Aluminium foil insulation: what the aluminium is doing

How a reflective surface works

A polished aluminium surface has an emissivity of roughly 0.03–0.05 against about 0.9 for plaster, timber or wool. Facing an air space, it emits and absorbs a small fraction of the infrared a normal surface would. That is the entire mechanism: it needs a space to act across, and it does nothing about conduction through material it touches. Air gap guide.

True aluminium vs metallised film

Some products use a true aluminium foil outer layer; many use a polyester or polypropylene film coated with a thin aluminium layer. Both can achieve low emissivity when new; true foil is more robust against creasing and has different fire behaviour, while films are lighter and tear-resistant. Emissivity degrades with dust and oxidation, which is one reason a sealed air space matters. Certificates state the tested product, so compare certificates rather than marketing about the metal.

Products that use it

• Multifoil quilts: several reflective layers separated by wadding, certified with air spaces.
• Bubble foil: one or two reflective faces on a bubble core — a radiant barrier.
• Foil-faced PIR board: aluminium facing on a rigid core.
• Reflective membranes: low-emissivity face on a VCL or breather membrane.
• Foil tapes: sealing joints so the layer is continuous.

Where aluminium foil insulation helps

• Roof slopes and loft rooms, as part of a certified multifoil build-up.
• Summer overheating: reflecting radiant gain under a roof.
• Outbuildings, behind radiators, under metal roofs.

Where it does not

• Pressed flat between two surfaces.
• As the sole insulation on a loft floor or in a bedroom ceiling.
• In a cavity wall.

Questions homeowners ask