How it works
A vacuum does the work.
No gel, no trick. Two walls and nothing between them.
Two walls
An inner and an outer bottle of 18/8 steel, welded at the neck and the base.
The gap
Air is pumped out of the gap and the base is sealed. Heat needs something to travel through. A vacuum gives it nothing.
The lid
Most heat leaks through the lid. Ours is double walled too, with a silicone seal and a small opening.
The test
Filled with ice water at 7am and left at 32 degrees in the shade. 4 degrees at 7am the next day.
Hot
Same physics the other way. Twelve hours for tea, because tea starts hotter than ice starts cold.
Cold at 4pm.
And at 7am the next day, if you forget it.