On gen 1 do u block upper or lower grill

I was reading about this feature on the C-max. Apparently Ford and GM both have some models with automatic powered grille louvres.

My take on this is that the primary objective is to improve aerodynamics, with the secondary benefit (esp on a hybrid) faster warmup and better engine temp control. Given that, blocking the outside seems like a better strategy than blocking the radiator behind the open grille. I'd guess that that would make aerodynamics worse...

I'm using pipe insulation in both lower openings and will be watching temperatures as I drive from Canada to Florida next week.