It is usually considered that the necessary level of lighting has been achieved if the surface of the window is 1/5 to 1/10 of the surface of the room and if it is glazed with insulated glass. This number is derived from the fact that the necessary lighting that needs to be achieved (expressed in lux) and ranges from 60 to 600 lux.
So, in a room of 10 square meters, the windows should occupy an area between 1 and 2 square meters.
The entry of natural sunlight into the interior of the house is a basic factor that improves the comfort of any living space, directly affects the mood and energy of people living in that space, and also improves the health aspects of it. In a well-designed house, natural light can emphasize the quality of the space, provide visual comfort and reduce energy consumption, so it is very important to correctly determine the size of the window. The entry of as much natural light as possible into the rooms, nowadays, has become one of the main requirements when designing houses in order to provide users the necessary comfort and quality of life when staying indoors, because a modern man spends most of the day indoors.
What is the standard glazed area for the room?
Construction norms and rules regulate the area of window openings in relation to the room area (in percent) of typical public and residential buildings, geographically located in the band from 45° to 60° north latitude. If the house is south of 45 ° north latitude, on this value should be applied coefficient 0.75, and when the building is located north of 60° north latitude coefficient 1.2.
As previously mentioned, the necessary lighting that needs to be achieved (expressed in lux) depends on the purpose of the room and ranges from 60 to 600 lux. It is usually considered that the necessary level of lighting has been achieved if the surface of the window is 1/5 to 1/10 of the surface of the room and if it is glazed with insulated glass. This formula gives approximate, but values quite close to the calculated ones. So, it basically means that in a room of 10 square meters, the windows should occupy an area between 1 and 2 square meters.
Things are not so simple though. The amount of light passing through the windows is also affected by other factors (shadow falling on the windows, position in relation to the sun, whether they are placed on a vertical wall, sloping wall, or ceiling…). This calculation, also, considers only the amount of light that is necessary, but windows need to meet some more requirements.
The following parameters must be taken into account when determining the window size:
- providing the necessary lighting in the space
- providing the necessary ventilation
- thermal gains achieved by the greenhouse effect, during the time when the windows are exposed to the sun
- heat losses for the rest of the time
- visual impression outside and inside
- balance of the need for a good view and for the privacy of the householders
- price range
Let’s explain all these things!
Window orientation in relation to sun and compass (cardinal directions)
The least light comes from the north, so the rooms oriented to the north are ideally intended for bedrooms. Houses in the city, particularly townhouses, have many design limitations, so this is often not possible. The solution is to increase the window area compared to the standard. Keep in mind that the northern lights are not direct but diffused and the quality of natural light on that side is constant in all seasons and all times of the day. Hence, north-facing rooms are especially suitable for art studios and painting studios, reading rooms, workshops or computer rooms, and bedrooms, because the sunlight does not pass into the room at its maximum intensity.
South-facing windows make the most of the benefits of solar energy, which is especially important in the winter months, so the orientation is used to increase the possibility of heating using the sunlight during cold weather. South-facing rooms are suitable for the living room or kitchen, or office if appropriately cooled.
A fully lit window measuring 100×150 cm heats a room like a 1kW radiator! In the summer when the sun is high, the window glass surfaces facing south can be protected from overheating by making balconies, canopies. Also, reflective glass that reflects solar energy can be used. Although these shades do not affect window size directly, you should consider them as a factor when choosing windows. Shading factors include balconies, terraces, canopies, as well as nearby buildings (especially in urban, densely populated areas) or large trees, and so on.
These features are often overlooked in the “Mediterranean” houses – by reproducing the numerous shelters specific for the hot climate, the architect forgets to increase the area of glazing, which is why the house is in constant twilight. In accordance with the requirements of modern construction standards, the sun should be “present” in residential premises for at least 2-3 hours a day.
What should windows’ height and position be?
To make the presence in the room comfortable, the bottom of the window should start at no more than 90 cm from the floor, and end outside the two-meter mark – this layout will open the view to both sitting and standing people. This applies to both vertical and dormer glass. Even if the light pouring from the ceiling is more than enough, the lack of a panoramic view will create an uncomfortable feeling of confinement.
The light penetrates into the most distant parts of the room through the upper half of the height of the window, so the height of the glazing should be at a height equal to approximately half the depth of the room. This tells us why rooms with high ceilings are better lit, and those with low ones are challenging in that regard.
Another interesting fact is that two windows placed at a certain distance illuminate the room better than one that has the same area as the sum of those two. However, the ratio of glass area to the total window area should be taken into account – some windows have more vertical and horizontal grilles that split glass into smaller pieces, but also block the sunlight.
How to determine the size of the attic window?
Although they do not open the panorama and generally do not give the possibility of a pleasant view, the value of roof windows should not be neglected, especially because the light coming from the roof allows, due to the angle of entry, up to three times more natural light than the glazed surface on the facade. Roof window manufacturers, in today’s market, also offer a range of additional products that can make the attic space almost magical.
If classic skylights are not enough, a flat roof window can be a good solution, as it will provide enough natural light, thus bringing life to the central part of the room. Leaving enough space between the furniture will create additional space for light to spread.
There are some rooms in a house that cannot be reached by daylight (hallway, bathroom, or pantry), and this can be solved by setting up so-called “light tunnels”. With light tunnels, it is possible to get daylight even in the darkest parts of your house, because it transmits light up to 6 meters in the room.
Does the size of the window affect ventilation?
Proper window position, orientation and its area which can be opened directly affects air ventilation in your home. Clean air in your home is not only a factor of well-being but also the basis of a healthy life. The type of ventilation that will be provided must be taken into account when determining the size of the window.
With transverse ventilation, changing the size of one opening does not significantly affect the air velocity, especially if the inlet opening increases, while simultaneously increasing both openings leads to an increase in air velocities in the room. In unilaterally oriented spaces, ventilation is usually poor due to a small difference in air pressure along with the opening. In the direction of the wind that is not directed towards the facade, opening two “side openings” causes a pressure difference sufficient to cause air movement in the room and provide moderate ventilation.
The size of these “side windows” has no effect on the caused ventilation, but only their distance. The flow of air through space depends on its direction of entry. Therefore, changing the position of the window vertically, directly affects the layer of air with the highest air velocities. As the best ventilation should be provided at the level of the space used, the height of the parapet is determined so that the main airflow is provided at that level.
Facade systems, or so-called building envelopes, are becoming denser, energy efficiency is increasing, health aspects in a residential building are becoming more important, and smart home solutions are becoming more intelligent. These improvements mean only one thing: controlled ventilation of living space is becoming an increasingly interesting topic in the construction industry.
These systems bring fresh air into the house, day and night, without the need to open windows. Dust and pollen do not enter your rooms or lungs. Headaches due to too much CO2 are a thing of the past. Definitely, a better feeling in your four walls with new ventilation systems that are incorporated into the windows. The only drawback is the price.
How does the size of the window affect the thermal balance in the room?
The area of the windows that are intended to be collectors directly affects the amount of energy that is collected in this way. Their surface should be determined by an accurate calculation, because oversizing a glazed surface, as well as downsizing, causes irrationality of the applied system. This is a fairly complex calculation, so it would be best to ask for advice from your windows manufacturer for exact numbers for your type of windows.
The heat balance of the window depends on several factors:
- heat losses by transmission through window elements, due to the difference between the external and internal temperature
- heat losses due to ventilation and blowing through the window-wall joint and the joints of the window parts (wing-box)
- heat gains by transmission due to temperature difference
- heat gains due to blowing and natural ventilation
- heat gains due to direct sunlight on the window