Abstract: A rectangular cardboard sheet has two parallel longitudinal scores, each spaced inward of a longitudinal sheet edge, and two lateral parallel scores, each spaced inward of a lateral sheet edge, forming a rectangular sheet central portion and rectangular bendable margins. The width of the sheet central portion is substantially the same as the distance between adjacent floor or ceiling joists of a building structure. The longitudinal sheet margins have several transverse slots dividing such margins into rows of separate tabs. Tabs at opposite sides of the sheet can be secured to adjacent joists such that the sheet central portion bridges between the joists, parallel to and spaced from the building structure ceiling or floor, for holding a layer of blowable insulation material against the ceiling or floor.