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.
Alternative solution: use an underfloor heating type which goes over your existing floor. Polyplumb do one called Overlay, which was what we had installed. 18mm thick panels are laid on the floor, which have grooves for the pipes to run in.
Obviously you're either a) going to need to put this in every room downstairs, or b) have a level difference between this room and the rest of your downstairs.
Best Answer
You may be able to use the existing radiator piping to some extent, but a number of other changes will be required:
Lower water temperature. Because the radiant heating runs under your entire floor, it is typically run at a temperature similar to a hot summer day (around 85 degrees Fahrenheit). Cast iron radiators are usually run at around 150 degrees F, which would be painfully hot to stand on. So if you are keeping any radiators, your system will need to support running at multiple temperatures. According to this piping schematic from hydronicpros.com the device that can both control your boiler and mix water for a lower-temperature section is called a "variable speed pump mixing control".
System balance. Changing the way a room is heated may take it out of balance with the rest of your home, leading it to get too hot or too cold. You'll want to get a professional to assess whether this will require additional thermostats / control modules, or whether there's a simpler way to balance things.
Under-floor access. For hot water radiant heat, you'll need to install PEX on the underside of the subfloor. I don't think you can do that without access from underneath. Probably not a problem for your kitchen but may add some work for the upstairs bath. Correction 2 Feb 2012: Actually, there are hydronic systems that can be installed above an original subfloor (inside a newly poured cement layer, under tile). These may cost more and would likely change the level of your floor, but are worth considering.
One alternative you may want to consider is electric radiant heating. Electric radiant is available in mats that are layered with thinset between the subfloor and tile. The mats can be custom made to the shape of your room, e.g. to flow around a kitchen island. The system will come with a temperature sensor and control that is typically mounted in a wall box next to the light switches for the room. In this case, you'd simply remove the radiators, and use electric heat for those rooms instead.