I've put in 1" Type 1 polystyrene (estimated at R3.75) around the exterior walls and I'm deciding which insulation to use in the framing.
The R12 works out to $.38 p/sqft The R14 works out to $.54 p/sqft (+40% over R12) The R20 works out to $.63 p/sqft (+65% over R12, + 17% over R14)
I need about 500 sqft of this stuff. The different R values come in different size packages, so that could come into play in terms of the cost difference.
Is it worth to spend 40%, or 65% more on the insulation batts? With R12 and the polystyrene, I've got over R15. But more insulation means more $aving on heating.