An insulation board assembly includes an insulation board body that has a front face, a back face, a top edge, a bottom edge, and two side edges. The back face has a drainage channel extending from the top edge to the bottom edge. The drainage channel is recessed inward of the back face by a channel depth. A drainage insert is positioned in the drainage channel proximate the bottom edge of the insulation board body. The drainage insert has a front face, a top edge, a bottom edge, and two side edges. The drainage insert defines at least one interior drainage passage extending from the top edge of the drainage insert to the bottom edge of the drainage insert. A reinforcing mesh may be secured to the insulation board body. The insulation board body may consist substantially of mineral fibre insulation.