We are renovating a home in Redmond WA(climate zone 4C).  The roof rafters are not deep enough to allow for sufficient insulation.  We will do a combination of closed cell spray foam and blown in loose fill on the interior side of the sheathing.   The roof will be standing seam metal.    What would be best way to add a layer of insulation on top of the sheathing to boost the overall R-Value as well as to provide a thermal break?