Vacuum insulated panel

A vacuum insulated panel (VIP) is a form of thermal insulation consisting of a gas-tight enclosure surrounding a rigid core, from which the air has been evacuated. It is used in building construction to provide better insulation performance than conventional insulation materials.

VIPs consist of:

Heat transfer occurs by three modes: convection, conduction and radiation. Creating a vacuum practically eliminates convection, since this relies on the presence of gas molecules able to transfer heat energy by bulk movement. A small decrease in pressure has no effect on the thermal conductivity of a gas, because the reduction in energy-carrying molecules is offset by a reduction in collisions between molecules. However, at sufficiently low pressure, the distance between collisions exceeds the size of the vessel, and then the conductivity does reduce with pressure.

Since the core material of a VIP is similar in thermal characteristics to materials used in conventional insulation, VIPs therefore achieve a much lower thermal conductivity (k-value) than conventional insulation, or in other words a higher thermal resistance per unit of thickness. Typically, commercially available VIPs achieve a thermal conductivity of 0.004 W/(m·K) across the centre of the panel, or an overall value of 0.006-0.008 W/(m·K) after allowing for thermal bridging (heat conduction across the panel edges) and the inevitable gradual loss of vacuum over time.

The thermal resistance of VIPs per unit thickness compares very favourably to conventional insulation. For instance, standard mineral wool has a thermal conductivity of 0.044 W/(m·K), and rigid polyurethane foam panels about 0.024 W/(m·K). This means that VIPs have about one-fifth the thermal conductivity of conventional insulation, and therefore about five times the thermal resistance (R-value) per unit thickness. Based on a typical k-value of 0.007 W/(m·K), the R-value of a typical 25 mm-thick VIP would be 3.5 m²·K/W (20 h·ft²·°F/BTU). To provide the same R-value, 154 mm of rockwool or 84 mm of rigid polyurethane foam panel would be required.

...
Wikipedia