When we insulate the joist boxes in apartment buildings or commercial buildings we are required to close the boxes over the foam with drywall or flame retardant foam board. Since your job is for utility purposes here's my thoughts. After you spray a couple of inches of foam into each box, cut measured pieces of foam board that will snugly fit above your wall, between the joists and up against the 1" rise of the sill plate behind the wall. Put a few dabs of construction adhesive on the edges and fit them into place. This will do a couple of things for you. It will create a larger dead air space in your now insulated boxes, extra R-value and also seal the top of your wall against the foundation. Smaller pieces can be glued in under the joists to complete the wall seal. If you want to further seal and joints in the foam board, use some HVAC aluminum tape.
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
You can control the depth of cut very accurately with a circular saw.
However, banging pipes are usually caused by water-hammer, and securing the pipes doesn't completely cure the problem. You (or a plumber) can install a shock arrestor (various types are available) near the valve that is causing the problem.