Vacuum Insulation Panels - The Next Step in Thermal Insulation

Author:  Dr. Roland Caps, Board of Directors R&E, va-Q-tec AG (As of: March 2015) To meet future energy needs and still achieve less carbon dioxide emissions from the combustion of fossil fuels into the atmosphere without nuclear power, the focus of politics is currently predominantly on renewable energy solutions. However, it is also important to reduce energy demand per se. Thus, the provision of thermal energy is responsible for a large proportion of energy consumption.

Wherever temperatures are to be kept constant, such as in buildings, refrigerators or freezers or heat storage units, it is possible through improved and innovative insulation technologies without sacrificing comfort to reduce energy demand. Novel insulation solutions based on vacuum insulation panels (VIPs) can help save valuable thermal energy without taking up much space. In principle, a VIP is similar in structure to a packet of vacuum-packed coffee powder. The core of the evacuated insulation board is made of an open-pored, compressible material of low thermal conductivity. The evacuated core is wrapped in a gas- and water-vapor-tight plastic film. Depending on the support core material, thermal conductivities of 1.5 to 8 mW/(m∙K) are achieved, i.e., up to twenty times better than the most commonly used insulating material, polystyrene foam. The air pressure inside must be reduced to values between 10 mbar and 0.02 mbar, depending on the size of the open pores of the core  in order to suppress the thermal conductivity of the air (for evacuated coffee powder, the pressure is typically 100 mbar).

Fiberglass fleece as core material

In Asia in particular, fiberglass fleeces are widely used as core material for vacuum insulation panels. When very thin glass fiber nonwovens are laid on top of each other, all fibers are optimally aligned perpendicular to the heat flow. Heat can then be transported across the fiber solid essentially only via point contacts between the superimposed fibers. These point contacts exhibit high thermal resistances - despite the atmospheric load of 1 bar and the associated relatively high density of the fiber VIPs of 250 to 300 kg/m³. Thus, one measures lowest thermal conductivities, which, including the thermal radiation conductivity, can be 1.5 mW/(m∙K) in the best case. However, these low values for fiber VIPs are only achieved when the gas pressures are below 0.02 mbar, thus almost completely eliminating the thermal conductivity of the air. This very low gas pressure required here is due to the much larger voids in the fibers compared to silica powder, which typically have diameters of 30 - 100 µm, depending on the fineness of the fibers.

While the low gas pressure in the 0.01 mbar range can easily be provided in a vacuum chamber during the production of fiber VIPs, the trick is to maintain the low air pressure and thus the low thermal conductivity during the service life.

This is where the choice of cladding films plays an important role. Aluminum composite films are very dense to the passage of air and water vapor into the interior of the VIP. In combination with getter materials that can absorb portions of residual air and water vapor, increases in thermal conductivities of less than 1 mW/(m∙K) per year (corresponding to an air pressure increase of less than 0.1 mbar per year) are achieved for fiber VIPs. However, the 6  to 10 µm thick aluminum foil at the edge acts as a thermal bridge, and its presence alone can effectively pull the thermal conductivity up by 5 to 10 mW/(m∙K), depending on the size of the VIP. As an alternative, therefore, ultra-thin metallized polyester films are used, which do not form a thermal bridge, but have much higher permeabilities to air and water vapor compared to aluminum composite films. Because of these problems, fiber-based VIPs have so far only been used for refrigerators and freezers, where their service life is limited to 10 to 15 years. A composite of aluminum composite foil and metallized foil is predominantly used as the shell, with one side with metallized foil facing the cold interior of the refrigerator, while the aluminum foil faces the warmer environment.  This significantly reduces the thermal bridge and achieves acceptable increases in thermal conductivity during the service life for the manufacturers of the refrigerators and freezers.

As other core materials for VIPs, other materials such as open-pored foams or perlite powder are also being investigated or have already been used in some cases, but have not yet gained any major significance.

Compared to other insulating materials, VIPs are considerably more expensive for the same insulating value. If one considers however the costs of the useful space occupied by the insulation, the cost calculation looks already differently. If the cost of vacuum insulation per unit volume is about the same as that of the useful volume (e.g. 2000 €/m³ for a freezer), but the vacuum insulation only occupies, for example, 20% of the volume of a polyurethane foam insulation, it only causes approx. 20% + 20% = 40% of the cost compared to polyurethane foam insulation (which is essentially the useful volume cost with negligible foam costs in the range of €100/m³).

Application areas

Household appliances such as refrigerators and freezers are in daily use and thus have relatively high power requirements. By installing slim and efficient VIPs, manufacturers of household appliances can achieve high energy consumption ratings and reduce the appliance's power requirements by up to 50%. For consumers, this means lower electricity costs and much more usable volume than conventional appliances insulated with polyurethane foam.

Other applications that VIPs are currently tapping into are hot water storage tanks. However, a sufficiently long service life can only be ensured in the long term by VIPs with cores made of fumed silica powders because of the relatively high temperatures around 80 °C.

In the building sector, especially in energy-related renovation, architects and planners are already using long-lasting silica-based VIPs. They achieve better thermal insulation in their building projects while gaining valuable usable space. VIPs are ideal for numerous fields of application such as roofing, flooring, parapet elements, balconies and conservatories, as well as other interior and exterior areas.