The present invention relates to an envelope (20) for a laminar structure providing adaptive thermal insulation, the envelope (20) enclosing at least one cavity (16) having included therein a gas generating agent (18) having an unactivated configuration and an activated configuration, the gas generating agent (18) being adapted to change from the unactivated configuration to the activated configuration, such as to increase a gas pressure inside the cavity (16), in response to an increase in temperature in the cavity (16), the envelope (20) being configured such that a volume of the envelope (20) increases in response to the increase in gas pressure inside the cavity (16), wherein the envelope (20) is made of a polymer composite material (8), the polymer composite material (8) including a fluid tight layer which is covered by a reinforcing layer comprising a polymer material, the rein-forcing layer being configured to limit formation of wrinkles in the fluid tight layer (8b) when subjecting the envelope (20) to one, or a plurality of activation/deactivation cycles.