I have a two story brick colonial with attached garage to one side of the main structure. The garage is consistently very hot (102 at the ceiling yesterday when ambient outdoor was 92). The temps down near the bottom of the inside walls and of the slab itself are close to outside ambient. There is a separate roof elevation above the garage that is vented, but has very poor airflow and no insulation on the floor of that roof space.

I plan on a new roof very shortly and getting some better ventilation in the peak, but I'm wondering if I still need to put in a layer or two of fiberglass (or similar) down at the floor joists to keep the radiant heat in that space from transferring into the garage below.

Thoughts?
Dan