Oh, it’s just a thing that gets to me as an HVAC tech. Everyone says heat rises. It doesn’t. Warm air gets pushed up by denser cold air.
Cold air does sink.
HVAC applications, functionally right.
Adding energy (heat) excited molecules, the orbits expand pushing each other apart, spacing apart.
Any mass becomes larger, expands, and that mass is less dense because it didn't gain 'Weight' (mass), but grew larger. Less dense by definition.
Heat expands metal, heat expands liquids, heat expands gasses, anything with mass.
A lava lamp works because the wax expands more and becomes less dense than the oil.
Mercury expands at a steady rate, so it's used for thermometers.
A warm 'Bubble' or column of air rises because it's less dense than the cooler air around it.
The less dense (heated) area rises in the ambient air column,
Physics, nothing much happens until something moves,
And the warmer rises (movement) and the cooler fills in underneath.
Cooler air does 'Sink', anyone that opened a fridge door in bare feet knows that.
BUT,
In that case, the 'Movement' is colder, denser air falling in the air column, being displaced from above with warmer room air.
Nature hates an imbalance and always tries to equalize.
In an insulated box, assuming the batteries do not touch the sides of the box, only the bottom would pull the majority of the heat from the battery. That is the primary reason for placing the heating pad on the bottom.
That might be a misspeak,
By that line of thinking, insulation UNDER batteries would keep cold from entering batteries entirely.
We all know that batteries would go ahead & radiate heat into the room...
Insulation all the way around would slow down the losses, but if warm air can escape, it's going to be REPLACED by cold air.
Seal up the cracks in the insulation, particularly the top since warm air is less dense and trying to rise,
The top is more effective as the 'Box', and the bottom as the 'Lid' will trap more heat.
I say 'Trap Heat', keep that warm air from rising and the inverted box makes the box fill with warm air before it reached the 'Lid' crack at the bottom.
