[Greenbuilding] Siding over foam insulation?

[Justin Close]

A, hopefully, quick question this time around.   :)

One of the bidders for our siding job on our house, in response to my 
question about installing an inch or two of foam board over my 
sub-sheeting on my walls, said that the warranty for the siding would be 
voided if installed in that manner.  (It is only 2x4 bays currently, 
hence the desire for thicker insulation.  And the foam sheets sounds 
easier/better than tacking on a 2" furring strip over each of the studs 
to increase the bay depth to 6".  Which in turn sounds easier than doing 
an offset stud, or something along those lines, to increase the bay depth.)

So my question is, what about installing the foam under the 
sub-sheeting?  It would go like this: remove all the current sheeting to 
expose the stud bays, insulate the walls, install foam sheets, install 
wall sheeting (and probably a wrap), install siding.  I would use long 
enough screws on the sheeting to reach through the foam to the studs, of 
course.  But then the siding would be going into the sheeting only.