There are three basic types of heat transfer: convection, conduction and radiation. The latter can be observed in common thermal images; for the former, an ice cube in a cocktail is the classic example. For buildings, the second form plays the most important role. Most of the heat that comes out of a closed house in winter is lost through conduction. It enters air-conditioned buildings in warm countries in the opposite direction via the same path.
Heat conduction mainly concerns the movement of the atoms: the more energy (heat) there is in a solid, the stronger the vibration of its atomic lattice. A solid in this case could be a brick, for example. When the air outside is cold and the heating inside is warm, there is a temperature gradient in the brick. The stronger vibration of the warm side feeds the weaker one of the cold side – the heat "drains away".
If there is no atomic lattice, nothing can vibrate. So there is no better way to prevent heat conduction than a vacuum. Unfortunately, the principle of the thermos flask cannot simply be applied to large elements. A completely empty, evacuated panel would be pressed together by the air pressure. Inner support is therefore required which should leave as much space as possible for the vacuum.
Supportive core plus emptiness
Vacuum insulation panels (VIP) have an open-pored supportive core, which usually consists of silicon dioxide. This material is fireproof, non-toxic and can be recycled. The core is covered with an aluminium-plastic film, which limits the heat radiation and maintains the vacuum in the panel. After installing the supportive core, frame and outer casing, the panel is connected to a vacuum pump and emptied of air.
The VIPs insulate four to five times better than the best conventional insulating materials. Conversely, they only need a quarter or even a fifth of the material thickness to achieve a certain insulation value. For this reason, VIPs are mainly used where space is limited or architectural requirements are particularly high, for example in the refurbishment of old buildings, ceiling and reveal insulation or in very high-quality properties. After all, their elaborate production naturally results in higher prices compared to mineral wool or plastic foam. Even during transport and processing, VIPs place higher demands than simple bulk goods. For the greater overhead, however, the client is rewarded with the best insulation values and an incredibly thin insulation jacket.
An absolute vacuum cannot be achieved for physical and technical reasons. A residual proportion of air molecules always remains, which can be larger or smaller depending on the vacuum technology used. Even the external sealing of the vacuum panel cannot guarantee complete hermetic sealing for an unlimited period of time. Minimal diffusion of air molecules cannot be prevented. Getters are integrated into the panels in order to keep the vacuum at the desired level for a long time despite these basic restrictions. They consist of a reactive porous material with a large surface area, which chemically bonds with most air molecules. It can consist of materials such as barium, lithium zirconium, vanadium or cobalt. They bind most of the gas molecules in the air, primarily nitrogen, oxygen and carbon dioxide. They can do nothing against the inert noble gas atoms, but their proportion in the air is low. Drying agents – usually calcium, barium and cobalt oxides – are also used to bind the water vapour contained in the air.