Overview
A daylight performance assessment considers the adequacy of natural light that would be provided within a new development. It is a predictive assessment which endeavours to quantify the amount of natural light that will be available within a space when the development is complete.
For low density developments, composed of traditional housing typologies (detached, semi-detached, terraced, etc.) a daylight performance assessment may not be required as a good understanding already exists about the levels of daylight which will likely be available within these conventional spaces.
Where daylight performance assessment begins to become important is in medium to high density situations where the pursuit of compact development begins to limit the access that interior spaces have to light from the sky. In these situations, a daylight performance assessment can provide the information that is needed for planning authorities to determine if acceptable levels of residential amenity would be provided.
What is considered in a daylight performance report?
Daylight performance reports can sometimes be carried out on commercial developments however they are more frequently commissioned for residential projects. For residential projects, planning authorities are typically interested in three separate aspects of daylight.
Planning authorities will wish to see information which shows that:
1) The habitable accommodation within a proposed development will be provided with reasonable levels of daylight amenity.
2) The individual units with a development will have access to reasonable levels of direct sunlight.
3) The outdoor recreation spaces will benefit from reasonable levels of sunlight amenity.
It is for this reason that a daylight performance assessment is usually organised into three component studies.
Against what standard is daylight performance assessed?
In Ireland and the UK two separate standards are relevant to the assessment of daylight performance.
When assessing the levels of daylight and sunlight that will be provided within new accommodation, the British Standard BS EN 17037 ‘Daylight in Buildings’ is typically referred to.
When assessing the levels of sunlight amenity that will be provided within proposed outdoor recreation spaces, a guide published by the Building Research Establishment (BRE) is typically referred to. The current version of this guide is called ‘Site layout planning for daylight and sunlight – A guide to good practice’ 3rd edition. The BRE Guide is often referred to as simply BR209.
What is measured in a daylight performance assessment?
What is measured in a daylight performance assessment depends on the particular study which is being considered.
For Internal Accommodation
When assessing the levels of daylight amenity that will be provided within proposed accommodation BS EN 17037 makes provision for two different metrics to be used. When relatively quick and simple testing is to be carried out a metric called the daylight factor can be used. When more detailed testing needs to be carried out a metric called Spatial Daylight Autonomy (sDA) is employed.
For Outdoor Spaces
A metric called sunlight exposure is used to assess the levels of sunlight amenity that will be provided to both residential units and outdoor recreation spaces.
To learn more about these metrics please refer to the following guide.
A concise guide to the metric used to assess daylight within planning.
How do you determine if the daylight amenity within proposed accommodation conforms with advisory minimums?
Depending on what metric you have used to assess daylight amenity in interior spaces (DF or sDA) the following performance criteria apply.
When using Spatial Daylight Autonomy to assess interior daylight levels
When using sDA (equivalent to Method 2 within BS EN 17037) to measure daylight levels, the minimum performance criteria recommended in BS EN 17037 (found in the UK’s National Annex) are as follows:
• For bedrooms, the minimum performance target is for an illuminance of 100lux to be provided over 50% of the rooms assessable area during 50% of daylight hours.
• For living rooms, the minimum performance target is for an illuminance of 150lux to be provided over 50% of the rooms assessable area during 50% of daylight hours.
• For kitchens, the minimum performance target is for an illuminance of 200lux to be provided over 50% of the rooms assessable area during 50% of daylight hours.
• Where a room include two separate uses BS EN 17037 recommends that the higher o the two targets should apply. Accordingly for open plan living rooms which include a kitchen the minimum performance target is for an illuminance of 100lux to be provided over 50% of the rooms assessable area during 50% of daylight hours.
When using Daylight Factor to assess interior daylight levels
When using DF (equivalent to Method 1within BS EN 17037) to measure daylight levels, the minimum performance criteria recommended in BS EN 17037 (found in the UK’s National Annex) are determined with reference to geographic location.
For London the following performance criteria apply:
• For bedrooms, the minimum performance target is for a daylight factor of 0.6% to be achieved across at least half of the assessable area within a room.
• For living rooms, the minimum performance target is for a daylight factor of 0.8% to be achieved across at least half of the assessable area within a room.
• For kitchens, the minimum performance target is for a daylight factor of 1.1% to be achieved across at least half of the assessable area within a room.
• Where a room include two separate uses BS EN 17037 recommends that the higher o the two targets should apply. Accordingly for open plan living rooms which include a kitchen the minimum performance target is for a daylight factor of 1.1% to be achieved across at least half of the assessable area.
For Dublin the following performance criteria apply:
• For bedrooms, the minimum performance target is for a daylight factor of 0.7% to be achieved across at least half of the assessable area within a room.
• For living rooms, the minimum performance target is for a daylight factor of 1.0% to be achieved across at least half of the assessable area within a room.
• For kitchens, the minimum performance target is for a daylight factor of 1.3% to be achieved across at least half of the assessable area within a room.
• Where a room include two separate uses BS EN 17037 recommends that the higher o the two targets should apply. Accordingly for open plan living rooms which include a kitchen the minimum performance target is for a daylight factor of 1.3% to be achieved across at least half of the assessable area.
Which method should be used to assess daylight amenity within proposed accommodation?
As detailed above BS EN 17037 provides two different methods for assessing daylight adequacy within proposed accommodation. The key characteristics of these methods are outlined as follows:
Method 1 - Daylight Factor Approach
• Daylight factor is a term used to describe the ratio of natural light shining at an unobstructed point outdoors to a point inside a room. A low daylight factors indicates that very little of the light available externally is capable of reaching a specific point inside a room.
• Daylight factor is calculated for a static scenario (one snapshot in time) and this scenario is defined by the presence of an overcast sky.
• While this form of testing is computationally efficient, it tends to underestimate the amount of natural light that will be provided in reality because it does not account for the additional light that would be provided by the sun.
Method 2 - Spatial Daylight Autonomy Approach
• Spatial Daylight Autonomy is a term used to describe the proportion of a room which is capable of achieving a particular illuminance for a set fraction of daylight hours, e.g. 50% of a room capable of achieving an illuminance of 100lux during 50% of daylight hours.
• Spatial Daylight Autonomy can only be determined by repeating the calculation at regular intervals (usually hourly) for all daylight periods in a typical year. To do this realistic climate date for the location in question is used as a key input to the assessment routine.
• Spatial daylight autonomy testing is computationally intensive however because it is capable of capturing the benefits of both skylight and sunlight the approach will generally render more favourable results.
As a general rule GV8 recommends that Method 2 testing be adopted when assessing the daylight performance within proposed accommodation.
How do you determine if the sunlight access available to proposed units conforms with advisory minimums?
When internal sunlight amenity is being considered, BS EN 17037 recommends that at least one room in a dwelling should have access to at least 1.5 hours of direct sunlight over the course of a specific test day (typically set as the 21st of March). A clear cloudless sky is assumed in testing.
How do you determine if the sunlight access available to proposed outdoor recreation spaces conforms with advisory minimums?
When external sunlight is being considered, the BRE Guide recommends that at least 50% of an external amenity area should be capable of receiving at least two hours of direct sunlight on the spring equinox (21st of March). A clear cloudless sky is assumed in testing.
How is testing carried out?
Daylight performance testing is typically carried out as a desktop study using computational tools. The key steps in a daylight performance assessment include:
• Collecting survey information for the neighbouring context
• Building a 3D representation of the neighbouring context
• Building a 3D representation of the proposed development including its internal layouts, window configurations and external recreation spaces.
• Assigning appropriate material properties to all surfaces in a scene.
• Labelling all assessment rooms, windows and recreation spaces so that their associated results can be easily identified within the final report.
• Selecting appropriate simulation parameters. (see detailed guide)
• Running daylight and sunlight testing using validated daylight analysis software.
• Preparation of a final daylight report.
In some cases, the outputs from this assessment will serve as a daylight report which will be submitted with a planning application. In other cases, where the design is at an earlier stage of development, the results will feed back to the design team informing future design decision.
What happens if it is not possible to demonstrate full conformity with daylight minimums in all cases?
In Ireland and the UK planning authorities tend not to regard daylight standards as mandatory regulation which have to achieved, and instead view them as advisory targets which should be pursued to the greatest extent possible.
Recognising that daylight performance is only one of many factors which influence the suitability of a development for a specific site, it is important that local authorities have the discretion to accept points of non-conformity in instances were wider planning factors countervail.
It is GV8’s experience that planning authorities tend to find it easier to accommodate points of departure in instances where:
• The development addresses an acute need in a given neighbourhood (e.g. affordable housing).
• The development is located on a strategic site close to a high frequency transport node.
• The development contributes positively to the neighbouring environment and community (e.g. expanding the public realm, activating street frontage, providing community facilities, etc.)
• Restricted daylight amenity is compensated with alternative type of amenities (e.g. larger units, higher specifications, health and fitness facilities, communal social spaces, etc.)
What information does a daylight consultant need to begin testing?
In order to run a Daylight Performance Assessment a daylight consultant will need to be provided with the following information:
• Either a fully developed 3D model of the proposed development (including all internal partitioning) or a full design pack of 2D drawings.
• Survey information for the neighbouring context if available; if survey information is not available it the 3D model of neighbouring context can sometimes be developed with reference to publicly available height and footprint data.
• Sufficient detail to be able to accurately represent the glazing system which will be used or each room. Key details geometric details to include width, head height, cill height, framing, dividers and reveal depth.
• Material specifications for the primary surfaces present on the exterior of the building, including balcony decks and soffits; this will effect the amount of light that is reflected into the building.
• Material specification for any shading or privacy devices that will be fitted to the exterior of the building.
• The diffuse transmittance of that glazing that will be used in all window systems.
• Details of trees that will be located close to the proposed development. Key details to include the height and species of the trees.
Additional Considerations?
Before a Daylight Performance Assessment can commence a number of important decisions need to be made regarding the scope and depth of the exercise. Some of the key points that need to be considered are detailed as follows:
Number of points assessed
In Dublin the current development plan calls for all rooms which have an expectation of daylight to be assessed however in other jurisdictions it may be permissible to assess only a sample of rooms. In circumstances where a floor plan repeats consistently from one level to the next it stands to reason that the ability to demonstrate conformity with guidelines at a sample of lower level will by extrapolation provide confidence regarding the daylight performance at upper levels.
The inclusion or exclusion of trees
The Guidance provided in BS EN 17037 and BR209 is not very clear on whether trees need to be included within a Daylight Performance Assessment. While guidance provided in BR209 suggests that the overshading caused by existing trees does need to be considered ambiguity currently exists regarding the need to include the new trees that are to be planted as part of the development. In circumstances where it is decided, in the interest of completeness, to include trees daylight performance testing will sometimes need to be carried out twice, once with branches in leaf to capture summertime overshadowing and once with branches bare to capture wintertime overshadowing.
Future development
BR209 recommends that the overshadowing caused by buildings which will occupy neighbouring sites at some point in the future should be included within performance assessments. In the absence of clear guidance regarding what constitutes a planned future building it falls to daylight consultants, in consultation with the design team, to use good judgement to represent these notional or speculative future developments within the computational model.
Overheating
The pursuit of high levels of view and daylight amenity can sometimes lead to inappropriate levels of glazing being specified. For single aspects apartments located in urban areas this can lead to an elevated risk of overheating. Recognising that an overheating apartment is arguably less desirable than an underlit apartment it makes sense in circumstances to expand the scope of assessment to include overheating analysis. In line with guidance provided in the National Annex to BS EN 17037 a dwelling where a daylight illuminance of 500 lx is exceeded on 50 % of the grid points for more than half of the daylight hours should be checked for overheating.
Dual testing
While daylight performance testing in the UK has been streamlined by the inclusion of a National Annex to their implementation of the European daylight standard EN 17037, it is regrettably the case that policy in Ireland refers to both the UK’s implementation and Irelands implementation I.S EN 17037. As the performance targets present in both standards I.S EN 17037 and BS EN 17037 are different consideration needs to be given to whether or not to run dual testing within a daylight performance report.
