Attic Insulation with PIR Boards — WT 2021 Compliance | BOKKA
Attic insulation — why it determines the comfort of the entire building
The attic is the building envelope component subjected to the greatest thermal stress — in summer it heats up from solar radiation, in winter it loses heat through the roof, which has the lowest thermal mass. Poorly designed insulation means rooms overheating above 30°C during summer, water vapour condensing on thermal bridges, mould growth, and heating bills up to 25–30% higher. In this article, we show how to design attic insulation compliant with WT 2021 (Polish Technical Conditions 2021) using termPIR® boards with low lambda, without losing usable headroom.
WT 2021 requirements for the roof and ceiling above the attic
In force since 1 January 2021, the Polish Technical Conditions impose strict requirements on roof envelope components:
| Envelope component | Umax per WT 2021 |
|---|---|
| Roof, flat roof, ceiling below unheated attic | 0.15 W/m²K |
| Ceiling above the top heated storey | 0.15 W/m²K |
| External wall | 0.20 W/m²K |
To achieve U ≤ 0.15 W/m²K using mineral wool with λD = 0.038 W/(m·K), a layer about 26–28 cm thick is required. Using PIR boards with λD = 0.022 W/(m·K), about 15 cm is enough — a difference that determines the room height and the geometry of the truss.
PIR boards — technical parameters for attic insulation
PIR boards are rigid polyisocyanurate foam thermal insulation compliant with EN 13165, faced on both sides with aluminium foil (gas-tight) or glass fleece. Key parameters:
- λD = 0.022 W/(m·K) for termPIR® AL and the eco variant termPIR® AL R-eco
- λD = 0.019 W/(m·K) for premium termPIR® MAX 19 AL — the lowest lambda on the market
- Fire reaction class (system): B-s2,d0
- Compressive strength ≥ 150 kPa (CS(10\Y)150) — load-bearing capacity during installation
- Dimensional stability, no dusting, resistance to fungi and mould
- Standard dimensions: 1200×2400 mm and 1200×600 mm; thicknesses from 20 to 250 mm
- Edge profiles: TAG (tongue-and-groove) — eliminate linear thermal bridges at joints
Minimum PIR thickness needed to meet WT 2021 (U ≤ 0.15 W/m²K) — indicative, for full coverage of the roof slope:
| Board | λD | Minimum thickness |
|---|---|---|
| termPIR® AL | 0.022 | ~150 mm |
| termPIR® MAX 19 AL | 0.019 | ~130 mm |
| Mineral wool λ=0.038 | 0.038 | ~260 mm |
Over-rafter method — eliminating thermal bridges
The termPIR® over-rafter layout consists of laying PIR boards on top of the rafters, beneath the battens and counter-battens. This is the reference solution for new buildings and for renovations combined with replacement of the roof covering, because it:
- eliminates thermal bridges in the rafter zone (timber has λ approx. 0.16 W/(m·K) — 7× higher than PIR)
- allows the roof truss to remain visible in the interior design
- does not reduce headroom in the room
- ensures continuity of insulation at the junction with the knee wall
The boards are laid on blind boarding or directly on the rafters with a vapour barrier on the room side. Joints are sealed with aluminium tape, and counter-battens are fixed with long carpentry screws penetrating the PIR layer into the rafters.
Under-rafter and between-rafter methods — fast renovation without removing the roof covering
When the roof covering is in good condition, the most effective approach is under-rafter or between-rafter termPIR® insulation. Thin boards (e.g. 100–120 mm) are fitted between the rafters with a 20–40 mm gap for ventilation beneath the underlay membrane, and a second layer (40–60 mm) is mounted below the rafters in a cross-layout. The resulting W I configuration (Layer I between-rafter + Layer II under-rafter) almost completely eliminates rafter thermal bridges.
Advantages of the method in renovation:
- requires no removal of roof tiles or sheet metal
- minimal loss of headroom (thinner layer than mineral wool)
- dry installation — no curing breaks
- immediate finishing with gypsum plasterboard on a frame is possible
For multi-layer assemblies, we recommend boards with the TAG profile and aluminium facing, which acts as a vapour barrier — eliminating the need for a separate PE film (provided all joints are carefully sealed with aluminium tape).
ECO and premium variants — when to choose which
- termPIR® AL — the universal standard, suitable for 90% of residential projects
- termPIR® AL R-eco — the same λD 0.022, lower carbon footprint (glass fleece inside, less Al) — the choice for buildings certified to BREEAM/LEED
- termPIR® MAX 19 AL — λD 0.019 — when limited structural space (low rafters, dormers, swallow-tail dormers) demands maximum performance per cm of thickness
FAQ — frequently asked questions
What thickness of PIR board is needed in an attic to meet WT 2021?
For the roof and flat roof, WT 2021 requires U ≤ 0.15 W/m²K. With termPIR® AL boards at λD = 0.022 W/(m·K), a layer of about 150 mm in full coverage is sufficient. For termPIR® MAX 19 AL (λD 0.019) — about 130 mm. In a between-rafter and under-rafter layout, two layers are added together (e.g. 100+60 mm). The final thickness depends on the envelope calculation accounting for rafter thermal bridges.
Can PIR boards insulate an attic without removing the roof covering?
Yes — the most common solution in renovations is the under-rafter or between-rafter method. The boards are installed from the room side, between and beneath the rafters, maintaining a 20–40 mm ventilation gap beneath the membrane. Thanks to the thin layer (15–20 cm in total), no significant headroom is lost, and installation is fast and dust-free — with no intervention in the roof covering.
Does the aluminium facing of a PIR board replace a vapour barrier?
Yes — the aluminium facing of termPIR® AL boards is gas-tight and acts as a vapour barrier provided all joints are sealed tightly with aluminium tape and all service penetrations and connections to the structure are sealed. An unfinished detail at the junction with the wall plate or the knee wall negates the effect — in such places, elastic tapes and low-expansion foams are additionally used.
Are PIR boards fire-safe in a habitable attic?
PIR boards in a system with aluminium facing achieve fire reaction class B-s2,d0 per EN 13501-1 — they are not readily ignitable and have limited smoke production. This is safer than EPS (class E). Once covered with gypsum plasterboard (type F — fire-protective), the envelope achieves full EI 30/60 fire resistance depending on the layout. The boards do not drip, do not produce dust, and are neutral for occupants’ health.
How does termPIR® AL differ from termPIR® MAX 19 AL for attic insulation?
The key difference is lambda: 0.022 vs 0.019 W/(m·K). termPIR® MAX 19 AL achieves the same U-value at about 15% lower thickness — translating into 2–3 cm of additional headroom or easier concealment of insulation in low dormers. MAX 19 is the choice wherever structural space is limited and energy requirements are high (WT 2021+ homes, NF15, passive house projects).
Planning attic insulation and need help selecting the thickness for a specific truss? Contact our technical advisory team — we will prepare a termPIR® board specification, U-value calculations for the envelope, and a quote for your project.
