Corrugated cardboard is perfect for radiator reflectors because it is free, lightweight (you’re going to hang it from the radiator brackets) and it provides some insulation – the proportion of heat from the radiator that passes through the reflective layer, the cardboard layer and the cold film of air behind the home-made reflector panel will be minimal. The corrugations make the cardboard stiff in one direction and fairly stiff in the other, and even in the stiff direction there will in most cases be enough flex to slip a panel into place where there is a shelf mounted above the radiator. Suitable cardboard cartons are given away for free at supermarkets, wine merchants, electrical stores and garden centres, or pressed flat and left outside High Street shops on rubbish collection day. For preference, choose sturdy boxes that have held something heavy like wine or tins of beans, the cardboard is stiffer than the boxes used for packets of cornflakes. If you can find a box that has been used for something large like a fridge or washing machine, so much the better because one side of the box may be large enough for a radiator and then you won’t have any folds in the cardboard. However, don’t go for a super-thick cardboard unless you have lots of space behind your radiators. See the next step for advice on working out how much you need.
For the reflective material, you need to find something that is cheap but will not lose its shine. Kitchen (aluminium) foil is not ideal because once the sheet is unrolled and exposed to the air the aluminium quickly develops a tenacious oxide coating that dulls the surface, meaning the radiant heat from the radiator will not be reflected as efficiently. (Yes, I know that most of the heat loss from a central heating radiator is by convection, not radiation, but it’s radiated heat we are concerned with here because the convected heat is going to continue rising up and heating the room whether there is a reflector panel behind the radiator or not.) In the photos I have used proprietary, foam-backed reflecting sheet that is meant to be cut to size and then stuck to the wall behind a radiator with double-sided sticky pads. That’s what I did when I bought it about 15 years ago, and within a year the pads had lost their stick and the foam sheets curled up, stopped doing their job and gathered dust so had to be removed. Possibly you have some of this stuff lying around too. If not, here are some suggested alternatives:
The silver emergency blankets or “space blankets” that you see being wrapped round the shoulders of marathon runners when they stagger over the line. These survival blankets are made from very thin and lightweight, but quite strong, mirror-finish polyester film. They typically measure about 200-210cm x 150-160 cm, which is enough for 4 decent-sized radiators or 2 large ones even if you wrap some of the film around to the back of the cardboard – which is best when using a thin film material as your reflector. They can be bought singly from Clas Ohlsen in the UK for £2.29 at present, and probably from numerous outdoor/mountaineering shops too. There is a supplier on UK Amazon selling 5 of them for under £5.
Reflective horticultural film. This can be bought by the metre/yard from gardening suppliers that cater to those who grow, shall we say, “alternative” crops. Try Googling “hydroponics supplies”. The film is used to keep energy bills down by reflecting light and heat, and presumably has the added bonus of helping indoor growers of illicit crops hide what they are doing from law enforcers’ heat-sensing cameras. There are various types of film available, including silver mylar. Read the supplier’s information carefully before choosing which one to go for, each has its own pros and cons. For example, the silver mylar is highly reflective (95-97%) but somewhat difficult to handle and has to be kept dry, which means you won’t be able to wipe it down with a damp cloth when it gets dusty.
Car sunshades and windscreen protectors. These can often be bought cheaply in markets and from pound shops. Look for silverised ones – heat and visible light are just different parts of the electromagnetic spectrum, so something that looks silver and shiny will tend to be a good reflector of heat too. Sunshades and windscreen protectors are much thicker than the plastic films used for space blankets and horticultural purposes, which makes them easier to handle. Aldi were selling 180 x 80 cm windscreen protectors recently for £1.99.