Primarily in the winter frequent ventilation is required even though the outdoor air is cold. Fresh air comes into the building, quickly warms up inside the home and absorbs water molecules, which is then extracted again. That is why ventilation is much more efficient in winter than in the summer where the atmosphere tends to be warm and humid. For this reason it is enough to simply open a window in the winter for about 5 minutes, longer periods would cool down interior walls and the home. Make sure to turn off your source of heat if windows are being opened to prevent enormous heating bills. Repeat short ventilation periods regularly, every 1 – 2 hours.

The bedroom also requires fresh air in the morning: Breathing air contains moisture and even during peaceful sleep the body releases sweat.

A mechanical ventilation system is recommended if residents do not have the possibility to open their windows. At certain times it is also not a good idea to open the windows, for example in the winter or transitional seasons where it is cold and draughty outside. Even in old buildings state-of-the-art centralised ventilation systems can be implemented.

Mechanical ventilation systems

Air supply and extraction is easily controlled and adjusted to current needs, even if residents are not at home. Decentralised fans individually supply fresh air in separate rooms. Some appliances do this fully automated without any human intervention at all. Humidity sensors for example can be set to react to a relative humidity of 60% and automatically switch on the fan.

Small rooms like bathrooms can often be supplied with air from adjacent rooms due to their small size. In this case it is enough to install a simple extractor fan. It is recommended to add or opt for a model with time delay to facilitate operation. Controlled residential ventilation offers a variety of benefits such as recovering heat energy, absorbing noise and particulate matter from the outside. The quality of air is kept at a good level consistently. There are also options that combine manual and automatic ventilation.

How to ventilate the cellar

Well insulated walls give off less heat to the outside. However, refurbishing old buildings is often so uneconomical that residents are willing to put up with higher heating bills. Warm walls are more able to prevent dew from settling than cold walls. However, most cellars are old constructions with leakage and cracks, trying to use a normal heater is almost futile. Particularly if the room is used for storage, the room is kept cool, and therefore stays humid. Proper ventilation can help extract moisture and protect the masonry even in the winter.

A cellar or basement requires a different kind of ventilation than a normal room in the household. It is a common misconception that warm exterior air in the summer can dry a cold cellar faster in the summer. This possibly causes even more moisture in the building because the warm air from the outside concentrates more moisture than cool air. If this warm exterior air now enters the cellar vault, the air temperature decreases and water is being released. It will condense on the inside, because the floor of the cellar is the coolest place due to its proximity to the soil. It is on the ground where you can see the condensation best. The water then spreads through the brickwork, a musty odour becomes noticeable and mould begins to grow.

⮩ Ventilate at night or in the cool hours of the morning and evening during summer time. Laundry should be left to dry outside if possible.

In the winter there should not be constant and permanent ventilation. On one hand to prevent pipes from freezing over or heated rooms from cooling down. On the other hand, exterior air can be quite rich in moisture, particularly during winter time. However, if the outdoor air is colder than the temperature in the cellar and the air is dry instead of humid, ventilation can be turned on. Cool air is let into the building, is heated up slightly, absorbs water vapour and then leaves the building.

Indoor and outdoor air humidity can be measured with a thermo hygrometer that can be attached inside the celler and on the outside of the building envelope. Instead of using a thermometer you can also try a different method with less exact results. Take a glass bottle from the celler (must have temperature of the cellar) and put it outside. If condensation is produced on the bottle, the bottle (and the cellar) are cooler than the air outdoors. In that case, do not ventilate. Instead of manually monitoring your ventilation system, you can also opt for automatic ventilation with temperature sensors and automatic shutters that prevent cold air from infiltrating.

Summary
In old buildings and cellars without modern retrofitting you should not be stingy with the heating during colder seasons of the year. Warm air binds more water molecules and can be extracted to the outside of the building simply by opening windows and airing the home. If you do not have the time or possibility to manually open windows and doors, you can prevent mould by implementing a mechanical ventilation system.