How do I recognise too much humidity in the building?

Excessive humidity in living spaces leads to increased mold growth and thus ultimately to a deterioration of the building fabric. In addition, mold can also significantly affect the health of residents. To avoid these problems, it is important to maintain the correct values. In this article, we explain how to ensure this and what the optimum humidity level is.

What is humidity, how does it occur and when is it too high?

Excessive humidity should be avoided at all costs. In order to address this issue in more detail and provide tips on how to avoid excessive humidity, we first need to take a closer look at the term "excessive humidity". What exactly does too high mean? It is precisely on this point that opinions differ. This is also evidenced by the fact that various studies on this topic almost all come to a different conclusion. We are passing on our experience from our many years of work and do not wish to deny anyone their opinion or results.

Excessive humidity is essentially dependent on two factors: the amount of water in the air and the temperature. Warm air can absorb more water and therefore moisture than cold air. This is because the air molecules "slide further apart" when it is warm and, put simply, more "water" fits into the spaces between the molecules. Now, the moisture "dissolved" in the air is not really a problem at first - it is the water that condenses out of the air due to temperature changes. Or the level of this moisture and its (possibly negative) effect on various things.

So when does the moisture stored in the air leave its host? When warm air with a lot of moisture cools down, the relative humidity first rises. At some point, however, the point is reached at which the maximum possible amount of water has been absorbed. If the air cools further, water precipitates from the air in the form of condensate.

This water is then usually the real problem for us and our home. So if we have a certain temperature and humidity in the room (e.g. 21° C and 45 % relative humidity), a cool surface can cool the air. This can be the surface of a component, e.g. a wall or a window, which is cooler than the rest. As a result, the relative humidity rises at this point. Often to levels of well over 80 % or even to the point where we can see condensation (i.e. water).

If a window becomes so cold that water condenses out of the room air (usually at the edges of the window), this is usually a sign that the humidity is too high. Excessive humidity in a building or apartment therefore depends on the coldest point in the room. In order to be able to make precise statements about the exact value, you would now have to know the temperature of the coldest surface in the room and calculate back what relative humidity must not be exceeded at a certain temperature.

What is the ideal humidity level in the various rooms?

The following table shows the ideal values for humidity in the various rooms:

Typical entry paths of increased humidity

Everyone generates moisture directly and indirectly. We sweat, breathe, cook, wash, water our plants and much more. But everyone lives very individually and sometimes produces more and sometimes less moisture. Some people have pets or aquariums. Sometimes there is a sauna or even a swimming pool in the basement. Others like to cook for long periods of time. All of this has an effect on the humidity in the building.

What all individual behavior patterns have in common, however, is that we produce much more humidity than we think. A family of four without aquariums, swimming pools or pets and with a separate laundry room, for example, produces around 10 to 15 liters of water per day in the living area. With an aquarium and co. it can quickly be many times that amount. Drying laundry just once can mean an additional 5 to 8 liters. This moisture must be ventilated out of the home so that the humidity can be kept constant.

Why is humidity so high in new buildings in particular?

In a new building in particular, the humidity can also come from other sources. This can be residual building moisture, for example. Residual building moisture is caused, among other things, by large quantities of cement, screed, paint or simply rain penetrating the building during the construction phase. This is completely normal and occurs in every building.

A detached house contains 15,000 to 20,000 liters of water. In an average apartment, there are still around 10,000 to 15,000 liters. This moisture slowly escapes from all building components (usually spread over the first 18 months after moving in or completion) and is thus released into the room air. If this residual building moisture also reaches the occupants of the home, the effects add up.

It is therefore quite normal for a lot of humidity to build up in a new building, especially in the early days. This high humidity can cause mold growth or damage to the building and/or furnishings. This moisture can quickly condense and run down windows or doors, or furnishings can start to mold and/or smell musty. There are also other ways in which moisture can get into a building. However, these are usually damages (e.g. leaks or similar) that have nothing to do with normal use. In such cases, a specialist is needed to clarify what exactly has happened and what needs to be done to rectify the damage.

Detecting and measuring humidity - that's what matters

There are hygrometers that display the relative humidity more or less accurately. The simplest models are analog indicators with a horsehair or plastic hair connected to an indicator needle. These devices are quite common and cost from around €5 in DIY stores. More expensive devices with a digital display and a real sensor cost around €15-20. Unfortunately, you can't tell how good the devices really are by their price. Normally, tolerances of approx. 5-10 % can be expected in the displayed data. However, devices with information such as "too low", "good" or "too high" should usually be viewed critically and are often not meaningful. In fall and winter, normal values are between approx. 40 % and 45 % relative humidity at a room temperature of around 21° C. In summer, this can sometimes be significantly higher. In summer, sometimes significantly higher.

When we dry laundry, cook a lot, shower or take a long bath, the relative humidity can quickly rise to over 60-70%. If we sleep with the window closed at night, 75% relative humidity is not uncommon. A fairly good method of detecting excessive humidity without a measuring device is to open a window or door with a glass insert, in addition to visible condensation on window panes (in which case the humidity is clearly too high).

In winter, when it is cooler, you can see how much or how long a window pane mists up from the outside when you open the window or door. As soon as you open a window (in winter), the outer pane of glass mists up to a greater or lesser extent. You can see this particularly well on a patio door.

A particularly large amount of condensation usually forms in the upper area of the window or door. The more pronounced the condensation or the longer the window pane remains steamed up, the higher the humidity in the room. If the entire window pane is slightly white or even small droplets of water form on the surface when you run your finger over it, it can be assumed that the humidity inside the room is very high. If the window remains steamed up for much longer than two minutes, this is also a good indication of high humidity. If the windows, the window frame or door locks/door handles or hinges mist up from the inside without the window or door being opened, there is almost certainly too much humidity and the room should be aired urgently.

Reducing humidity - how it works

The easiest way to reduce humidity is to air the room regularly. However, one problem here is that most people go to work during the day and air can only be exchanged in the morning and evening. Modern ventilation systems provide a solution here. They permanently extract the humid, stale air, lead it outside and direct fresh air from the outside to the inside. If it is a ventilation system with heat recovery, the heat from the stale air is also transferred to the incoming fresh air, so that additional energy can be saved.