DIY radiator insulation panels

Radiators mounted on outside walls waste a lot of their heat into the wall itself. It's possible to considerably reduce this by putting some sort of insulation between the radiator and the wall, and there are many products sold to do just this at varying costs and levels of effectiveness but the cheapest and best way is to make your own.

How bad is the problem?

Radiators deliver heat to the room in two ways. by emitting infra-red radiation and by directly heating the air in the room. We can find out how much heat is emitted in the form of infra-red radiation using the Stefan-Boltzhmann Law :

q = ε σ (T_h⁴ - T_c⁴) A_c

Where ε = emissivity of the object, σ = 5.6703 10^-8 (W/m²K⁴) - the Stefan-Boltzhamnn Constant, T_h = hot body absolute temperature (K), T_c = cold surroundings absolute temperature (K) and A_c = area of the object.

Taking a 'typical' 0.6m x 1.2m radiator, coated with white paint with an emissivity of 0.8 and rated at 1140W with the room at 20°C and the radiator at 70°C we find that q = 0.8*5.6703 10^-8 *(343 ^4 - 293^4)*(2*0.6*1.2) = 422W.

So about 40% of the heat is radiant, the remaining 60% is released by conduction meaning that 20% of the radiator's heat is potentially lost from the back of the radiator straight into the wall by radiation, but an additional 30% of the heat could be lost to the wall by conduction. This is a gross simplification because it assumes that the surface of the wall behind the radiator remains at 20°C, but it does serve to illustrate that relying on a radiant barrier alone is going to be a lot less effective.

How can I fix it?

There are many products sold for the specific purpose of insulating behind radiators, some are cheap, and some are expensive, so we need to know which one is the most effective.

Radiant Barrier Only

Heatkeeper manufacture polished aluminium panels that stick to the wall behind the radiator and aim to reflect heat back into the room. They claim to reduce the heat lost into the walls by 20%, which seems reasonable enough given the above calculation but then go on to make the totally bogus claim that fitting them behind all your radiators will reduce your heating bill by 20% even though not all radiators are on outside walls. In spite of Heatkeeper's claims to the contrary, placing insulation behind radiators on interior walls is a complete waste of time and effort -- the energy simply has nowhere to go except into the house. In fact that stored energy is beneficial as it adds to the thermal mass of the house and will reduce temperature swings as the heating cycles on and off.

All this wouldn't be so bad if it weren't for the price. £40 (+p&p) for 10 panels 0.58m x 0.45m (£15/m²)for something that only addresses half the problem, and you'd get the same effect with a bit of kitchen foil or survival blanket. I think we can do better than that...

Foil Faced Bubble Insulatioin

In the middle of the performance spectrum we have the rolls of foil-faced bubble insulation (FFBI) as sold in DIY stores, £20 for a 7.5m x 0.6m (£4.50/m²).

On the plus side it's easily available, can be cut with scissors and only needs a bit of double sided tape to stick it to the wall. Each roll is enough to do about 4 radiators, so overall it's fairly cheap.

The downside is that it's not really a very good insulator, there's a lot of marketing hype around FFBI 'radiant barrier' but in most real world situations the silver coating doesn't make a huge amount of difference because amount of heat transferred by radiation is only significant when the temperature difference between the two sides of the radiant barrier is quite high. This is one of the few situations in which radiant barrier might really be quite effective, but it's basic insulation value is only RSI 0.26 and even taking the foil facing into account it probably only manages RSI 0.7 or so. Normalising the cost by RSI gives about £6.50/m²/RSI

Foil Faced Polyurethane

The best performing option comes in 2.4m x 1.2m x 12mm sheets in the form of Cellotex TB3000 foil faced polyurethane insulation board. It's easy to cut with a sharp knife, thin enough to slide easily behind radiators and if you cut it right you don't even need double sided tape.

It has a basic insulation value of RSI 0.5, and applying the same correction factor for foil faced bubble insulation of an extra RSI 0.44 for the effect of the radiant barrier this could be as much as RSI 0.94. And the best news is that it's only about £12 per sheet (£4.20/m²), normalising cost by RSI give 4.60/m²/RSI -- the cheapest of the lot.

The bad news is that sometimes it's hard to get. I was lucky enough to find that my local Travis Perkins would deliver a single sheet to my door for £12, but I think I might have got lucky, if you find a good source be sure to mention it in the comments section.

Gallery