This is dependent on a number of factors.
- The color of the roof?
- The size of the roof?
- The ventilation in your attic?
- The amount of overall sun in your area?
I don't ever understand placing shingles over the old ones, but this will not effect the roof's reflectiveness. This and the ventilation are probably the two biggest factors.
For more information:
Read "Three Ways to Make Your Roof More Energy Efficient" for tips fromAmy Westervelt.