What is the best way to protect wood studs and fiberglass insulation when you are soldering inside wall stud bays ? I have to solder a 1.5 inch elbow and there is only about one half inch between the wood stud and the elbow. Someone told me that aluminum foil is a good heat shield. I have some thin sheets of metal that I could use but if the metal is resting on the studs would'nt that burn the wood studs ?

Duce, there is a liquid gel type heat barrier solution that comes in a spray bottle that protects wood, insulation. And really anything from the heat of your torch. There are different brands now available and your local plumbing supply store should be able to fix you up.

Be sure to liberally squirt all the surrounding surfaces that you want to protect. One piece of advice, becareful not to get any of the spray on your actual copper pipe. The pipe will not heat and soldiering will be near impossible.

A thin sheet of metal can be used, but the fire retardent gel is safer. Here is an example of one brand. It's the fourth item down the page.
Fire Retardants, preventers, extinguishers and heat barriers

They now make a liquid solder that I was told about and it works great I used it to install my hot water heater 3 years ago and still no leak but you have to buy it at a plumbing supply house and its not cheap but it stops you from having to solder anything I personally highly recommend using it.