Abstract: A universal air-vapor barrier box for electrical outlet boxes is convertible between a fixed mode and a foldable mode. The barrier box is manufactured in the fixed mode with top, bottom, side, and back walls and top, bottom, and side flanges. The back wall has a groove that extends between the top and bottom walls. Grooves in the top and bottom walls are aligned with and continuous with similar grooves in the top and bottom flanges. In the fixed mode all the walls and flanges are unitary members. The barrier box receives an electrical outlet box, and the two are nailed together to a building stud or rafter. With many electrical outlet boxes, there is sufficient room within the barrier box when it is in the fixed mode to swing a hammer and drive the nails. With large electrical outlet boxes, the grooves in the top and bottom walls and flanges are cut through to place the barrier box in the foldable mode.