Measuring attic temps is not as straightforward as I thought. For years I had a temperature probe stuck up through the attic floor, about 12 inches above the insulation and I thought I was "measuring the attic temperature". Well that point is very different from other places to measure. In addition to that I have a remote temperature sensor hanging in the air just below the radiant barrier, and another placed above the RB.
As I write this at 10am in July, the measurement is 88F below the RB and 93F above it. The point above the attic floor is 84F. I did a study on typical temperatures in Sep-Oct 2007, when the reported peak was still low-mid 90's. Generally saw 125F commonly above the RB in late afternoon, with about 110F below the RB and 95F at the attic floor. On such day (10/4/2007 I think) the official high was reported at 92F but my back porch saw only 86F. All that looks pretty nice plotted on a graph but it's hard to figure out what it all means. Whenever someone say "attic with RB should be within 10 degrees of outside temperature" I listen to them but don't know whether I need to do something else to live up to that claim. IMO there is no one number which clearly describes attic temperature.
This is as good a time as any to say I REALLY admire the toughness of those Texas AC guys who have to go up into these attics on summer days!
Hope this helps -- Pstu