The floor area of an attic is not automatically the roof insulation area. Before ordering PIR, EPS, XPS or mineral wool, identify the elements covered by the design and the products specified for each location. Slopes, horizontal ceiling sections and walls may form separate zones with different thicknesses and installation arrangements. Measurements should show the geometry of each zone, openings and interruptions in the layer. This guide explains how to prepare an understandable area schedule, distinguish geometric quantities from order quantities and avoid errors when handing over measurements. The numerical example calculates area only; it does not specify insulation thickness or make any material suitable for a roof slope.

Different insulation materials can work together in one building element when each solves a specific problem. PIR can keep the build-up thin, mineral wool can contribute to fire protection or acoustics, and XPS suits certain applications exposed to moisture and compressive loads. The order of the layers, how they interact and the details of their installation determine whether the combination works. Adding their thermal resistances is not enough. In a roof retrofit, serviceable insulation can often stay in place while a new layer improves the whole assembly. In a floor, thermal insulation and a resilient layer for impact sound have different jobs. Combining materials is a design decision.