Insulating Existing Walls Both fiberglass and cellulose can be blown into existing walls with great effectiveness. Blow-in insulation into existing walls means cutting or drilling a hole into either the exterior or interior wall to accommodate the insulation machine’s nozzle. If the exterior of the wall is a brick veneer or stone, cellulose is the choice material. It is able to be blown through a 1” nozzle. Cutting a hole into the wall for the nozzle needs to be precisely in between wall studs t