Pitched roof thermal insulation

There are three basic methods of thermally insulating roofs: insulation above the rafters, insulation between the rafters, or a combination of the two methods. In the case of above-rafter insulation, which is the best insulation method in structural terms, a continuous cover of insulation panels is installed across the entire roof area. This means that the rafters can have a smaller cross-section. In the case of between-rafter insulation, the specified thickness and quality of the insulation material dictates the thickness of the rafters. As a result the rafters are in any case larger than would be structurally necessary. Furthermore, thermal bridges are created by the rafters and walls, which makes the insulation less effective. It's rather as if you were to cut strips out of your winter overcoat. The system known as combination insulation rectifies the weak points of between-rafter insulation, thus making slimline roof structures possible.