What is reflective foil?
In construction, reflective foil is usually a plastic film with a thin aluminium layer, intended to block radiant heat transfer. It comes as a standalone membrane, as a facing on mineral wool or insulation boards, in multi-layer multifoil insulation, on underfloor heating panels and as radiator foil. Some versions are micro-perforated, which makes them vapour permeable.
How it works
Heat crosses an air space in three ways: conduction, convection and radiation. Ordinary building materials have an emissivity of about 0.9 to 0.95, so they radiate a lot of heat. A bright aluminium surface is far lower, with foils ranging roughly from 0.02 to 0.20. The lower the value, the less heat is radiated across the gap.
The key point is that the foil only works when it faces an air gap. If the aluminium is pressed directly against another material, there is no radiation to reflect and the foil adds practically nothing thermally.
What it achieves
Using the calculation rules of EN ISO 6946, an ordinary vertical air cavity of 30 to 50 mm has a thermal resistance of about 0.17 m²K/W. With reflective foil on one side, a gap of at least 20 mm and a cautious emissivity of 0.10, this rises to about 0.57 m²K/W. That is a useful bonus, but it does not replace a proper insulation layer. Only in floors, where heat flows downwards, can foils on their own reach genuinely high values.
Application and tips
- Leave at least 20 to 25 mm of air in front of the reflective face. Narrower gaps lose performance quickly.
- Design with ε = 0.10, not the laboratory figure, to allow for ageing, dust and workmanship.
- Keep it clean. In masonry cavities, mortar and dust can cover the foil. Well-sealed voids, such as in timber frame construction, are more favourable.
- Check the vapour function. A continuous aluminium layer acts as a vapour barrier. On the cold side of the construction you need a perforated, vapour-permeable version.
- Be sceptical of claims. Figures suggesting a thin multifoil performs like a thick insulation board do not stand up physically. Ask for independently tested values for your specific build-up.
Related terms
- Multifoil insulation
- Insulation value (Rd value)
- Cavity wall
- Vapour control layer
- Reflection, absorption and transmission
- Glass wool
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