Abstract: A lost form for concrete which comprises insulting slabs of foam material reinforced by a core and held in place in an upright orientation by horizontal base elements, horizontal connecting elements disposed between the tiers of slabs and vertical elements bridging pairs of slabs of each tier. The horizontal elements have a ladder-like configuration with longitudinal members engaging in formations in the lower and upper edges of the slabs and transverse members extending horizontally between and spacing apart the slabs. The vertical elements also have a ladder-like configuration with vertical longitudinal members received in confronting grooves of the slabs and vertically spaced horizontal transverse members which bridge the slabs. The elements are retained in the cast concrete wall and hold the slabs thereagainst.