Vacuum insulated panel
Encyclopedia
A vacuum insulated panel (VIP) is a form of thermal insulation
consisting of a nearly gas-tight enclosure surrounding a rigid core, from which the air has been evacuated. It is used in building construction to provide somewhat better insulation performance than conventional insulation materials.
, 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 through the insulator. It also greatly reduces conduction, as there are far fewer collisions between adjacent gas molecules (or between gas molecules and atoms of the core material).
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 .
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.
However, thermal resistance per unit price is much less than conventional materials. Aerogels are more difficult to manufacture than polyurethane foams or mineral wools, and strict quality control of manufacture of the membranes and sealing joins is important if a panel is to maintain its vacuum over a long period of time. Air will gradually enter the panel, and as the pressure of the panel normalizes with its surrounding air its R-value deteriorates. Conventional insulation does not depend on the evacuation of air for its thermal performance, and is therefore not susceptible to this form of deterioration.
In addition, VIP products cannot be cut to fit as with conventional insulation, as this would destroy the vacuum, and VIPs in non-standard sizes must be made to order, which also increases the cost. So far this high cost has generally kept VIPs out of traditional housing situations, However, their very low thermal conductivity makes them useful in situations where either strict insulation requirements or space constraints make traditional insulation impractical.
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
consisting of a nearly gas-tight enclosure surrounding a rigid core, from which the air has been evacuated. It is used in building construction to provide somewhat better insulation performance than conventional insulation materials.
Construction
VIPs consist of:- Membrane walls, used to prevent air from entering the panel.
- A panel of a rigid, highly-porous material, such as fumed silicaFumed silicaFumed silica, also known as pyrogenic silica because it is produced in a flame, consists of microscopic droplets of amorphous silica fused into branched, chainlike, three-dimensional secondary particles which then agglomerate into tertiary particles. The resulting powder has an extremely low bulk...
, aerogelAerogelAerogel is a synthetic porous material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density and thermal conductivity...
or glass fiber, to support the membrane walls against atmospheric pressure once the air is evacuated. - Chemicals (known as getterGetterA getter is a deposit of reactive material that is deliberately placed inside a vacuum system, for the purpose of completing and maintaining the vacuum. When gas molecules strike the getter material, they combine with it chemically or by adsorption...
s) to collect gases leaked through the membrane or offgassed from the membrane materials. These are added to VIPs with glass-fiber or foam cores, because cores with bigger pore size require a higher vacuum (less than about 1 mbar) during the planned service life.
Thermal performance
Heat transfer through a volume occurs by three modes: convectionConvection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
, conduction and radiation
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
. Creating a vacuum practically eliminates convection, since this relies on the presence of gas molecules able to transfer heat energy by bulk movement through the insulator. It also greatly reduces conduction, as there are far fewer collisions between adjacent gas molecules (or between gas molecules and atoms of the core material).
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
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
(k-value) than conventional insulation
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
, 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 .
Comparison to conventional insulation
The thermal resistance of VIPs per unit thickness compares very favourably to conventional insulation. For instance, standard mineral woolMineral wool
Mineral wool, mineral fibers or man-made mineral fibers are fibers made from natural or synthetic minerals or metal oxides. The latter term is generally used to refer solely to synthetic materials including fiberglass, ceramic fibers and stone 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
R-value (insulation)
The R-value is a measure of thermal resistance used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux through it or R = \Delta T/\dot Q_A.The R-value being discussed is the unit thermal resistance...
) 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.
However, thermal resistance per unit price is much less than conventional materials. Aerogels are more difficult to manufacture than polyurethane foams or mineral wools, and strict quality control of manufacture of the membranes and sealing joins is important if a panel is to maintain its vacuum over a long period of time. Air will gradually enter the panel, and as the pressure of the panel normalizes with its surrounding air its R-value deteriorates. Conventional insulation does not depend on the evacuation of air for its thermal performance, and is therefore not susceptible to this form of deterioration.
In addition, VIP products cannot be cut to fit as with conventional insulation, as this would destroy the vacuum, and VIPs in non-standard sizes must be made to order, which also increases the cost. So far this high cost has generally kept VIPs out of traditional housing situations, However, their very low thermal conductivity makes them useful in situations where either strict insulation requirements or space constraints make traditional insulation impractical.