adds the overall construction cost.