A duct board or duct liner (10) comprises: an insulating layer (12) formed from fibrous material bound with a resin binder, an outer facing layer (18) adhered to an outer surface of the insulating layer, and a water repellant mat facing (14) adhered to an interior surface of the insulating layer opposite the outer surface to form a duct board material. The mat facing (14) provides sufficient water repellency to repel a mixture of about 40% isopropanol and about 60% water. The duct board or duct liner material (10) may be formed into a tubular shape capable of conducting air, with the mat facing on the interior of the tubular shape.