Thermal Insulation: Vacuum-insulated panels
Vacuum-insulated panels achieve higher insulation values with thinner construction
Thermal insulation requirements are becoming increasingly exacting, to reduce both energy costs and CO2 emissions.U-values of 0.6W/m²K, which became the norm following the energy crisis of the 1970s, now seem extravagant. But does this mean that external wall thicknesses will just get thicker and thicker? Not necessarily.
Vacuum-insulated panels (VIPs) achieve much higher insulation values than conventional insulants, such as mineral and glass fibre and cellular foam. This means that walls, floors and roofs can be thinner and the ratio of gross internal area to gross external area will therefore be higher. An additional benefit of reduced wall thickness is that more light can enter buildings through openings.
VIPs can also be used to achieve higher insulation values where there are limitations on construction thickness, for example in conservation projects, where new and existing surfaces have to align. Reduced ground floor and roof insulation thicknesses also means lower buildings or increased room heights.
But VIPs are new to the construction industry, so there are a number of caveats. Cost, product life and performance are the main areas of uncertainty.
What are VIPs?
Like thermos flasks, VIPs reduce heat conduction and convection. Originally developed for use in the manufacture of refrigerators in the 1980s, they comprise factory-assembled, gas-tight enclosures with rigid fibre, powder or foam cores, from which the air has been evacuated. These cores hold the vacuum and prevent the enclosing membrane from collapsing due to negative
pressure. Centre of panel U-values for VIPs are less than 0.005W/m²K, and are approaching 0.001W/m²K. Thermal resistance is 11.8-19.7R per centimetre, compared with 1.7-3.1R per centimetre for foam insulation and 0.8-1.4R per centimetre for glass fibre batts.
There are a number of potential applications in construction: masonry cavity walls; external walls with insulated render; prefabricated cavity wall units; panels, which are framed into curtain wall systems in the same way as double glazed units; roofs; and ground-floor construction.
Capital costs of VIPs are significantly higher than those of conventional insulation. Architect Ian Abley of construction research company Audacity helped to organise the International Vacuum Insulation Symposium, held in September 2009. He advises specifiers to ask manufacturers to quote prices in pounds per unit of thermal conductivity. These costs can be offset by gains in net area, room heights, insulation and by savings in construction and running costs.
Unlike glass fibre insulation, the lifetime of VIPs is a concern. Because it is impossible to completely prevent air from filling the vacuums in VIPs, their maximum life is currently 30 years. For this reason, they are inappropriate for use in cavity expected to last 100 years. It is difficult to control quality. Core materials must be carefully handled and the internal gas pressure is critical. According to manufacturer va-Q-tec, 90 per cent of VIP failures involve invisible micro-leakages or pin-holes.
Abley outlines other points needing attention. Although the core material of VIPs can achieve a class-0 fire rating, the enclosing membrane can be a fire risk. Panels may need to be lined in order to control reverberation noise and wall ties, which could be fibre-reinforced plastic, need to be integrated with panels and avoid cold bridging.
Pool Architects’ Seitzstraße apartments, Munich
Perhaps the best example of VIPs in construction is Pool Architects’ apartment block in Munich, completed in 2005. The building has 20mm-thick VIPs sandwiched between the inner leaf and 80mm rendered insulation. According to architect Martin Pool, this high-performance insulation specification provides 125m² of additional area.
VIPs reduced the thickness of the roof by 250mm and Pool was therefore able to increase the height of all rooms by 50mm. VIPs insulating the rooms below balconies reduce the level change at the thresholds by 120mm.
• Porextherm Dämmstoffe manufactures VIPs for use in the construction of walls, roofs, ground floors and as curtain wall panels.
• Another German company, va-Q-tec AG, manufactures va-Q-vip panels. The German Institute for Building Technology recently granted this product the first approval for a VIP. Panels are coated with a black glass-fibre textile.
• Nanopore Insulation LLPP in New Mexico manufactures VIPs for various applications, including shipping containers. It states that the core materials are silica, titania and carbon, and product
lifetimes are up to 20 years. Panels are encased in a metalised-plastic barrier film.
Manufacturers and researchers continue to refine VIPs, discussing quality control, testing procedures and certification. There is particular interest in developing panels with 100-year lifetimes, and in reducing the cost of microporous core materials.
Stainless steel casings will achieve better fire ratings and there is enormous potential to integrate VIPs into prefabricated cavity walls. Researchers have also investigated the possibility of vacuum insulation that can be cut to fit and materials that can retain their properties when machined or accidentally damaged. According to Abley, ‘This remains an aspiration, but a good example of novel thinking about insulation.’
For now, specifiers will have to wait for further developments, but they will do well to watch them closely.