Here's a chart I found on the web. Hope it helps someone!
Insulation Type R-Value per inch of thickness
--------------------------------------------- ----------------
Fiber glass blanket or batt 3.2
High-performance fiber glass blanket or batt 3.8
Loose-fill fiber glass 2.5
Loose-fill rock wool 2.8
Loose-fill cellulose 3.5
Perlite or vermiculite 2.7
Expanded polystyrene board 3.8
Extruded polystyrene board 4.8
Polyisocyanurate board, unfaced 5.8
Polyisocyanurate board, foil-faced 7.0
Spray polyurethane foam 5.9
Polyicynene spray in foam insulation 3.8
Mike...