Abstract: A universal air diffuser comprised of a fiberglass plenum having an open side and means for attaching a duct connector on a closed side opposite the open side. The closed side of the fiberglass plenum is covered with a close-fitting non-porous material which reinforces the fiberglass to allow cutting holes for attachment of air ducts. A plurality of shallow grooves on the exterior of the closed surface provide cutting lines for hole patterns to cut holes for various standard size ducts. The reinforcing material is a fiberglass-reinforced foil which maintains the integrity of the hole cut in the surface and provides a non-porous surface for adhering to duct tape.