If he was a building inspector with a county/city, he should have also mentioned the R-value required for your specific area.
"In Zones 1 through 3, open cell (vapor permeable) foam can be used successfully to enclose building spaces with moderate or dry interior humidities. In higher interior humidity spaces, closed cell foam or other roof designs are recommended." Taken from; see/read about Fig. 8: http://www.buildingscience.com/docum...on?full_view=1
Without having the added cavity insulation, your situation is slightly different than these test results; http://www.buildingscience.com/docum...d-roof-systems
Before I explain further, where are you located? Nearest large city...
What is the humidity reading in the room most of the year?
Clothes taking longer to dry?
Clean the dryer screen in HOT water if using fabric softener sheets.
They leave a residue that impedes air-flow, costing you money.
Clean the ducting in the last six months? 17,000 dryer fires annually!