All of our models are designed to be recessed in between studs except for the Model NE-242, which is a surface mounted unit.
If you choose to surface mount a unit designed to be recessed, you will need the trim kit to finish out the sides of the unit so they will be flush. Just keep in mind that the unit will protrude approx. 7" - 8" rather than 3" - 4" depending on which model you choose.