A flexible disk for securely holding insulation to an air duct made from a resilient material, and having a convex upper surface and a diameter of from 1.5 inches to 4.0 inches. A washer for receiving a disk fastener is centrally positioned in the disk top, which is thicker than the top. A rim is located at the circumference of the top which has a planar bottom for contacting the insulation. When the disk is properly attached to an air duct by a fastener, it deforms to a concave configuration by which it resiliently exerts force on the underside surface of the disk and the rim which in turn presses downward on the insulation, securely holding the insulation in place without crushing the insulation.