Reflective foil works by blocking the flow of radiant energy. Radiant energy is what you feel when you stand out in the sunshine. Something like 85% of the heat that is lost through the walls during the winter is of type radiant. The number is more like 95% through the roof. Foil type insulation is most effective when it is installed with an air gap on both sides. Some tests that I have done indicate that the foil does a better job than the 1-1/2" of fiberglass insulation that was stock in our Airstreams. There are other tests being done now too. Take a look at the following threads:
Insulation - real test results...
Insulation effectiveness tests
The following thread has some tips and tricks that I used for foil insulation installation:
http://www.airforums.com/forums/f46/...cks-30196.html
Malcolm