(…) Another factor to consider when specifying the R-value for a roofing project is the fact that installation of the insulation with a metal fastener (screw) through the entire thickness of the insulation produces a thermal bridge that reduces the effective R-value even more. A study published in 1986 shows that installation of a metal fastener with a metal cap creates and R-value reduction of 3% for 2 inch thickness
up to 8% for 6 inch thickness.2 This is based on a typical fastener density of one fastener per 2 square feet of board area for a fully adhered membrane application.(…)
via Are you getting what the R-value specified and paid for?.