My parents home is an old bungalow. The mostly unfinished attic has 2x4 ceiling joists 24" on center. I put in R-11 years ago, but need to upgrade due to high oil prices. How can I cost-effectively do this? Joists are full -- other options, foam board over insulation but cost is an issue -- cut in holes and blow into floor joists (there is a subfloor of 8-inch boards in place, but there is a staircase coming up to the attic so I would not be able to insulate there) and part of the attic is finished (unsure if I can get insulation under this floor). What is the best way to say double the R-value of what is in place? Thanks for your help.