The reflectix site provides a great range of information about this and about how it differs from other types of insulation.
Fiberglass attic insulation vs reflective.
Next i compared adding an additional r 19 to my existing r 19 level of attic insulation vs installing radiant barrier and the results actually surprised me a little.
If your attic has a lot of nooks and crannies your best bet may be to use loose fill insulation made of fiberglass or cellulose.
The sun heats up the roof and then the heat is transferred by radiant heat across the attic space until it hits the attic insulation.
What is the right insulation for an attic.
The key to using reflectix properly is that you should take a little time to understand how reflective insulation works in the first place.
Radiant barriers and reflective systems.
Both types of heat are trying to enter your home on a hot summer day.
Radiant barriers reflect radiant heat.
The radiant barrier resulted in roughly 4 times more savings than the added insulation.
Which is better will depend on the topography of the specific roof you re insulating.