Building Regulations are becoming ever stricter on the energy efficiency of buildings. In the near future, meeting these higher standards will require thicker versions of currently available materials, and buildings will need to be designed to accommodate them. This, in turn, could eat into the living space that homes can provide for their occupiers or the saleable space for commercial properties.

The effectiveness of an insulating material is called the lambda value, measured in W/m-K. The lower the lambda value, the lower its thermal conductivity and the better its insulating properties.

Heat transfer through a volume occurs by three modes: convection, conduction and radiation. The most basic insulators rely on the principle of blocking the convection of heat.

This is achieved by creating a matrix of fibres or bubbles, as in mineral fibre or polystyrene (EPS). The voids in this matrix are then filled with a buffer zone of “still” air. Therefore a product’s insulation capacity is limited by the conductivity of air, which has a lambda value of 0.025W/m-K.
Added to the conductivity of the matrix itself, this produces typical lambda values of 0.03W/m-K or higher.

Aerogels operate on the same principle. By restricting the movement of air molecules in very small spaces, their insulation performance has been recorded to be as low as 0.012 W/m-K.

Replacing the air with a less conductive substance improves the insulating performance still further. Systems such as closed-cell PIR or closed-cell phenolic replace the air with a different gas, or blowing agent. The thermal conductivity of the blowing agent is significantly lower than air and therefore the overall performance is improved, obtaining minimum lambda values of about 0.02W/m-K.

However, using gas still adds unnecessary conductivity. In recent years, performance improvements in these products have been restricted to incremental steps, limited by the physical properties of the gases used.

Panel Development

Significant further improvement requires that the gas be removed altogether. A vacuum — a space that has been evacuated of air — has no thermal conductivity, and only radiation can pass through it. This is the principle used in the vacuum flask, invented by Sir James Dewar in 1892.

Vacuum-insulated panels (VIPs) themselves are not a new technology. The first rubber-enclosed panel was patented in 1930, while the first patent on a nanostructured VIP was filed in 1963. In 1999, the US Department of Housing and Urban Development evaluated the market potential for VIPs in residential buildings, evaluating 27 different constructions. In 2005, VIPs were used in the facade of a seven-storey mixed-use building in Munich, designed by architect Martin Pool. It was the first privately financed building to be entirely insulated with VIPs, and the first time the technology had been used in a building more than two storeys high.

Kingspan’s Optim-R VIP solution was launched in March 2012. Today’s VIPs are rigid boards with a microporous core made of three components: fumed silica, silicon carbide and fibre. This core is evacuated of air, encased and sealed in a thin, gas-tight envelope.

Advantages of VIPs

The key advantage of VIPs is their low thermal conductivity: declared lambda values of 0.007W/m-K can be achieved. This is three times better than existing high-performing materials and up to five times better than first-generation insulation materials.

The other principal benefit of VIPs is their thinness compared with other kinds of insulation panel. The panels specified for the Munich building, for example, were 20mm thick and covered with expanded polyurethane, while, externally, a render was applied. The total thickness of the facade was just under 120mm. By halving the thickness of the insulation, the useable floor area within the building was increased by 50sq m. Kingspan’s Optim-R VIP comes in thicknesses of between 20mm and 60mm. The company has compared its insulating properties with other commonly available insulation products and discovered that 200mm of a typical glass mineral fibre is equivalent to just 40mm of Optim-R, to achieve the same performance.

The thinness of VIPs offers obvious advantages in an era in which ever-tougher insulation requirements mean that greater thicknesses of insulation are needed, and living and working spaces could become proportionately smaller as a result.

There are additional benefits for refurbishment projects, where retrofitting improved insulation can lead to unforeseen costs if it is thicker than the building was designed to take. Extra measures are often needed to accommodate thicker walls, floors or roof — for example, the extension of eaves.

Multiple uses of VIPs

The high thermal efficiency and minimal thickness of VIPs may be an advantage in a number of other applications where space or structural depth is an issue. It should be remembered that, regardless of the application, VIPs should not be cut or penetrated because this would remove the thermal advantage of the vacuum.