Insulated drinkware works by blocking three separate routes for heat to escape, and how well it does that depends on the construction.
The Three Routes
Conduction, which is heat moving directly through the material of the wall. A single-wall mug loses heat this way continuously, which is why the outside gets hot.
Convection, which is air movement carrying heat away from the surface.
Radiation, which is heat emitted as infrared regardless of any air movement.
What Double Wall Does
- Two walls with a gap between them
- The gap slows conduction, because air conducts poorly
- The outer wall stays cool, which is why you can hold it
- A vacuum in that gap removes conduction and convection almost entirely
- Which is why vacuum-insulated performs far better than plain double wall
Where a listing states double-walled plastic rather than vacuum-insulated stainless, of the kind on some models at https://copco-usa.com/, that is a meaningful difference in how long a drink stays hot rather than a wording preference.
Plastic Double Wall Versus Vacuum Steel
Plastic double wall keeps a drink warm for a while, keeps the outside comfortable to hold, and is light and cheap.
Vacuum-insulated stainless holds temperature for hours rather than a while, and costs several times as much. It is also heavier and it can make coffee taste metallic to some people.
Neither is wrong; they suit different journeys.
Preheat It
The single most effective thing you can do, and almost nobody does it.
Filling the mug with boiling water for a minute and tipping it out before adding the drink means the walls are not absorbing heat from your coffee.
It makes a substantial difference on a cold morning.
Fill It Full
A half-full mug has a large volume of air inside cooling the drink. Full holds temperature considerably longer.
The Lid Is the Weak Point
Most heat escapes through the lid, which is usually thin plastic with an opening in it.
Closing the drinking slider between sips is worth doing, and it is the difference people notice most.
Cold Works the Same Way
Insulation blocks heat in both directions.
Never Microwave One
Steel cannot go in, and sealed double-wall plastic can build pressure.