I have built an addition to the side of my house, with a new metal roof over 5\8 osb. The rafters are 2 x 10 spanning 15 feet, and my plan is to hang drywall to the underside for ceiling. I bought r30 bats for insulation, because lowes guy said it would be ok. Thes bats are supposed to be 9 1/2 inches thick, and my concern is ventilation. Should I take the r30 back and get something thinner? or how otherwise can I make sure of air flow?