“Radiators still have to be turned on and reach a high temperature for a lengthy period of time to heat a home for the foil to have the desired effect, so this will have a slight impact on the warmth in a room, but the costs will end up being basically the same,” says Steele.
Article source: https://www.huffingtonpost.co.uk/entry/heating-hacks-that-dont-work_uk_63624b17e4b045895a982445