A method of insulating ducts is described including laminating a reinforcement fabric to a fibrous insulation board, lifting at least a portion of the reinforcement fabric from the fibrous insulation board, applying a first layer of a mastic coating to the fibrous insulation board, embedding the reinforcement fabric into the first layer of mastic coating, and applying a second layer of the mastic coating to the outside surface of the reinforcement fabric. The reinforcement fabric is laminated to the fibrous insulation board using an adhesive material.