Radiator reflector

A radiator reflector is a thin sheet or foil applied to the wall behind, and closely spaced from, a domestic heating radiator. The intention is to reduce heat losses into the wall in the belief that this will increase the output of the radiator and thus reduce fuel expenditure.

Although the foils are termed "reflectors", they do not have much effect on radiated heat or its reflection. As radiators work at a relatively low temperature, the Stefan–Boltzmann law means that they are weak radiators of heat. Most heat from a domestic radiator is as convection currents of heated air. Where a reflector foil also has some insulating ability against conduction (i.e. losses through the wall), it may have some useful effect. This is most pronounced when the wall itself has poor insulation performance: in a wall constructed to modern standards of insulation, even this effect may be reduced to a negligible benefit.

The effect of placing a 10mm combined insulation and reflection behind radiators is about the same as that of 15mm insulation without a reflective layer. When the wall thickness behind the radiator is at minimum 1980 German standards this will reduce total heat losses of a building by about 4%. For a (by 1980s standards) well-insulated building heat losses can be reduced by about 1.6%.

It is widely believed that a literal radiator reflector of ordinary aluminium kitchen foil is useful. This highly reflective foil is used with the shiny side facing towards the back of the radiator. However, as little of the radiator's heat is released by radiation anyway, there is little advantage in reducing losses to it. There is also a risk that made from kitchen foil may soon become inefficient, as aluminium oxidizes very quickly and then loses its reflective quality.

A more effective DIY radiator reflector is a thin insulating layer (against conduction) of a lightweight insulator such as expanded polystyrene foam veneer or 3mm polyethylene foam, as used for laminate flooring underlay.

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