Radiant Barriers can be installed on the attic rafters, the attic floor, or both.
Installed to the rafters (and exterior-facing gable walls), Radiant Barrier provides substantial summertime performance by reducing attic heat 30-40◦ F, on average. This means less heat impacting your living space, better performance of your existing fiberglass insulation (less likely to reach heat saturation), and less stress on your attic-installed HVAC system. Rafter installation is, however, higher in cost than floor installation and it provides limited benefit to the living space in winter.
Installed to the attic floor (and interior-facing walls), Radiant Barrier provides substantial year-round performance by reflecting heat away from the living space in summer and helping to trap the heated air in the living space in winter. It costs less than installing on the attic rafters. It does not, however, reduce heat in the attic itself which will continue to impact your attic-installed HVAC system.
Installing Radiant Barrier to both the rafters and the attic floor (and exterior and interior-facing walls), also known as a ‘Combo’ installation, will cost more but will provide the benefits of both methods of standard installation and will provide a double-layer of protection for your living space in summer