PHI study on “Living Comfort”: Positive impact of Warm Edge spacer bars on thermal comfort
Warm Edge spacer bars are well known for their positive impact on comfort and the avoidance of mould formation. In order to be able to verify these advantages with data, the Passive House Institute in Darmstadt carried out its “Living Comfort” study, which deals with the question of how comfort and hygiene criteria for windows can be defined and adhered to in different climatic zones.
What is comfort anyway? Can comfort be physically measured and proved?
People perceive a space as being thermally comfortable when it has a certain “operational temperature”. This comprises the average value of the air temperature and the surrounding surfaces. The temperature difference between air and surfaces should be a low as possible because large temperature differences give rise to the unpleasant impression of draughts. In the case of cold air draughts, the air falls onto cold surfaces, causing uncomfortable air movements.
It has been proved that a pleasant feeling arises when the temperature difference between the indoor air (“operational temperature”) and the colder surrounding surfaces, such as external walls, ceilings, floors or windows, is less than 4.2 Kelvin. Or: in order to feel comfortable, surfaces must be no more than 4.2 Kelvin colder.
Location-related statements on the thermal comfort can be made on this basis. The location-related conditions of the coldest daily average of a reference year in a certain location and the maximum temperature difference can be used to calculate the heat transfer coefficient U, below which thermal comfort is guaranteed.
For example, if the operational indoor temperature is taken to be 22 °C and the outside temperature -16 °C, this gives a U-value of 0.85 W/(m²K). This value is known as the Passive House comfort criterion for the cool-temperate climate and applies to the installed window.
If the non-installed window is considered on its own, the limit value in the cool-temperate climate is 0.80 W/(m²K) in order to ensure a buffer for the built-in thermal bridges.
The arithmetical proof of comfort therefore shows that the maximum U-value of the component depends on the outside temperature – and thus on the local climate.
Is it possible to arithmetically prove when mould occurs?
Another important factor where comfort and health are concerned is the avoidance of condensation and mould. As a general rule, moisture gathers at the coldest point in the room – which is frequently around the edge of the glass. Consequently, mould often forms here because the high moisture level provides the best conditions for it to grow. When combined with condensation, structural damage can also occur.
Important to know: Mould starts growing above the dew point temperature. Surface temperatures must therefore be above the so-called mould temperature to effectively protect a building’s structure and the health of its occupants.
That can also be calculated: The so-called temperature factor fRsi has become established as an indicator for the hygiene-related conditions on the edge of the glass. If this value is at least 0.7 in a cool-temperate climate, it can be assumed that no mould will grow at normal levels of indoor humidity. An effective solution for avoiding mould is therefore to increase the temperature on the edge of the glass.
The following table stipulates the comfort and hygiene requirements for the different climatic zones:
What specific examples did the study look at?
It is now clear that the window must always be considered in conjunction with the building’s location. The study therefore looks at, for example, the suitability of different combinations of window profiles, glazing units and spacer bars for five basic climatic conditions, namely Arctic, cold, cool-temperate, warm-temperate and warm climates.
The following table summarises the results of the study. It shows which investigated combinations of window frame, glazing unit and spacer bar are suitable for the various basic climatic conditions in terms of comfort and hygiene.