Thermal comfort begins with the window

Swisspacer reports that a new study by the PHI into ‘Living Comfort’ proves the positive effect of warm edge spacer bars.

When constructing and refurbishing residential properties, comfort plays an important role for the occupants; for the right feel-good climate, the relationship between temperature and air humidity also has to be right from a construction physics perspective.

The impact that the quality of the windows and the choice of spacer bar has on comfort is shown by a new study conducted by the Passivhaus Institute.

In its study Living Comfort, the Passivhaus Institute in Darmstadt, Germany, dealt with the question of how comfort and hygiene can be defined and met for windows in different climate zones.

“The windows are, as a general rule, still the weakest elements in the building in thermal terms,” Professor Benjamin Krick, managing director of the Passivhaus Institute, said.

“Also important for how they perform in terms of thermal comfort are the location of the building and its climatic environment.”

The study differentiates between Arctic, cold, cool-temperate, warm-temperate and warm climates. The investigation was based on various plastic windows with double, triple or quadruple glazing. Aluminium, stainless steel and Swisspacer Ultimate warm edge spacer bars were used.

“The choice of spacer bar has a big impact on the thermal quality of the window and thus on comfort and hygiene,” Krick said. “With highly energy-efficient composite spacer bars, windows achieve the requirements by appropriate means and therefore represent little effort for the window maker.”

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. Large temperature differences between air and surfaces 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 and the surrounding surfaces, such as external walls, ceilings, floors or windows, is less than 4.2 Kelvin.

From these underlying conditions, the Passivhaus Institute calculated the so-called ‘Passivhaus comfort criterion’ for various climate zones. For the cool-temperate climate that prevails in Germany, for example, the derived U-value is 0.85Wm²/K. For this, the team in Darmstadt assumes an operational room temperature of 22°C, and an outside temperature of -16°C. If this characteristic value is not reached, a heat source is needed under the window to avoid the unpleasant sinking of cold air.

An important factor where comfort and health are concerned is the avoidance of condensation and mould. Moisture gathers at the coldest point in the room, which is frequently around the edge of the glass. However, mould starts growing above the dew point temperature. Surface temperatures must, therefore, be above the ‘mould temperature’ to effectively protect a building’s structure and the health of its occupants.

“The temperature factor f_Rsi has become established as an indicator for the hygiene-related conditions on the edge of the glass,” Krick said. “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.

Increasing the temperature at the edge of the glass is an effective solution.

“The temperature at the edge of the glass depends on the spacer bar, on the thickness of the glazing unit, and on the installation of the pane in the window,” Krick said.

That is why the most meaningful solution is the combination of high-quality thermal spacer bar, triple glazing with the widest possible gap between the panes, and a well-insulated window frame.

The PHI study shows that the choice of spacer bar has an important effect on the window’s thermal quality. From the warm-temperate climate and cooler, the general recommendation is to use triple glazing in combination with highly energy-efficient spacer bars such as the Swisspacer Ultimate.

The values required for comfort and hygiene could then be achieved even with the uninsulated window frames widely found in these regions. With well-insulated window frames, the requirements for warm-temperate and cool-temperate climates can be met both with stainless steel spacer bars and with the premium space bar from Swisspacer. The requirements of the Arctic climate necessitate the use of the Swisspacer Ultimate in conjunction with a particularly well insulated frame.

“When you look at the results of the investigations, practically everyone benefits,” Krick said. “The occupier of the building, who profits from greater comfort and mould-free glass edges, and the window maker, who can simply improve his products.

“Damage that can be caused by moisture is also avoided. That increases customer satisfaction and reduces complaints. Due to the high energy saving, the climate is relieved by the lower CO₂ emissions.”

The study was commissioned by Swisspacer. The Passivhaus Institute in Darmstadt and Swisspacer have been working together on studies that provide information for the market. Swisspacer started participating in the Passivhaus Award this year and has announced its own special prize, which will be awarded for the first time in 2021.