Understanding U-values and insulation performance

U-values measure how much heat passes through a building element — lower is better. UK building regulations set a target U-value for walls, roofs and floors in new work.

PIR (polyisocyanurate) boards deliver the best thermal performance per mm, making them the go-to where space is tight. Mineral wool is more forgiving to fit and offers better acoustic performance.

EPS and XPS boards are common for floor insulation because of their compressive strength. Whichever you choose, always check the manufacturer's declared lambda value and detail junctions carefully to avoid cold bridging.