air-jacket

from The Century Dictionary.

• noun A space surrounding a steamcylinder or other vessel within which is circulated hot air or gas, or within which ordinary air is confined without circulation: used to diminish loss of heat by radiation through the walls of the cylinder and to lessen cylinder-condensation of steam back to water.
• noun A closed space, usually annular, about some part of a machine or piece of apparatus, designed to secure uniformity of temperature.
• To provide with an air-jacket. See air-jacket, n., 2.
• noun A jacket inflated with air, or to which bladders filled with air are fastened, to render the wearer buoyant in water.