To limit the probability that the effective thermal resistance will be unacceptably low near the eaves of sloped roofs, which could lead to excessive thermal transfer through the building envelope, which could lead to excessive use of energy for heating and cooling, which could lead to an unacceptable effect on the environment.
To permit a reduction in the effective thermal resistance near the eaves of sloped roofs, recognizing that the insulation level near the perimeter is constrained by roof slope, truss design and required ventilation in the attic.