Lightweight composite panels, compositions used to make composite panels, and methods for manufacturing composite panels. The composite panels include a lightweight foam core sandwiched between thin protective
layers (e.g.,
fiber reinforced cementitious layer, thermoset
polymer layer, e.g.,
polyurea or
polyaspartic, or
magnesium oxide). Lightweight composite panels can be used in place of conventional wallboards and panels, including for various uses such as interior
drywall, backerboard for tile and other interior finishes, including those exposed to
moisture, exterior sheathing, floor underlayment, soffits, roofing decks, shaft liners, and the like. Lightweight composite panels can be
cut, drilled, and screwed onto structural elements of buildings, such as wall frames comprising wooden or
metal studs, roof frames comprising boards, studs, or trusses, floor joists, concrete floors, foundations, and the like. Exterior sheathing panels can include a drainage layer and / or a factory installed finish. Interior panels that replace
gypsum boards can include a plaster layer.