A thermally insulated, airtight building envelope plays a major role in determining how comfortable and pleasant an indoor environment is for us and how active and productive we will be this environment. Comfort cannot be measured objectively, as humans perceive their environments very differently. Nonetheless, it is still possible to create an indoor environment where most people will feel comfortable.

The following main criteria for thermal comfort should be taken into account:

  • Room air temperature
  • Temperatures of building element surfaces
  • Relative humidity
  • Air movement in the room

The indoor air quality is the primary factor that influences our comfort level. However, acoustics, noise protection, the light situation and colour design also affect our perceived comfort.

Good indoor air

Temperature of the indoor air

The air temperature has the strongest influence on our comfort level. A temperature range of between 20 °C and 23 °C in homes is regarded as comfortable in winter, while temperatures of up to 26 °C are still regarded as pleasant in summer. In this context, airtightness has a crucial influence on the effectiveness of thermal protection in both winter and summer

Temperature of the wall surfaces

The influence of the temperatures of surrounding surfaces in a room on perceived comfort is often underestimated. After all, we are in permanent radiative exchange with these surfaces. To ensure thermal comfort, the surfaces of windows, walls, ceilings and floors should have a temperature of between 18 and 19 °C. Temperature differences between various components should also be as small as possible. This helps to avoid unpleasant draughts.

Relative humidity

In Central and Northern Europe, people feel most comfortable in winter at indoor temperatures of between 20 °C and 23 °C at indoor relative humidities of between 40% and 60%. Of course, we don’t have a sense organ that can directly perceive relative humidity. However, secondary impressions such as perspiration or a feeling of ‘closeness’ in the air give us a quick, reliable signal that something is not right. For example, dry mucous membranes in wintertime tell us that the indoor humidity is too low and that we should do something about this. Faulty airtightness is often the cause of excessively dry indoor air in wintertime. This can even result in illness.

The relative humidity of air expresses the moisture content of air as a percentage of its maximum possible moisture content.


Draughts (air movements in rooms)

A carefully installed airtightness layer helps to prevent unpleasant draughts. Continuous air speeds of over 0.3 m/s are perceived as unpleasant by humans. We react even more sensitively if the draught is cold and if it consistently comes from the same direction. The causes of draughts include leaky windows, air currents from electrical sockets, poorly executed joints between building elements, and air-conditioning systems with incorrect settings.

Gesünderes Kinderzimmer

Indoor air quality

Good indoor air is essential if we are to feel comfortable in indoor environments. Ideally, the air should have a neutral smell, and contain a lot of oxygen and low levels of harmful substances, such as VOCs, which can be given off by construction products, cleaning agents or furniture. For this reason, only construction products that have been independently tested and approved should be used if good air quality is to be achieved. Sufficient ventilation is also important. Humans find air with CO₂ concentrations of around 0.1 to 0.15 vol.% unpleasant. Concentrations of around 2% can be tolerated for short periods. Breathing difficulties start to occur at concentrations of 3% to 4%.

Find out more about how you can ensure good indoor air

Systems for achieving ‘feelgood’ indoor climates