Poly Foam Board and fiberglass batt – GreenBuildingAdvisor

I’m not sure what you mean by “poly foam board”? Polyiso? “Poly” usually means “polyethylene”, but polyiso is polyisocyanurate — a very different material!

I see no issues running rigid foam over the exterior of your studs under your siding. That’s “exterior rigid foam”, which is continuous insulation, and you’ll be fine with that much in any climate zone you could be in in your area. I see no reason to cut polyiso to fit between joists in the attic though, that’s just a waste of money and effort. You’d be better off just adding a third layer of fiberglass, likely for less money and certainly easier to install, or, better yet, switch to loose fill (blown) insulation instead and just use that. Blown cellulose is the cheapest and best option here, blown fiberglass is a bit less weight per unit R value, costs more, but can tolerate things like roof leaks better.

Bill