Both types of heat are trying to enter your home on a hot summer day.
Fiberglass attic insulation vs reflective.
Air migrates from warmer spaces toward colder spaces and insulation is designed to help prevent or at least slow that migration.
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.
The reflectix site provides a great range of information about this and about how it differs from other types of insulation.
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.
Radiant barriers reflect radiant heat.
Remember traditional attic insulation has r value that works to slow conductive heat.
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 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.