Vinyl expands and contracts with temperature changes and foam does not, or at least not to the same extent. That’s why the vinyl has horizontal nail slots and provision for expansion and contraction on each end of the panels.

In addition to that, there has to be a way for moisture that condenses behind the vinyl to escape and evaporate into the atmosphere. If you look at the bottom of each panel you will see a little weep hole. That allows drainage of the condensation. Preformed foam panels sometimes block those holes and damage to the sheathing, especially if it’s OSB, may occur. I have heard of a few lawsuits because of this.

The only real advantage to this is that it helps to hide the overlapped seams and provides a firm surface if you push on it. Big deal! So who goes around pushing on your siding?

I always used 1/2″ Dow Board, (blue board) in 4′ x 10′ sheets. It gave a better insulating quality, allowed proper air circulation behind the vinyl and smoothed out irregular siding surfaces. If the customer insisted on a firm backing, I would cut strips of foamboard that fit behind the flat part of the vinyl. (Dutch Lap) But, that was only done at the entry way or the electric meter area.

Just remember this most important thing::: Vinyl is HUNG to a surface, not NAILED. Leave the nail or screw heads about 1/4″ away from the sheathing or foam.