Installed 9" of spray foam insulation (Sealection 800) in roof system (R=38) in a home in Massachusetts. Bldg Inspector says we need a vapor barrier, but manufacturer says no barrier is needed with this product. Anybody have any knowledge in this area? Does open cell/closed cell make a difference?

It is a code requirement to have one, that is why the inspector is telling you that regardless of the manufacturer requirements. We switched to semi-permeable vapor barriers on projects because of their breathability and mold issues.

MemBrain Smart Vapor Retarder (http://www.certainteed.com/CertainTeed/Pro/Contractor/Insulation/Prodindex/Residential/MemBrainProdIndex.htm)

Code is slower evolving that technology and new materials. You may not need a barrier but if code requires it, code trumps the manufacturer. If you provide enough documentation from the manufacturere the inspector might relent.