Flat Roof Insulation: PIR, EPS, Mineral Wool Compared

The insulation layer is critical to a flat roof's thermal performance and condensation behaviour. The right choice depends on the roof construction type, required U-value, available depth, and budget.

PIR (Polyisocyanurate) Board

PIR board (brands include Celotex, Xtratherm, Kingspan) is the most widely specified insulation for warm flat roofs. It has the best thermal performance per millimetre of thickness (lambda value ~0.022 W/mK), making it ideal where overall roof depth is restricted. It's rigid, easy to cut and lay, and compatible with all mainstream roofing systems.

EPS (Expanded Polystyrene)

EPS has a lower thermal performance than PIR (lambda ~0.033–0.038 W/mK) but is significantly cheaper. It's commonly used on inverted warm roofs (where it sits above the waterproofing) and as tapered insulation for falls. Requires a filter layer above to prevent fines from the aggregate ballast washing into drains.

Mineral Wool

Mineral wool (rockwool or glasswool) is more commonly used in cold roof constructions, placed between joists. It has good acoustic performance and fire resistance but is not suitable for use above the membrane in a warm roof system due to water absorption.

Tapered Insulation

Tapered insulation — usually PIR or EPS cut to a wedge profile — creates the drainage falls within the insulation layer itself. It's commonly used in re-roofing projects where the existing deck has inadequate falls, and it simultaneously improves drainage and thermal performance.