Spray foam (specifically closed cell spray foam) is used in open cavities and exposed crawl spaces, walls, and rooflines. Closed Cell spray foam is a mixture of two liquids that when it is sprayed it adheres and hardens within seconds to become a very dense and high R-value (7 per inch) insulator. Spray Foam is a two-part polyurethane mold resistant, fire retardant, rodent resistant, airtight (unlike typical batting) and has a lifetime guarantee.
Injection Foam is used within closed cavities and contained spaces to fill the already poorly insulated or non-insulated space. Injection foam is a water-based material that expands slowly to fill every crevasse and crack until the wall is entirely filled. Injection foam can be used in walls that have existing batting, loose fill fiberglass, wires or anything else.
Injection Foam (R-5 per inch) can be used in wall cavities that are less than 1.5 inches where as dense packing walls with loose blown fiberglass or cellulose is not possible with spaces this small.
Injection Foam also differs from dense packing walls; dense packing of walls with fiberglass or cellulose is more common, which is normally used for closed spaces such as ceilings and walls that are greater than 3 inches. Both processes of injection foam and dense packing involves drilling 1.5-2 inch holes in the wall/ceiling to pump material into the cavity until it is fully insulated. (Holes are patched when finished, ready to paint).