The applicable building energy code that determines the minimum insulation requirements for commercial roofs with insulation entirely above the deck in Michigan is the Commercial Michigan Energy Code (based on ASHRAE 90.1.-2013 with state-specific amendments). The minimum insulation requirements apply both to new construction and roof replacements on existing buildings.
The minimum requirement for insulation installed entirely above the roof deck is R-30 for climate zones 5 and 6, and R-35 for climate zone 7. This code became effective on September 20, 2017.
Sample roof assembly diagrams and additional information can be found in PIMA’s Michigan code fact sheet.