Blown-in insulation is a loose fill type insulating material that is blown under pneumatic pressure into the spaces between joists and wall studs in an existing structure. Water vapor barriers are applied first, and are available in the form of a moisture resistant spray-on paint. Blown-in insulation is most commonly used to insulate interior walls. Because special equipment must be used for its application, blown-in insulation should be applied by individuals trained in its use.