In this article “R” Fairy Tale: The Myth of Insulation Values, David B. South from the Monolithic Dome Institute describes the application of insulation layers.
“We use three inches for most of our construction. Two inches will do a very superior job. We have insulated many metal buildings with one inch of urethane and the drop in heat loss is absolutely dramatic. Obviously the first quarter inch takes care of the wind blowing through the cracks. (It usually takes an inch to be sure the cracks are all filled.) The balance of the inch adds the thermal protection.”