Removing your headliner and insulating/sound deadening

Heat barriers are usually radiation heat not conductive or convection. If there is no air gap, reflective materials do nothing. This is the same reason it's pointless to install attic insulation with the reflective side down. Assuming this is the type of insulation that you have in the roof liner, I'm not surprised that you didn't notice a difference.

 

here’s what I used in the headliner
https://www.secondskinaudio.com/heat-insulation/heat-wave-pro

Maybe there’s a slight difference but I honestly didn’t find it noticeable there was.

 

Sitting baking in the sun all day, sure. It won't keep it cooler in direct sun.

The difference is how it retains cool and hot after you've affected the cabin externally. It will stay cooler longer, or warmer longer after running the heating or cooling system.

 

Yea I guess I'm just not sure how to really measure that difference.