Abstract: In metal buildings, a plurality of purlins project down into the building and, in effect, serve as heat exchange fins, which conduct heat into the building in warm climates, and heat out of the building in cold climates. In order to counteract this phenomenon, each purlin is covered with a separate blanket of insulating material, which is held in place with a flexible sleeve that is laminated to the blanket. Two additional flexible sliding sleeves are used to complete installation at each end of the purlin.