Is it worth doing this? If you've got cavity wall insulation I would have thought that the radiator would heat the inner wall but the heat wouldn't get past the cavity wall insulation. So the inner wall would retain the heat and act a bit like storage heaters, generally adding to the comfort and warmth of the house. The heat wouldn't be lost. What do people think? Mike
Foilfaced R 7.2" Polyisocyanurate foamboard will help it is the highest R value foam made, it also has a foil radiant barrier, you do loose heat heating a cold wall, the heat goes outside. Regular foil will reflect only the radiant portion of heat of what is emitted, and it doesnt insulate.
Good article on this in this week's Sunday Telegraph. It says the same thing about the storage properties of the wall, but goes on to point out that since many heaters are below windows, which may have condensation, the warm wall does not stay wet from dribbles running down.
The heat may not be totally lost, but rather than increasing the room temperature, it increases the wall temperature. If you think of your CH system as money spent to heat a particular thermal mass, what you really want to do is to reduce the thermal mass that you are spending money to heat.
You only benefit from 'heating the wall' if you switch your heating off earlier at the end of the evening and rely on the walls to keep the rest of the room warm - but given that your heating system has spent more money at the start of the evening to heat up the walls you're probably not ahead overall.
I suspect in most cases, the 'benefit' will be to heat rooms when everyone is asleep; IOW, it could be considered a reasonable approach for bedrooms, though a better quilt would be cheaper overall.
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You seem to have overlooked the point of the question - if there is cavity wall insulation would the heat that reaches the wall be lost. The short answer to that is that however well you insulate some heat will always still leak out but with cavity wall insulation the amount is probably too small to worry about.
You also seem to have a fundamental misunderstanding of the situation. Foil does insulate (dictionary definition "To prevent or reduce the transmission of ... heat ..."). Furthermore in the restricted circumstances of the wall behind a radiator the main route of heat transfer of heat from radiator to wall is radiation, direct conduction is restricted to the radiator brackets and most of the convected heat rises up and escapes into the room at large rather than through conduction from air to that bit of the wall.
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