Interior Spray Foam Insulation – Farnam, NE First, technicians will mask off all adjacent surfaces to prevent over-spray on areas such as windows and doors. All electrical and plumbing work must be completed before we will apply polyeurethane foam to the walls. Any electical boxes will be masked off before spraying to prevent overspray. Technicians will install an average of two inches of spray foam to the walls. This two inches of SPF will provide an R-Value of around 13.4 to the entire sprayed suface.