A Vacuum Insulation Panel consists of a rigid, highly-porous core material encased in a thin, gas-tight outer envelope, which is evacuated and sealed to prevent outside gases from entering the panel. The combination of porous material and the vacuum results in an extremely high thermal resistance, which allows for thinner insulation in applications where space is at a premium.
Core materials for VIPs require a completely open-porous structure to allow evacuation and must have a sufficient high compressive strength to withstand the mechanical pressure load. Core material classes comprise basically three structures as micro porous powders, fibres and foams. Each structure has its inherent solid mechanical properties. The minimum density to withstand atmospheric pressure loads is distinctive as well as the centre of panel (COP) value for the thermal conductivity which depends on the solid backbone of the core material. The core material should have the lowest possible thermal conductivity to ensure the VIP's best performance. Optimum materials have a micro porous structure and are able to absorb humidity; otherwise getters have to be added. The most common core materials are fumed silica and glass fibres. In a few cases open-cell polymer foams, e.g. special polyurethane or polystyrene foams, are used.
Gas impermeability is the main factor influencing the selection of envelope materials for VIPs. In terms of a VIP's life expectancy, its impermeability to water vapour is also important. Furthermore, a low thermal conductivity is also vital in order to keep the thermal bridge effect as low as possible along the edges of the panels. Because of the mechanical loads that occur during construction, the envelope materials should also have sufficient puncture resistance.
The envelope materials used for thermos flasks, namely stainless steel, aluminium and glass, are also generally suitable for flat Vacuum Insulation Panels. In combination with filler materials such as foams and fibres, it is actually only these materials that achieve the required high gas impermeability that is required for durable products. Most manufacturers use aluminium-metallised high barrier plastic laminates, aluminium composite films and stainless steel films or sheets to provide the VIP envelopes.
The thermal conductivity of a well-evacuated dry VIP with a fumed silica core is about 0.004 W/(m•K) after production; and it is about 0.020 W/(m•K) at ambient pressure. By comparison, thermal conductivities of conventional insulation materials are typically between 0.030 and 0.040 W/(m•K).
Vacuum Insulation Panel technology provides:
- extremely low insulation thicknesses (10 mm to 25 mm), increasing the volume capacity of applications for which a lack of space for insulation thickness is an issue;
- stable, long-term thermal performance when installed correctly and protected from damage and penetration;
- high performance thermal insulation panel with a thermal conductivity 8 to 10 times lower than other conventional insulation materials (3 mW/(m•K)); and
- new design and construction possibilities.
Estimations of payback period (PBP) should include the value of the floor space made available by the thinness of the VIPs. Construction materials must have extremely long service lives. Since VIPs reach the end of their service life when the gas pressure rises above 50 to 100 mbar, much research and development is focused on the envelope of the VIP. Further research into panel construction could maximize the service life cycle to many decades or even a century.