Abstract: An insulating insert is disclosed, that fits into the cores of concrete building blocks. The insert is collapsible widthwise, near its ends, and lengthwise to fit both standard and odd-dimensioned cores. In the preferred embodiment, the invention basically comprises: a generally rectangular body with flat top, bottom, front and rear panels; two segmented endwalls; and opposing slots (T-shaped in cross section) that start at the endwalls and extend from the insert's top to bottom. The horizontal portion of each T-slot permits the insert's end portions to be collapsible widthwise to easily accommodate different-shaped cores, even sash blocks; and each vertical portion permits the overall length of the insert to be reduced to fit smaller cores.