Abstract: The present invention relates to an envelope (20) for a laminar structure (100) 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) having, in the unactivated configuration of the gas generating agent (18), a flat shape with a thickness (d) of the envelope (20) being smaller than a lateral extension (A) of the envelope (20), the envelope (20) being configured such that the thickness (d) of the envelope (20) increases in response to the increase in gas pressure inside the cavity (16), the cavity including at least a first sub-cavity (16a) and a second sub-cavity (16b) at least partially stacked abo