Insulation between storeys may serve thermal and acoustic purposes, but these are not interchangeable criteria. If the aim is to reduce footstep noise, comparing PIR, EPS, XPS and mineral wool means examining a specific floor assembly rather than thermal conductivity alone. The supporting floor, proposed resilient layer, loads and junctions at walls and services all matter. This guide helps describe the problem and collect information for selecting a solution with a designer or acoustics specialist. It also explains why a test result for another assembly is not a promise of identical performance after renovation. It does not prescribe one board or thickness for every floor.

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.