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Daylight Factor: What it is and How to Calculate it

The daylight factor is the index that defines the correct ventilation and lighting of a dwelling. Discover why it is important and how it is calculated.

Living in healthy environments with low concentrations of pollutants is important to prevent allergies, respiratory problems, and cardiovascular diseases.

To never lose sight of the living wellness objective, to maintain buildings optimally, and to provide the opportunity to verify that each space has the necessary amount of light and air, building and urban planning regulations have introduced the daylight factor.

Do you know what it is, why it’s so important, and how to calculate it with a building design software? Find out this and much more in the article.

What is the daylight factor?

The daylight factor, acronym D.F., defines the amount of sunlight and air that enters a room in relation to its floor area. In fact, in each environment, we obtain this ratio as:

D.F. = useful window area / floor area

The D.F. ratio is divided into:

1. I.F. or Illumination Factor, referring to the useful illuminating surface, namely the sum of the glazed surfaces through which light passes;
2. V.F. or Ventilation Factor, referring to the useful ventilating surface, namely the sum of the glazed surfaces through which air passes.

Assessing the amount of ventilation and lighting in an environment is an important requirement to define the habitability and suitable public and commercial use of a building.

What is the daylight factor for?

Control over the D.F. and compliance, room by room, with its limit value must be verified to ensure that a dwelling always has a certain level of:

• healthiness;
• hygiene;
• security;
• energy saving.

In fact, this ratio allows us to understand the quantity of light that a space receives and its daily air changes.

Two-room apartment with large openings that comply with the air-to-light ratios required by law | Render produced with Edificius

How is the daylight factor calculated?

The daylight factor must be calculated for each room of a dwelling, and to do so, it is necessary to know the net floor area and the useful window area:

• the net floor area or useful habitable area is the surface of the room that is actually walkable, i.e., net of walls, pillars, partitions, loggias, porches, balconies, terraces, verandas, gaps, doors, and windows;
• the useful window area is the openable surface, i.e., the surface occupied by the fixture measured gross of the window frames facing open spaces.

To obtain the D.F., simply add up the useful window area of the windows present in the room and divide what is obtained by the net floor area, if this ratio is greater than 1/8 and thus greater than 0.125, the room has an adequate daylight factor and complies with regulations.

To better understand, let’s proceed with an example: let’s imagine calculating the D.F. of a dining room characterized by a rectangular plan of 6.00 m x 4.00 m and illuminated and ventilated by two windows of 1.40 m x 1.50 m. How to proceed?

• the first step is to calculate the total window area:

Ww = Ww1+ Ww2=(1.50 x 1.40)m2+ (1.50 x 1.40) m2= 2.1 m2+ 2.1 m2= 4.2 m2

• then proceed to know the useful floor area, which in this case is:

Ff= (6.00 x 4.00)m2= 24 m2

• the D.F. will be:

D.F.= Ww / Ff= 4.2 m2 / 24m2= 0.175 > 0.125

As seen, since the ratio is greater than the minimum ratio established by law, the room has an adequate daylight factor.

Example floor plan for RAI calculation of a room |Edificius

Why is it important to properly ventilate and illuminate living spaces?

Having an adequate D.F. value is not only mandatory by regulation (in the case of new constructions, building renovations, indoor restructuring, or change of use of rooms and spaces) but also brings a series of benefits to the indoor environment:

• proper air recirculation: not only useful for eliminating stale air but essential for safeguarding comfort and health. Closed or poorly ventilated environments, often due to too low a ratio, are the cause of the formation of steam, carbon dioxide, and indoor pollution that can lead to headaches, breathlessness, weakness, and lack of concentration;
• adequate room lighting: the use of large and well-exposed windows allows reducing the use of artificial lighting and heating, resulting in economic and energy savings;
• short and continuous air changes: which do not generate heat loss and guarantee environments free from germs and bacteria, the cause of moisture stagnation and mold formation.

As we have seen, verifying the D.F. during the development of a design is very important and can influence the acquisition of the building permit itself, automating its analysis is not impossible, you just need to use a building design software.

Have you modeled your building construction with walls, columns, and fixtures? Just define the rooms it consists of, characterizing them based on their properties, which will create tables and charts to analyze them all. Among these, it will be possible to check the floor area, the window area, and calculate the D.F. ratio room by room automatically.

You just have to try it for free and experience all its potential!