Spray foam is a type of insulation material that is applied using a spray gun. It is made by combining two chemicals, polyol resin and isocyanate, which react when mixed and expand into a foam. This foam is then sprayed onto walls, roofs, and other surfaces where it hardens and creates an airtight seal. Spray foam insulation is known for its excellent thermal insulation properties, as it can provide a high R-value per inch of thickness.