Abstract: A preformed insulating insert is disclosed, that fits into the cores of masonry building blocks. In the preferred embodiment, the insert is generally rectangular having an upper and lower face, front and back surfaces and sidewalls. Elongated internal cavities emanate downwardly from the upper face toward the lower face and compression slots extend upwardly from the lower face toward the upper face. The elongated interanl cavities and the compression slots terminate at or beyond a common horizontal plane within the body of the insert, thereby providing lateral compression over the full height of the insert while maintaining optimum thermal resistance.