An insulation sheet for insulating a wall, floor, ceiling or roof cavity is flexible, compressible and resilient and has lateral edges extending the length of the sheet. The lateral edges of the sheet are formed with contours along the lengths of the lateral edges, which with the flexibility, compressibility and resilience of the insulation sheet, increase the effective width of the sheet, relative to a conventional insulation sheet of the same length, width, thickness and density with straight lateral edges extending perpendicular between the major surfaces of the conventional sheet, with no or substantially no increase in the amount of insulation material forming the sheet relative to the insulation material used in the conventional insulation sheet. The contours of the lateral edges are formed by reciprocally oscillating or angling cutting blades in a direction transverse to the feed of a sheet past the cutting blades.