USING EN 17037 TO ACHIEVE GOOD DAYLIGHTING
It’s one thing to understand what daylighting is and why good daylighting is important. At the level above that, it’s possible to design for both visual needs and non-visual needs. But how can you actually measure outcomes? How can you aim for a level of performance aligned with the goals you choose to set? One way is to use the European Standard for daylighting, EN 17037.
KEY LEARNINGS
- Name the four areas of performance covered by EN 17037.
- Understand how the standard defines performance recommendations generally.
- Be aware of the minimum, medium and high levels of performance for all four areas.
AREAS OF PERFORMANCE COVERED BY EN-17037
EN 17037 gives designers flexibility
The standard defines a minimum level of performance for each of the four areas. It then goes on to suggest enhanced standards of performance, which designers are free to pursue if it is desirable and feasible to do so.
Simplified and detailed methods of assessment are also provided for each area, making the standard accessible and usable for a wide audience of professionals.
How can EN 17037 be applied when seasons, climate conditions, sunlight angles and hours of daylight vary from country to country? The answer is that although it provides common methods of calculation for the four performance areas, when the daylight calculations are carried out, climate-based modelling techniques use local, project-specific conditions to ensure that solutions are appropriate.
Below is an overview of EN 17037’s daylight performance indicators which, together, can help designers to determine if they are designing for healthy/appropriate levels of daylight. A more detailed examination of daylight performance parameters can be found in the evaluate section of this guide.
PERFORMANCE INDICATOR 1 DAYLIGHT PROVISION
How much natural light is needed?
EN 17037 recommends that daylight openings “should have appropriate areas to provide sufficient daylight throughout the year”. A space is considered “adequately” daylit if a target illuminance level is provided across a defined area of the ‘reference plane’ within a space, for at least half of the daylight hours or in other terms based on the metric ‘Spatial Daylight Autonomy’.
EN 17037 recognises that different professionals and project types will have ranging capabilities (cost, time, software) and different needs or benefits for calculating daylight at different levels of accuracies. Accordingly, it offers two different calculation methods using validated software, both of which still account for local climatic conditions (availability of daylight at the site location), the properties and geometry of the space/s and the characteristics of the daylight openings.
Method 1, is most simple and calls for a target Daylight Factor (DT) to be met. Daylight Factor defines the daylight availability in terms of the relationship between the light available at different positions inside with that available outside. The target Daylight Factors for each performance level have been set to achieve ‘comparable’ results to Method 2’s Spatial Daylight Autonomy targets set out in Table A.1 (but through a simplified calculation method)1.
EN 17037 provides adjusted targets for Daylight Factor according to median external diffuse illuminance across different geographical latitudes (in simple terms, it accounts for the fact that a cloudy sky in Rome will be brighter than a cloudy sky in Oslo). Table A.3 and A.4 give the target daylight factors for all the capital cities within the EU – an example extract of this is shown below.
Table A.3 – Values of D (daylight factor) for daylight openings to exceed an illuminance level of 100, 300, 500 or 750 lx for a fraction of daylight hours Ftime,% =50% for 33 capitals of CEN national members2
Method 2 uses the metric Spatial Daylight Autonomy, which more precisely addresses the changing light throughout the day and seasons by using sub-hourly local climate data rather than median external illuminances on a cloudy day (as Method 1 does). It is expressed as a percentage of time (% of daylight hours), when the target illuminance for a point in that space is met or exceeded by daylight alone. A benefit of this method is that it also allows dynamic shading devices to be accounted for within the modelling and calculation, further increasing accuracy of results.
Table A.1 – Recommendations of daylight provision by daylight openings in vertical and inclined surface3
Though EN 17037 gives target illuminances, it does not distinguish between what is ‘adequate’ for different types of space or different tasks that an occupant might carry out. Practically, some spaces or tasks can be adequately designed for based on daylight measurements in the room generally, while some tasks require daylight measurement in the specific area where the task will be carried out. Because of this, to deliver highly optimised daylighting solutions it may be necessary to consult supplementary sources of information and recommendations, depending on the nature of the project.
PERFORMANCE INDICATOR 2 ASSESSMENT OF THE VIEW OUT OF WINDOWS
Why do we need external views? EN 17037 acknowledges that visual contact with the outside, and the natural world specifically, is an important psychological aspect linked to daylighting.
Providing daylight alone is not enough, as it does not satisfy the user’s natural need for external views. Have you ever spent a long time in a room without windows? It can be an unsettling experience. Windows supply information about orientation, give experience of weather changes, and generally allow us to follow the passage of time over the day.
Studies show that, over the long term, windowless rooms can cause harm to our wellbeing4.
Having a good view: why are ‘layers’ important? To assess whether a view is ‘good’ is of course very difficult to do objectively. EN 17037 draw upon existing research that shows to counteract monotony and feelings of being trapped or closed in, views should ideally include layers of sky, city or landscape, and a view of the ground5. This forms the foundation of its assessment of views.
To provide these layers, the size and position of window solutions needs to be designed carefully, taking into account the eye level of the building occupants.
EN 17037 has three levels of recommendation for providing a view out, which apply to vertical, horizontal and inclined openings. The content of the view (the number of ‘layers’) is an important contributor to the quality of a view, along with the sight angle and the distance of the view. These are all elements that can be assessed objectively through established calculation methods (detailed within EN 17037).
Table A.5 – Assessment of the view outwards from a given position6
Lisa Heschong, 2019
“Daylight and window views have often been considered as interchangeable topics. However, they are distinct design choices, and they also have distinct impacts on the occupants of buildings.”
Window views
Nature views
Window views
Nature views
PERFORMANCE INDICATOR 3 ACCESS TO SUNLIGHT
What is the right level of exposure?
Calculating access to sunlight is both an aesthetic and a health issue. Access to sunlight is generally desirable, and EN 17037 notes that dissatisfaction is likely to occur if direct sunlight is permanently excluded.
Particularly in hot climates, however, too much exposure can result in thermal and visual discomfort, with the former also negatively impacting on a project’s energy use goals, it is crucial that the different needs are balanced through smart execution of design solutions.
Access or exposure is defined by a number of hours for which a room receives sunlight on a given reference day. EN 17037 offers the following recommendations for minimum, medium and high levels of sunlight exposure.
Table: Recommendation for exposure to sunlight7
Architects: HK Associates.
PERFORMANCE INDICATOR 4 PREVENTION OF GLARE
How can we avoid visual discomfort?
If the lighting in a space is unsuitable or inadequate, making it difficult to see properly, it negatively influences our health and wellbeing, and the intended uses of a given space. Poor brightness and inadequate contrast, high luminance differences, and flickering light(s) can all cause unnecessary eye strain, eye irritation, tiredness, headaches, and can lead to accidents and/or injuries.
EN17 037 advises that a glare assessment should be made for spaces where reading, writing and the use of display devices (or an equivalent activity) will take place, and where a person has no choice over where they position themselves. It further recommends the use of shading devices within any space that has daylight openings to reduce the risk of glare and avoid a direct view of the sun or a reflection of it.
Daylight Glare Probability (DGP) is used to assess protection from glare in relevant spaces and threshold values are provided for the different levels of performance. Table A.7 defines the threshold values which should not be exceeded for more than 5% of the occupied time (8h -18h Monday – Friday) throughout the year.
Although the equation for calculating daylight glare probability is quite complex, EN 17037 offers 2 different verification methods for protection from glare. This includes a simplified method whereby ensuring adequate shading devices are specified in a design (detailed in guidance tables that take into account “sunshine zones” and orientation) the defined threshold value is assumed to have been met eliminating the need for actual calculation8.
Table A.7 – Proposed different levels of threshold DGPe< 5% for glare protection9
Architects: Architecture-Studio & Jean Nouvel.
What other metrics can be used to measure daylighting? EN 17037 is one possible route to evaluating whether a project has achieved ‘good’ daylighting, and other assessment methods are available. Whichever approach is selected, a variety of metrics exist with which you can measure and assess daylight.
1 CEN European Daylight Standard (EN 17037:2018), Annex A.2. 2 CEN European Daylight Standard (EN 17037:2018), Annex A.2. 3 CEN European Daylight Standard (EN 17037:2018), Annex A.2. 4 Robbins, C. L. (1986) Daylighting Design and Analysis, New York: Van Nostrand Reinhold Company. 5 Boyce, P., Hunter, C. and Howlett, O. (2003) The Benefits of Daylight through Windows, Lighting Research Center, Rensselaer Polytechnic Institute. 6 CEN European Daylight Standard (EN 17037:2018), Annex A.3. 7 CEN European Daylight Standard (EN 17037:2018), Annex A.4. 8 CEN European Daylight Standard (EN 17037:2018), Annex E. 9 CEN European Daylight Standard (EN 17037:2018), Annex A.5.