Abstract: A building unit for constructing a wall, a ceiling, a floor, or a roof includes a metal rail, and insulating panel, and an elongated linear stud. The metal rail has a flat surface and at least one flange extending from the flat surface. The insulating panel has a front surface and a back surface bounded by opposing edges. It is made of rigid foamed plastic and it has at least one primary linear passageway extending through it between and perpendicular to two opposing edges. At least one of the opposing edges can be attached to the metal rail. The elongated linear stud has two ends that can be fixed to the rail. The stud fits into the primary linear passageway and is at least as long as the primary linear passageway. Another similar metal rail may be attached to the other end of the stud. The panel may include a second linear passageway that is at right angles to the primary linear passageway. A linear metal strap fits into the second linear passageway.