You have existing insulation of 4"-5" (between R13 and R18) the only way I know to add to the insulation value is to add to the insulation itself.
If you foam the underside of the roof you might keep the attic space cooler, but what you want to do is to insulate the house to keep it cooler/warmer so add to the existing insulation and let your exaust vents take care of the heated air.
Another option for a DIUer is cellulose insulation. It gets blown in on top of your existing insulation and fills any cracks or spaces in the fiberglass.
According to this chart you should have between R50 & R60
Step One - Calculate Your Need | GreenFiber.com
If you use the chart on the same page to factor raising your insulation value to R60 it would take 160 bags (for a 2000 sq ft building) and last time I bought the cellulose it was $5.15 a bag (2 yrs ago to insulate my son's attic) it would cost you under $1000 ...and that is factoring in the rental of the blower machine (if they don't offer it's use for free), coveralls, gloves and dust masks.