You want the pipes to be next to the heated wall with insulation between them and the garage wall. Adding insulation between the heated wall surface and the pipe means it won't get heat from the house...insulation doesn't make heat, it just slows the transfer, and you need it between the pipe and the cold side. Maybe some foam panels on the garage side. You could add a second layer with a slot for the pipe, open to the house side. Keep in mind, on the garage side, it needs to be fire-rated drywall...typically 5/8" thick to meet code.