You should assume that all doors with double-pane glass ... is filled with air or with a gas, usually argon or krypton. It is this sealed space that gives the window most of it's thermal performance.

If it’s single-pane glass, it will feel cold. If it’s double- or ... or an inert gas like argon or neon. That layer of air or gas does the insulating. “Insulated windows act as effective ...