Introduction: Cheap Radiator Reflectors to Lower Your Fuel Bills

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Central heating radiators do a good job of warming the wall behind them, which isn’t a great use of energy particularly if the radiator is on an uninsulated outside wall.  A reflector positioned behind the radiator will save some of this wasted energy, meaning that the room should heat up quicker and the boiler will consume a little less fuel in maintaining the temperature.  The savings will not be great, but even a small saving is worth having – if you can get it cheaply.  And since this project makes use of cardboard boxes that can be obtained for free, it is very cheap.

The UK’s Energy Saving Trust (an independent organisation that works in partnership with the government and other bodies to provide energy efficiency advice in the residential sector) currently recommends two manufacturers’ radiator reflector products (see here:  Buying enough of the cheaper product to suit 6 “average” radiators (which might just be enough for a small house, if you only treat the radiators on outside walls) will cost £18 including delivery.  But if you live in a solid-walled old house like mine, your radiators (and energy bills) will be rather bigger than “average” and you would need to spend considerably more.  This Instructable shows how to make cheap and cheerful reflectors from corrugated cardboard and sunshades, windscreen protectors or metallic plastic film.  They scarcely show when they are in place and they can easily be removed for cleaning – it’s a good idea to do that at least once a year, at the start of the heating season.

There is some merit in fitting reflector panels behind radiators on internal walls too, and on external walls that are insulated, but the savings will not be as good so do the external uninsulated walls first.

Materials and tools
  • Cardboard cartons or other source of corrugated card
  • Reflective silver coated material
  • Electricians’ tape (preferably in a colour that matches the decor) or decorative silver sticky tape
  • Double sided tape or a stapler
  • File spines/binders of the slide-on type (optional)
  • Bamboo kebab skewers or wooden cocktail sticks (may not be required)
  • Craft knife and straight edge
  • Rule/measure
  • Big cutting mat or a pile of newspapers
  • Pencil
  • Pen that will write on your reflective material

Step 1: Notes on Materials

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:

  1. 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.
  2. 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.
  3. Car sunshades and windscreen protectors. These can often be bought cheaply in markets and from pound shops. Look for silverised ones – radiated 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.

Step 2: Working Out What Size to Make Each Reflector

Radiators are hung on the wall using two or more vertically mounted brackets.  The top of each bracket will be a few inches below the top of the radiator, and the outermost brackets will be a few inches from the ends of the radiator, so that they are hidden from view.  You are going to make a reflector panel that is also suspended from the brackets, but quite how will depend on how near the brackets are to the ends of the radiator and its top edge, and also on whether there are three or more brackets rather than just two. 

The simplest arrangement is where there are just two brackets, they end close to the top edge (say less than 6 or 7cm, 2.5 or 3”) and are no more than about 15cm (6”) in from each end.  This calls for a simple rectangular panel of a width that will just fit between the brackets and a height a little less than the height of the radiator.  A tab protruding sideways from the top of each side edge hangs the panel from the brackets.  See diagram 1 (the first PDF).

Where there are one or more brackets between the ones at each side of the radiator, you can either cut slots in the reflector panel so that it will slide down over the intermediate brackets, use pieces of file binder to “bridge” across them, or make a separate panel (each with tabs) for each inter-bracket section of the radiator.  See diagram 2.  Slots are good where there is a big gap between the top of the brackets and the top edge of the radiator, file binders otherwise.  You can also use file binder pieces at the ends instead of cutting tabs, as long as the gap between the top of the brackets and the top edge of the radiator is small.  The small file binders that are meant to hold up to 25 sheets of paper are best.  They can be packed out with strips of cardboard at the back if they don’t grip tightly enough. 

If there’s a big gap outside of the endmost brackets, you might want to make the reflector panel extend beyond them to stop as much heat loss as possible.  You won’t be able to take the reflector right to the edges of the radiator or it will be visible from the sides, it’ll need to stop a little way short.  Cutting slots is the way to achieve this as the file binder solution works best when a section of the panel that is supported by file binder at one side is also supported (by file binder or otherwise) at the other side.  See diagram 3 (the second PDF).  Using slots has another advantage over file binders, tabs or skewers (see below) – the panel will be held tautly in the horizontal direction.  This can be important if the gaps between brackets are large as the cardboard has a tendency to bow rather than stay flat, and can touch the back of the radiator somewhere in the middle, reducing its heat output.  Using the cardboard with the corrugations running horizontally should avoid the bowing effect, but it is not easy to find cardboard boxes that are large enough.

If a radiator is attached to the wall by brackets that stop some distance below the top, cutting slots in the reflector panel will avoid a bare expanse of wall above the brackets.  Alternatively, protruding tabs can be cut below the top edge or else lengths of kebab skewer (or stiff wire) can be used to join sections together below the top edge and act as hanging points.  See diagram 4. 

For each radiator you want to treat, decide how many cardboard panels you need and how they are to be suspended from the brackets.  This will depend on the size of the radiator, the number and position of its brackets and how big the cardboard pieces (and possibly also the pieces of reflective material) are that you have available.

Step 3: Cutting and Fitting the Cardboard Panels

Assuming you are making use of cardboard boxes, the first job is to disassemble a box and open it out flat.  Avoid cutting it if you can, just ease apart the glued joints with your fingers or a blunt tool.  If possible, cut the panel from the sides of the box so as not to have any horizontal folds in it, but it may be necessary to use some of either the top or the bottom of the box in order to get a panel that is tall enough to suit the radiator.  The height of the panel needs to be about 25-50mm (1-2”) less than the height of the radiator so that it is not visible from the front.  However, if the bottom of the radiator comes below the top of the skirting board, the panel may need to be shorter so that it sits neatly against the wall on top of the skirting board or just above.

As for the width of the panel, that depends on whether it is to extend beyond the outermost brackets or not.  If it is going to, then cut a rectangle that is about 50-100mm (2-4”) less in width than the width of the radiator, otherwise cut it about 12mm (1/2”) narrower than the gap between the outermost brackets plus another 100mm (4”) for tabs if that is how you are going to suspend it.  For wide radiators you may need to use two or more panels with each one ending at a suitable bracket.  If your cardboard is wide enough then don’t forget to leave an extra 50mm (2”) at the side wherever a tab is needed.  But you can always use the kebab skewer / coffee stirrer or file binder solutions otherwise.

Draw out the rectangle with a pencil and straight edge, then cut it using the craft knife against the straight edge.  (Cut on a pile of newspapers to save your table/bench/floor, unless you have a large cutting mat.)  Cut out the tabs if required, making them stretch a little further down the sides of the panel than you think to start with so that you can make adjustments if necessary to get the top of the panel level and the right distance below the top of the radiator.  Mark and cut any slots, again ending them below the point you think they need to, you can always extend them later to allow the panel to drop further down.  Make the slots wider than you think too, they need to be at least 12mm (1/2”). 

Try fitting the panel in place and adjust it as necessary until it fits well and hangs at the right height.  It shouldn’t be too tight a fit between brackets because it will get a little bigger when the reflective material is added.   Mark the top of the panel (if it doesn’t have slots or tabs – it will be obvious which way up it goes if it does), the side that will be towards the radiator and the left and right.

Step 4: Neatening the Edges

If you are using a thin reflector film you can skip this step because you will be taking it over the edges of the cardboard.  But with a thicker material such as a sunshade, it is best to neaten the edges of the cardboard with tape.  It looks better if the reflector panel is visible (and the top edge, at least) is likely to be visible to someone standing close and looking down), it stops dust getting into the corrugations (assuming they run vertically), it makes it easier to wipe the edges clean, and it may even stop a little convective heat loss by sealing the vertical airflow path through the corrugations.

Apply electricians' tape, silver-coloured festive gift-wrapping tape or a similar plastic or metallic tape around all the cut edges, including the slots and tabs, folding over the edge and smoothing it down on both sides. Take care to make the top edge that will be towards the room neat.  Ideally, use tape that is silver to match the reflector material or else choose a colour to match the wall behind the radiator.

Use strong tape such as carpet tape or duct tape to strengthen the tabs and the top of each slot.  Apply it to both the front and the back.

Step 5: Adding the Reflective Material

Lay your reflective material on the floor wrong side uppermost and lay the cardboard panel on it face down (ie the side that will be towards the wall uppermost).  Draw round it with whatever kind of pen you can find that will work on the material in question, ignoring any tabs – just draw a rectangle.  Don’t bother about cutting slots just yet.  Add a border of a couple of inches all the way round if you are using a thin film such as a space blanket.  Cut around the line. 

If you have cut out the reflector around the cardboard without any border, turn both cardboard and reflector over and check the reflector doesn’t protrude beyond the edge of the cardboard anywhere.  Trim it if it does, or the panel may not fit. 

You can either stick the reflector to the cardboard using double sided tape or staple it on.  Either way, be sure to get it the right way up as there may be a small difference. 

For a non-film reflector, put double sided tape all the way around the edges of the cardboard just inside the edging tape, and also put it around all 3 sides of any slots.  Put a few diagonal strips across the middle of the panel too.  Then peel off all the backing paper from the tape and press the reflector material down onto it.  If your panel has slots, cut away the reflector material from the back where it covers them.  Alternatively, just staple around all the edges.  You may need to borrow an office stapler intended for thicker stacks of paper than the average domestic one.

If you are using film, start by running a strip of double sided tape along the top edge of the back of the cardboard panel and sticking the overlap of the top edge of the film onto it.  Then lay the panel right side up and stick some diagonal strips of tape across the middle of it and around any slots.  Stick the film down onto it pulling it smooth as you do so, but don’t pull so tight that the cardboard bows.  Turn the bottom overlap over the bottom edge and stick it in place with more tape.  Then do the same with both side edges.  Finally, cut up the middle of each slot and fold back the film on the back to either side, sticking it down with tape.  Before considering using a stapler instead of tape to attach film, try stapling an offcut to a piece of card and then tug on it to check whether it will tear or not. 

Step 6: Finishing Touches

All that remains is to provide a means of suspension if you have made panels without tabs or slots.  If the radiator brackets end near to the top of the radiator then use file binders if you have them – cut an A4 binder in half and slide half on each end of a panel, leaving enough sticking out to catch on the top of the bracket.  If you don’t have a binder or the panel needs to be suspended higher up because the brackets are relatively low, then cut a bamboo kebab skewer in half or use a wooden cocktail stick.  Get someone to hold the panel in place behind the radiator at the right height and mark the point at each side edge that is level with the top of the bracket.  Then push the pointy end of the skewer/stick in at that point, between the layers of the corrugated card, perpendicular to the edge.  BE VERY CAREFUL not to skewer yourself in the process, bamboo is sharp, wearing an old pair of leather gloves is a good idea.

Leave your finished panel flat on the floor under a heavy rug overnight if you can, to get it good and flat and ensure that all the tape sticks well. 

Finally, slip the panel into place, pushing it back far enough for there to be a good gap behind the radiator.  If it bows and touches the radiator in the middle, stuff something like a cork or a small offcut of wood between radiator and reflective panel to hold it back against the wall.  Enjoy the moderately enhanced warmth.
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