Currently designing a sitting area under an outdoor pavillion and breezeway which will have a 3:12 metal roof over a T&G ceiling similar to a classic farmhouse porch. 

In our climate zone 3B where comfort is measured by how to stay cool, we want to avoid a roof assembly which radiates heat toward the underside with a hot sun shining and ambient temperatures in the nineties and higher. Is a simple thermal break between the roofing and ceiling by way of an inch of rigid insulation enough? Is it unnecessary?