Comment by kuschku

> a thermos is specifically good at insulating because not only does it have a vacuum gap, it's also got two layers of metal (inner and outer) to absorb and reflect thermal radiation.

Not necessarily. There are many modern thermos "cups" that are just a regular cup, except with two layers of glass and a vacuum. Even the top is open all the time. (e.g. https://www.ikea.com/us/en/p/passerad-double-wall-glass-8054... )

It's still good enough to keep your coffee hot for an entire day.