I'm thinking about removing 1 inch of fibered polyiso that I installed at least 10 years ago. I did not tape the r- board, and also used typar over it, then vinyl siding. The walls have blown cellulose. I keep thinking about all the many layers of interior paint that was put on the plaster walls over 80 years. Also, at least 1 coat of vapor barrier paint. Since I am redoing this job would it be ok to install 1 or 2 inches type 1 eps, then 2 inches of type 2 eps . How permeable would this be. I would tape all joints and use a rain screen/1x4 furring boards? Or, should I tape the polyiso and then install 2 more inches of eps. This is in central Iowa on the 5-6 zone border line. I sure would listen to all opinions on this. Thankyou

If the polyiso has not created moisture issues in the wall, adding EPS over it will not make it any worse.

If the only polyiso is only the R-board, putting 3" EPS on the exterior will slow the exterior drying capacity a bit, but with 1" Type-I (R3.8) plus 2" of Type-II (R8.4) the OSB part of the R-board would have R12 on the outside, and ~R17-R20 of insulation on the inside- a ratio that would be good enough for dew-point control even in US climate zone 6 even without the vapor barrier paint on the interior side.  If you backed off to only the 2" of Type-II it would be good enough for climate zone 5 without the v.b. paint, but with the v.b. paint you're good to go pretty much anywhere. With 1-2" of Type-I plus 2" of Type-II EPS the combined foam layer would have a vapor permeance of about 1.2-1.4 perms (more permeable than 1" of XPS) and plenty of drying capacity for the OSB to dry into the rainscreen gap.

Tape all layers of sheathing & foam as you go, and take care to air-seal the edges.

The key to moisture control in this assembly is to correctly address the window flashing & drain-plane aspects.  It needs to lap correctly to the housewrap, and if the windows are roughly co-planar with the R-board, it's better to use a crinkly type housewrap between the R-board & foam to guarantee a tiny space to allow any bulk water incursions to drain.