PVC and insulation

Hi, all

My roof is getting pretty old. It's a bit of a low slope, probably 3/12? and is torchdown.

I want to put on a PVC roof and add some insulation. When I called a couple of companies and said this, they said add an inch. So my real question, how much good will that do? 2/3 of the house is torchdown on one side of a 2x6, and sheetrock for the ceiling on the other. Is one inch going to help substantially? Two inches?