Abstract: A preformed masonry building block having a U-shaped preformed insulating insert of molded expandible polystyrene disposed in each cavity thereof. Each web of the building block is provided with a vertical insulating slot which is at least three fourths (3/4) of an inch wide and which has a volume equal to at least one half (1/2) a volume found by multiplying slot width by web height and thickness. Each end web has a slot adjacent the end of each leg of the adjacent insert and the legs project into and fill the slots. The intermediate web has a slot on each side thereof adjacent the body portions of the inserts and a projection on the body portion of each insert enters and fills its adjacent slot.