Vacuum Insulation

Trying to minimize heat transfer can be difficult sometimes. Even substances with a very low thermal conductivity still let some conductive thermal energy pass through. However, what if we are able to just remove all substances in the insulating layer, thereby removing conductive heat transfer to the equation? This is the logic behind vacuum insulation.

VIP opened up and compared to conventional insulation

Image 5: On the left image, is a VIP cut open to components. On right image, VIP compared to an extruded
polystyrene panel with a similar thermal conductivity. (Alotaibi, Riffat 2013)

Using vacuum insulation in buildings generally takes the form of vacuum insulated panels (VIP). A VIP will be a porous, sealed panel that has been evacuated to have a very low air pressure between the pores. A small pore size is generally better for insulation purposes, as a larger pore size requires a lower pressure to achieve the same thermal conductivity as a lower pore size.
Thermal conductivities with respect to pressure chart

Image 6: Thermal conductivity vs. pressure of air for materials with different pore sizes (Baetens et al. 2009).

The main appealing aspect of VIPs are their high thermal performance, which is often several times higher than a conventional insulator of a similar thickness. This means that VIPs can be used in places where not much space is available.

Performance levels of insulators & VIP comparted to glass wool
Image 7: on the left is a comparison of the conductivity of some insulators, on the right is a VIP (in foil) compared to the thickness
of glass wool needed for an equivalent thermal conductivity (Baetens et al. 2009).
 
However, VIPs have several disadvantages compared to more conventional insulators. One is the fragility of the panels. If a panel is punctured, it will lose its low pressure environment, and since VIPs are generally made much thinner than other insulators, a punctured VIP will become a very poor insulator. Because the seal on a VIP needs to be maintained, they must be very carefully installed and once they are manufactured, as they can't be modified without damaging the foil seal. Another disadvantage of VIPs are their service life. Over time, VIP's thermal conductivity will increase as a result of gas penetration and water vapor penetration. While the service life of a VIP depends on what type of core is used, a service life of about 60 years is expected for a fumed silica core with a multilayered aluminum envelope, and a service life of 10-15 years for an areogel with a metal foil envelope (Alotaibi, Riffat 2013). This compares to other insulators like mineral wool and fiberglass, which both have a service life of about 100 years. The last disadvantage of VIPs are their higher price compared to other conventional insulation.

VIPs are a relative newcomer to the insulation landscape, but they can carve out a definite niche in places where other insulators won't fit.

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