Here is an evaluation by Vilnis Vesma, an Energy Management consultant. I pass it on without comment, other than the usual disclaimer that I have no connection with him.
RADIATOR REFLECTIVE FOIL
Putting a foil reflector behind your radiators "dramatically cuts heat loss by reflecting warmth back into the room" according to the sales blurb at Nigel's Eco Store, a well-meaning online source of environmentally-friendly products. "Cuts heat loss by up to 50%" according to the Screwfix web site.
Are they telling the truth? In the case of Screwfix clearly yes: there will be some benefit, and "up to" 50% is not a challenging claim. But are the savings "dramatic"? I have worked it out and in the UK climate at current gas prices the annual saving per square metre of foil is about £2.50 at most. This assumes the radiator is on an uninsulated outside wall and provides 24-hour heating seven days a week. For a moderately well-insulated wall (e.g. a filled two-inch cavity) the savings plummet to about 40p per square metre of foil per year, less if heating is intermittent.
The full analysis is at www.vesma.com/downloads/radiator_foil.pdf.
[Screwfix is a large UK, Eire & Germany on-line and trade supplier of Tools, Plumbing & Electrical equipment.]
Re: Radiator Foil - exaggerated energy-saving claims
and most radiators work by convection so cold air in at bottom and hot out the top
i too have a hard time see much savings as most heat is radiated upwards anyway