Air is a very good insulator, for instance the thermal conductivity of air is 0.24, which is much better than old fashioned glass fibre at 0.44 (the lower figure the better). Above and beyond that a sealed air gap next to a reflective surface improves the R value of the air, there is a useful EN standard called EN6946 that you can use to calculate the R value of an air gap if you have the emissivity (reflectiveness) of the foil.
A sealed air gap will be up to 200% improved if the emissivity of the foil is good. Air is a good insulator but its important that the air is kept still; convection currents in air reduce performance. This is why glass wool was used an an insulator for a long time, it works to keep air from moving around. If you compare the thermal conductivity of glass to glass fibre it is very poor (think how cold single glazing is); the same glass made into glass wool is a lot better because although its an open structure it’s trying to keep the air still. Foam board is the same, doing what glass wool does but more efficiently.
Multi layer foil insulation takes the technology further, combining sealed air gaps with reflective surfaces to not only create efficient air gaps but to create super efficient low emissivity air gaps. Each layer in Superfoil multi foil insulation creates an improved performance air gap and beyond that creates improvements between its self and the plaster board below and the secondary rain barrier membrane above.
In a roof EN6946 states that the efficiency starts to reduce in air gaps below 13mm, in walls its 20mm. Battens are incorporated in the construction detail of Superfoil multifoil to ensure the minimum air gap is maintained. A multilayer foil can reduce a timber frame wall from 300mm, sized to accommodate the amount of glass wool needed, to that needed purely for structural strength, which may be only 100mm. A significant saving on timber and labour is achieved.
Air is a good insulator and Superfoil multifoil insulation takes the greatest advantage of that. Air is also probably the cheapest form of insulation available! Superfoil multilayer foil insulations maximize the contribution of air, by applying science and technology to create a leading edge insulation.