An alternative that might work in some rooms is a ceiling fan that has a low speed (we have some 6-speed models in our home where the lowest speed just circulates the air slightly). This would help move air around in the room (and thereby helping to move some heat away from the radiators) and has the added plus that you can use it on higher speeds for summer cooling.
We have forced hot air heat, and our master and living room have high ceilings, so we run the fans year-round.
You are correct that having a hot radiator on what's effectively a cold space - the porch - is very inefficient and, presuming nobody's on the porch most of the time, wastes a lot of energy.
And you are correct that the hot water going into that radiator is being cooled in the process.
However, it is unlikely that the water is going to any other radiators, but rather it's going back to the boiler to be reheated. Nevertheless, you are right that stopping water flow to this radiator will save you on your heating bills.
But - if you stop the flow of water through the radiator, no amount of insulation will keep the pipe from freezing, because with no place for the cold water to go, the hot water in the rest of the system WILL NEVER reach the pipe you insulated. It's just like turning on the hot water in a cold bathroom - the water runs cold for a while even though the hot water is available elsewhere in the house.
On a cold enough day, the stopped pipe with water in it will freeze, no matter how hot the rest of the system.
The best solution to this is to remove the radiator completely. To accomplish this, you need to trace both the input and output pipes for the radiator in question back to an interior space that doesn't ever drop below freezing. The entire system will then need to be drained, and the input and output pipes cut off and capped so that the useless radiator can be removed.
EDIT:
Since you're looking for a short-term, this winter solution, here's what you can do for now to minimize the waste.
- DO NOT TURN OFF THE RADIATOR
- DO turn down the radiator to as low as you can get it without shutting it off. You don't want it to have a chance of freezing.
- Wrap the radiator in blankets to keep in the heat it does generate and conserve energy as much as possible.
Best Answer
Statistics? I'd argue this is a misuse of the word, unless you are asking what is the fraction of photons reflected by the reflective surface. Since that truly is a probabilistic question, we could indeed compute statistics on the distribution of such reflections.
Yes, this is not just a theory. Heat is transmitted in several ways. One such way is convection. So the radiator heats the air around it, which will rise, displaced by heavier cool air from the rest of the room. These convective air currents cause a transmission of heat from the radiator into the room.
A second way that heat is transmitted is by radiation. Infrared photons will carry off energy (heat) from the radiator. Some of these photons land on the wall behind the radiator. If the wall absorbs them, then it will get warmer. So the trick is to put a reflecting surface in the way. A SHINY metallic surface is such a solution, a heat mirror. Thus a shiny metal foil will suffice. Even a regular mirror, which is simply a piece of glass with a metallized reflective back surface that reflects light, will suffice.