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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[note 1] 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%.[note 2]
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 reflectors 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[note 3] or 3mm polyethylene foam, as used for laminate flooring underlay.
There are only two radiator reflectors approved for use in the UK Government's Carbon Emission Reduction Target (CERT) Scheme administered by Ofgem (the UK Regulator of energy companies)– Radflek and Heatkeeper (also called Novitherm).
- The Stefan–Boltzmann law states that the effectiveness of a thermal radiator is proportional to the fourth power of its absolute temperature. A hot-water radiator at 77 °C (350 K) has only 1/4 the radiated power of a stove at 220 °C (493 K), or 1/16 that of a radiant element at 427 °C (700 K)
- N. König: "Der Einfluß von wäremreflektierenden Folien in Heizkörpernischen auf den Heizenergieverbrauch eines Hauses" https://www.ibp.fraunhofer.de/content/dam/ibp/de/documents/Publikationen/IBP-Mitteilungen-optimiert/058.pdf
- Widely sold in 2mm sheet rolls as an insulator beneath wallpaper.
- "Radflek radiator reflector". Radflek.