Builder is suggesting instead of 4 inches. closed cell spray foam under deck of a truss supported roof , go with a ventilated attic ( cold roof) by using  gasketed can lights, in ceiling , spray entire top surface of can lights drywall and bottom board of truss with 1 inch close cell SF. Topped with r38 blown cellulose . My understanding was you need two inches of closed cell to achieve vapor control barrier…but maybe this will be effective air barrier AND allow moisture in house  to diffuse into attic (is the later good or bad???)