Daylight Simulation

What is a daylight simulation?

A daylight simulation is a computer model that predicts how much natural light reaches the rooms of a building design before it is built. It takes the geometry of the building, its windows and surfaces, the site and the surrounding buildings, and a sky condition. It then calculates light levels at points or on a grid across each room. The output is a set of values, maps and images that a designer can compare with a target.

In Slovak the subject appears under several names. Decree 259/2008 uses denné osvetlenie for daylight and preslnenie for the favourable effect of sunlight in a building. The computer work itself is called simulácia denného osvetlenia.

How does a daylight simulation work?

Most tools follow the same steps. The designer builds a three-dimensional model with the room surfaces, the glazing and frames, and any fixed shading. The model sits on the site with its true orientation. The sky is either a standard overcast sky or a weather file for a full year. The software then traces how light enters through the openings and bounces between surfaces.

The result depends on the inputs. Wall and floor colours, the light transmission of the glass, the frame share of the window and the position of blinds all change the numbers. A model with wrong inputs gives a precise but wrong answer, so the report lists the inputs next to the results. The method for annual metrics assumes operable blinds, which makes the blind setting an input to state.

Which daylight metrics does a simulation report?

One simulation can report several metrics, and each answers a different question. The table sets out three that come up often.

Metric	What it measures	Typical use
Daylight factor	The ratio of indoor to outdoor illuminance under a CIE overcast sky, with direct sun excluded	The metric behind the Slovak minimum values, see Daylight Factor
Spatial daylight autonomy (sDA)	The share of the analysed floor area above a set lux level for a set share of the occupied hours	Whether a room has enough daylight through the year, see Daylight Autonomy
Annual sunlight exposure (ASE)	The share of the analysed area that receives direct sun above a set lux level for more than a set number of hours a year	Finding areas with too much direct sun

The Illuminating Engineering Society (IES) describes sDA and ASE in its guide LM-83. Software documentation gives example thresholds. For sDA it uses 300 lux for at least 50 % of the occupied hours. For ASE it uses 1,000 lux for more than 250 hours a year. The daylight factor is a single static value. The two annual metrics need a calculation over a full year. A report should state the thresholds it used, because different schemes set different ones.

Which Slovak rules does a daylight simulation help answer?

The main Slovak rule on daylight in buildings is decree 259/2008 Z. z. on the indoor environment of buildings. Section 4 ods. 3 says that rooms where people stay for a long time need adequate daylight, as set in Annex 3. Section 4 ods. 4 says that a new building must not worsen the daylight in neighbouring buildings more than the technical standards allow. The general technical decree 532/2002 Z. z. says that the design judges daylight together with sunlight. The design also counts surrounding buildings, permanent greenery and the effect of the new building on existing development (§ 17 ods. 1).

Section 4 ods. 12 of decree 259/2008 says that daylight is measured only when calculated data cannot be used. Calculated values, including simulation results, therefore come first.

Annex 3 of decree 259/2008 sends the designer to the standards STN 73 0580-1 and STN 73 0580-2. For long-stay rooms other than residential rooms, it sets a minimum daylight factor. The value is 1.5 % (Dmin) for side lighting and 3 % (Dm) for top or combined lighting. This page did not read STN 73 0580-2, which covers daylight in residential buildings, so it gives no value for living rooms. Ask your designer for the value that applies to each room.

Sunlight in flats has its own rule. Decree 259/2008 § 5 ods. 2 says that the sunlight of flats follows a Slovak technical standard. Decree 532/2002 § 18 ods. 2 says that a flat must receive sunlight. The hours come from that standard, which this page did not read.

What does STN EN 17037 ask?

EN 17037 is the European standard Daylight in buildings. The Slovak standards office published it as STN EN 17037 + A1 (73 0581), titled Denné svetlo v budovách. The July 2022 edition is the Slovak version of EN 17037:2018 + A1:2021. It replaces the February 2020 edition. The full text is sold, so this page uses the free catalogue cover sheet and the scope text published by a national standards body.

The scope says that the standard specifies elements for an adequate impression of lightness by natural light and for an adequate view out. It gives recommendations for the duration of sunshine exposure in occupied rooms. It also covers how to limit glare, and it defines metrics and principles of calculation and verification. This page gives no target values from it, because it did not read them.

The decrees read for this page name STN 73 0580-1 and STN 73 0580-2 for daylight and do not name EN 17037. The standards office announced on 4 April 2025 that it would issue STN 73 0580-1 from 1 May 2025. It says the standard takes the requirements of EN 17037 into account. The national foreword of STN EN 17037 + A1 lists decree 259/2008 and decree 532/2002 among related legal acts.

No decree read for this page requires a daylight simulation as a document. If a building office or a client asks for a study, the designer can use a simulation as one input.

How is a daylight simulation different from a daylighting study?

People often use the two terms as if they meant the same thing, but they sit at different levels. A daylight simulation is a calculation tool and its results. A sunlight and daylight study is a written assessment of whether a building gets the sunlight and daylight that the rules ask for. The study may use simulations as one input. It also covers the site, including neighbouring buildings, the shadows the new building casts and the sunlight that reaches flats.

Aspect	Daylight simulation	Sunlight and daylight study
What it is	A calculation model and its results	A written assessment with a conclusion
Scope	Usually the interior of one design	The site, the building and its effect on neighbours
Output	Values, grids and images	A report that states whether the requirements are met

What do designers most often get wrong about daylight simulation?

The first mistake is to treat one number as the whole answer. A single value at the centre of a room says little about the corners, the area near the window or the winter months. Ask for results across a grid of points and state the time basis of each result.

The second mistake is to model the building without its surroundings. Decree 532/2002 names surrounding buildings and permanent greenery among the factors to judge with the design, so the model needs them.

The third mistake is to read a bright result as a good one. Large glazing can admit direct sun. Decree 259/2008 § 4 ods. 5 says that places of long stay must be protected from direct sunlight where glare could occur. An annual sunlight exposure result shows where this may happen. Direct sun can also add summer heat, see Summer Overheating Risk.