Double wall French presses

Double wall describes how a press is built rather than what it does for you. Two shells with a gap between them, and depending on the design that gap is either still air or a vacuum. It is worth understanding the difference because the two perform differently and both are sold under the same phrase.

Vacuum against air gap

A true vacuum wall is the better insulator by a wide margin, because with almost no air in the gap there is nothing to carry heat across it. A plain double wall with air in between still helps, mostly by removing the direct contact between hot glass and the room, but it is a fraction as effective. Where a maker specifies "vacuum insulated" they are claiming the better of the two; where they say only "double wall" they may not be.

Double-walled glass is a different product again

Borosilicate double-walled beakers exist and are the compromise option: you keep the ability to see the brew and gain some heat retention, at the cost of being more fragile than single-wall glass and much more fragile than steel. They also cannot usually be replaced as cheaply, because the beaker is the expensive part rather than a commodity tube.

How each one fails

A vacuum wall fails by being dented or dropped, which breaks the seal. There is no repair and no way to tell from the outside except that it stops keeping things hot. Single-wall glass fails by breaking, which is obvious and usually cheap to fix. Double-walled glass fails by cracking between the layers, after which it can trap water in the gap. Which failure you would rather have is a reasonable way to choose.

The same presses, from the other side

Every press here is also on insulated French presses, because double-walled construction is how insulation is achieved. That page is about the brewing consequences: heat retention, decanting, and why an insulated press does not let you leave coffee on the grounds. This one is about the build.