A vacuum insulated panel (VIP) is a form of

thermal insulation Thermal insulation is the reduction of heat transfer (i.e., the transfer of thermal energy between objects of differing temperature) between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with ...

consisting of a gas-tight enclosure surrounding a rigid core, from which the air has been evacuated. It is used in

building construction Construction is a general term meaning the art and science to form objects, systems, or organizations,"Construction" def. 1.a. 1.b. and 1.c. ''Oxford English Dictionary'' Second Edition on CD-ROM (v. 4.0) Oxford University Press 2009 and com ...

, refrigeration units, and

insulated shipping container Insulated shipping containers are a type of packaging used to ship temperature sensitive products such as foods, pharmaceuticals, organs, blood, biologic materials, vaccines and chemicals. They are used as part of a cold chain to help maintain pr ...

s to provide 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 silica Fumed silica (CAS number 112945-52-5), 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 agglom ...

, aerogel,

perlite Perlite is an amorphous volcanic glass that has a relatively high water content, typically formed by the hydration of obsidian. It occurs naturally and has the unusual property of greatly expanding when heated sufficiently. It is an industrial ...

, or glass fiber, to support the membrane walls against atmospheric pressure once the air is evacuated. * Chemicals (known as

getter A getter is a deposit of reactive material that is placed inside a vacuum system to complete and maintain the vacuum. When gas molecules strike the getter material, they combine with it chemically or by . Thus the getter removes small amounts of ...

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 occurs by three modes:

convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...

conduction Conductor or conduction may refer to: Music * Conductor (music), a person who leads a musical ensemble, such as an orchestra. * Conductor (album), ''Conductor'' (album), an album by indie rock band The Comas * Conduction, a type of structured f ...

and

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...

. 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 The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

(''k''-value) than conventional

insulation Insulation may refer to: Thermal * Thermal insulation, use of materials to reduce rates of heat transfer ** List of insulation materials ** Building insulation, thermal insulation added to buildings for comfort and energy efficiency *** Insulated ...

, or in other words a higher

thermal resistance Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow. Thermal resistance is the reciprocal of thermal conductance. * (Absolute) thermal resistance ''R'' in kelvi ...

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 wool Mineral wool is any fibrous material formed by spinning or drawing molten mineral or rock materials such as slag and ceramics. Applications of mineral wool include thermal insulation (as both structural insulation and pipe insulation), filt ...

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 VIP would be 3.5 m²·K/W (20 h·ft²·°F/BTU). To provide the same R-value, of rockwool or of rigid polyurethane foam panel would be required. However, thermal resistance ''per unit price'' is much less than conventional materials. VIPs 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. However, materials like polyurethane foam are susceptible to water absorption and performance degradation as well. 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. VIP performance is also temperature dependent—with increasing temperature, conductive and radiative transfer increase. Furthermore, typical panels cannot operate much above due to the adhesive used to seal the thin envelope.

Construction

Thermal performance

Comparison to conventional insulation

See also

References

Further reading