And if the RB was able to reduce attic temps, which in turn would keep the decking and shingles cooler, wouldn't I be at least "even" in terms of shingle life if not ahead by a bit?
I think the jury is still out on that verdict. But there is at least
some validity to your theory. You will have an increase in
radiant energy to the shingles, but since the temperature of the attic will be cooler, there will be an offset with greater
conduction and
convection energy loss from the shingles to the attic and there will also be an increase in radiant energy loss to the sky since not all radiant energy will be absorbed by the shingles on either the inbound or outbound path.
If there was a significant measurable increase in shingle temperature or decrease in shingle life, rest assured the shingle manufacturers would be denying warranty coverage on that basis. I have not heard of that, has anyone else?