Unless a major remodel of your house or building will take your walls down to the studs, retrofitting existing walls with spray foam behind sheet rock is a dubious task. Injectable polyurethane foams do exist and are installed by drilling hundreds of small holes through the interior drywall, five to eight penetrations per cavity. Destructive to wall aesthetics, the work leaves behind a significant amount of drywall repair and does so without complete assurance your cavity wall has been sufficiently insulated, especially if insulation is already present in the cavity. The difficulty of retrofitting existing walls is a prime reason to have your walls applied with spray foam during initial construction or during a major remodel where drywall is removed from the studs.