PolyIso board is the best form of board insulation for R-Value. Spray foam in board form, it boasts an R-Value of 6.5 per inch. For best results, cut to fit your cavities and air seal the edges with can foam. Currently priced at:
Half inch - $6 per sheet
1 inch - $11 per sheet
1.5 inch - $15 per sheet
2 inch - $20 per sheet
2.5 inch - $25 per sheet
3 inch - $30 per sheet