An insulation panel is formed comprising a binderless, heat bonded glass fiber insulation board encapsulated within a partially evacuated, gas-tight envelope. The insulation board is formed by laying down discrete length glass fibers into a thin glass fiber mat with the glass fibers lying predominately in and randomly oriented in planes extending substantially parallel to the major surfaces of the thin glass fiber mat. The thin glass fiber mat is layered to form an uncompressed thick glass fiber blanket. The thick glass fiber blanket is heated to a temperature between about 25.degree. C. below and about 20.degree. C. above the strain temperature of the glass fibers; compressed to its final thickness and density; and cooled to form the binderless, heat bonded glass fiber insulation board having glass fibers lying predominately in planes which extend parallel to the major surfaces of the insulation board. The insulation board is encapsulated within the gas-tight envelope to form the insulation panel.