Masonry block, including slump stone and cinder block, having hollows or spaces therein, are widely used construction materials. Such masonry blocks are especially useful in constructing exterior building walls, with the blocks normally staggered or offset to one another as they are stacked by the mason with mortar between each block to form a mortar joint. A wall so constructed out of the masonry block is hollow, since the cinder block is of the type which normally has a pair of rather large spaces defined by the interior sides of the cinder block composition walls. Insulation of such walls becomes increasingly important as energy and heating costs spiral. Heretofore, it has been common to attempt to so insulate the walls by blowing an insulating material, in the form of rock wool, glass fiber compositions, or foamed plastics into the block spaces. However, such a method is not altogether satisfactory, since the materials must be directed into the walls from the top, and because the blocks are offset and staggered from one another, the spaces are usually incompletely filled, thereby leaving significant voids which reduce the overall effectiveness of the insulation.