Abstract: The invention provides an insulator for use in combination with a structural block having top and bottom surfaces that are connect by side surfaces. The side surfaces and the top and bottom surfaces define a continuous periphery that at least partially forms a mortar joint with adjacent blocks. The periphery has a generally vertical height bounded by the top and bottom surfaces, and a width bounded by the side surfaces. The insulator has a pre-formed portion of insulating material having top, bottom and side surfaces. The top and bottom surfaces define a height greater than the generally vertical height of the periphery of the block. The side surfaces define a width greater than the periphery of the block, so that the insulator extends into and insulates, on at least a portion of all surfaces forming the block periphery, the mortar joint between the block and adjacent blocks.