Encapsulated glass fiber insulation is made by forming an encapsulating or covering material (152) over the top surface of a glass fiber mat or core (11) with the formed covering material extending adjacent the side surfaces. The covering material is greater in width than the sum of the widths of the top and two side surfaces, so that the edges of the formed covering material extend beyond the mineral fiber core. A vacuum is applied below the mineral fiber core via vacuum duct (186) to draw the formed covering material extending beyond the side surfaces under the core.