Owner-builder/designer  who passed my CA title 24 On my own. R19 in the walls standard around here but upped them to R21. That helped. Ended up using in stock Roxul R23 in the master suite to make it even better. Great stuff. Much easier to achieve a tight fit. But pricy. Was getting ready to special order r21 fiberglass for the rest. Cheaper than Roxul but up around $.90 a square foot. 

Then see that Home Depot has unfaced Owens Corning r30 rolls at $.38 a square foot. More than a little research on line leads me to a chart OC had up that showed R values for insulation when compressed into a tighter cavity than intended. R30 in a nominal 2×6 (5 1/2″) cavity was rated R21. Bought a roll. You’d have your work cut out for yourself fluffing that stuff up to 9 1/2″ for the rated r30 !  Had some Johns-Manville R30 comfort therm face batt to compare it to. The latter was easy to fluff.

Tried the r30 in a test 2×6 cavity. It fit in nicely. Not too lofty. Will use 4mil plastic sheet for my vapor barrier. No questions about good coverage with that. Should be no problem screwing the drywall down. Moral of story: regular r30 equals r21 at a much better price point.

The above mentioned Owens Corning chart is no longer on their site (I will not speculate why but will leave that up to you)  but can be seen using this handy tool, the “Web Archive Way Machine”. Here is the url: 

http://web.archive.org/web/20110420013153/http://www.owenscorning.com/around/insulation/CompressionChart.xls