Pitched roof – over-rafter insulation with PIR boards — | BOKKA
Pitched roof insulation using the over-rafter method — 350 m² of termPIR® AL boards
This project involved the thermal insulation of a pitched roof on a single-family residential building, covering a total roof surface of 350 m². The investor opted for the over-rafter method, which delivers a continuous insulation layer above the timber truss structure — uninterrupted by rafters and free of the thermal bridges typical for between-rafter insulation systems.
Design challenge
The investor’s principal objective was to achieve thermal performance comfortably exceeding the requirements of WT 2021 for roof partitions (U ≤ 0.15 W/m²K), while keeping the insulation layer as slim as possible to expose the timber truss from the interior side. Additional criteria included rapid installation, fire class reaction-to-fire performance suitable for residential construction, and long-term stability of the λ value — with no settling, moisture uptake, or degradation throughout the service life of the building.
Product applied
termPIR® AL boards with a thickness of 120 mm were used — rigid PIR panels faced with gas-tight aluminium foil, with a declared thermal conductivity of λD = 0.022 W/(m·K). The boards were installed directly onto full timber sheathing, above the rafters, with tongue-and-groove joints on all four edges to eliminate linear heat losses at the connections. Mechanical fixing was carried out using long stainless-steel screws anchored into the rafters.
Final result
The over-rafter method combined with PIR insulation produced a homogeneous, continuous thermal envelope above the roof structure, with parameters approaching passive-building standards. A slim 120 mm layer with low lambda replaced more than 200 mm of conventional mineral wool, reducing installation time and leaving the truss fully exposed from the inside — exactly as specified in the architectural design.
Want to achieve a similar result on your roof? Get in touch — we provide regional sales coverage across Poland and will select the optimal termPIR® thickness and variant for your project.
