A copper lining is a thin copper foil applied to the outer face of the inner wall, sitting inside the vacuum gap between the two steel walls. It works as a radiant barrier that reflects heat back, improving heat retention by a small but measurable margin.

“Copper vacuum” and “copper plated” are common marketing terms for this construction. The copper never touches the drink, and it is not a structural layer.

The copper is also a visual selling point, since it can be seen through clear outer shells on some designs. The foil is thin enough that it adds little weight to the bottle.

Practical takeaway: Treat copper lining as a modest heat-retention and visual upgrade, not a substitute for a proper vacuum.