lag·ging 
(lăgĭng)
(lăgĭng)n.
1.
a. A covering used to provide thermal insulation, as from a steam pipe.
b. A similar covering used to provide acoustical insulation, as on an air duct.
2.
a. A plank or set of planks forming the outer surface of a centering for the construction of an arch.
b. Any of various structural members or materials used as a protective sheathing, as on the ceiling of a mine tunnel or around a boiler.
[From LAG2.]