Ventilation: the balance between winter and summer

There are clear differences in ventilation requirements between winter and summer.

In modern buildings, a good balance between humidity and air regulation is essential for a healthy and comfortable indoor climate. Vapour pressure and ventilation play a crucial role in this. Vapour pressure, or the amount of moisture in the air, varies depending on the temperature inside and outside. The difference in vapour pressure between indoor and outdoor air determines how moisture moves through a building. There are therefore differences in ventilation requirements between winter and summer.

Vapour pressure

Vapour pressure is the pressure exerted by the water vapour in the air. This pressure depends heavily on temperature: warmer air can hold more moisture than cold air and therefore reach a higher vapour pressure. Inside buildings, where the air is often warmer, the vapour pressure is usually higher than outside, particularly in winter when the outside air is cold and dry.

When a vapour pressure difference arises, moisture migrates from the area with higher vapour pressure to an area with lower vapour pressure. This phenomenon is called vapour diffusion. In winter, the vapour pressure indoors is higher than outdoors, causing moisture to be transported from inside to outside. In summer, the vapour pressure difference may be reversed, depending on outdoor conditions.

Vapour pressure differences in winter and summer

·         Winter: In winter, the outdoor temperature is often lower and the outdoor humidity is usually lower than indoors. This results in a low vapour pressure outdoors. Indoors, the air is usually warmer and more humid, resulting in a higher vapour pressure. A typical vapour pressure difference in winter might be, for example:

o   Indoors: 1244 Pa

o   Outdoors: 600 Pa or lower

o   Vapour pressure difference (Δp): 1244 – 600 = 644 Pa

This vapour pressure difference causes moisture to migrate naturally from indoors to outdoors.

·         Summer: In summer, outdoor temperatures are often higher, and so are the relative humidity and vapour pressure. The outdoor air may even contain more moisture than the indoor air. An example of a summer vapour pressure difference might be:

o   Indoors: 1244 Pa

o   Outdoors: 980 Pa

o   Vapour pressure difference (Δp): 1244 – 980 = 264 Pa

In summer, the vapour pressure difference is smaller and, in some cases, the vapour pressure outside may even be higher than indoors. This means that in summer there is less natural moisture migration from inside to outside and that there is a risk of moisture actually being drawn in from outside.

Ventilation requirements in winter and summer

Ventilation is important for bringing in fresh air, removing harmful substances and maintaining a balanced moisture level. However, ventilation requirements vary greatly between winter and summer due to differences in vapour pressure and temperature fluctuations.

·         Winter: In winter, the vapour pressure indoors is often much higher than outdoors. This leads to a natural migration of moisture from inside to outside through non-vapour-tight structures, such as walls or ceilings. As the air outside is drier, it absorbs this moisture. However, excessive ventilation in winter can be problematic: cold outside air entering the building contains less moisture, which can cause indoor humidity levels to drop too low. This can lead to comfort issues such as dry skin, irritated airways, or even damage to timber structures.

It is therefore often advisable to limit ventilation in winter to the minimum necessary to remove harmful substances, whilst avoiding unnecessary moisture loss and heat loss.

·         Summer: In summer, the situation is reversed. The outside air is warmer and often contains more moisture, meaning that the vapour pressure difference between inside and outside is smaller, or may even be reversed. This means that there is less natural migration of moisture from inside to outside, or that moisture may move from outside to inside. Furthermore, ventilation is important in summer to remove heat, as excess heat can build up inside the building. As a result, more ventilation is often needed in summer to keep both heat and moisture under control.

Increased ventilation ensures that excess moist air is expelled outdoors and that the indoor climate remains comfortable and healthy.

Ratio of ventilation between summer and winter

Based on the difference in vapour pressure, we can make a rough comparison of ventilation requirements between summer and winter. For example, if we assume a vapour pressure difference of 644 Pa in winter and 264 Pa in summer, the ratio is:

Ratio = 644 / 264 ≈ 2.44

This suggests that, based purely on vapour pressure differences, the ventilation requirement in winter should theoretically be greater than in summer. In practice, however, this is often not the case. In winter, the large vapour pressure difference causes a natural migration of moisture, meaning that less active ventilation is required to maintain a balanced indoor climate. Furthermore, excessive ventilation in winter would lead to unnecessary heat loss and excessively dry indoor air.

However, in summer, despite the smaller vapour pressure difference, more ventilation is often required due to the heat and the need to remove moisture in order to ensure comfort.

Other Considerations

In addition to vapour pressure and ventilation, other factors play a role in determining the correct ventilation strategy for buildings:

·         Energy: In winter, it is important to limit ventilation to minimise heat loss. Ventilation systems with heat recovery can help to regulate ventilation efficiently without significant heat loss.

·         Indoor humidity: In winter, care must be taken to ensure that indoor humidity does not become too low. Air that is too dry can cause health problems and damage to the interior, such as cracks in wood or paint.

·         Mould growth and damp problems: Inadequate ventilation, particularly in summer, can lead to damp problems and mould growth. In areas such as cellars and crawl spaces, ventilation and vapour pressure relief are essential to prevent damp problems, regardless of the season.

In summary

A building’s ventilation requirements vary significantly between summer and winter, mainly due to variations in vapour pressure gradients and the influence of temperature and humidity. In winter, when the vapour pressure gradient is high, natural moisture migration through the building structure can help maintain a healthy level of humidity, thereby reducing the need for ventilation. In summer, when the outside temperature is higher and vapour pressure gradients are smaller, more ventilation is often required to remove both moisture and heat.

A well-balanced ventilation system that takes seasonal variations into account can help to keep the indoor climate healthy, comfortable and energy-efficient.