How Do I Remove Rigid Insulation From an Old Water Heater So I Can Make a Solar Batch Heater?

I cut away the outer metal sheeting of a used hot water tank to make a batch heater and discovered the insulation surrounding the tank is a hardened foam type that forms a solid rigid mass around the tank and is adhered to the water tank inside. Do you have any recommendations for removing this insulation easily so I can use the steel tank inside for my batch heater?

I’d just use a wood chisel to get most of the foam insulation off. Then I’d use a paint scraper to finish the job.

—John Canivan