Flat roof (reinforced concrete slab) — termPIR® AL

When to use this system

Standard insulation of a flat roof on a reinforced concrete slab — the most common build-up in Polish multi-family residential, public-use and industrial construction with reinforced concrete structure:

• Low U-value at minimum thickness — termPIR® AL has the best λ in the industry (0.022 W/(m·K)). At 200 mm we achieve U = 0.11 W/m²·K (well below WT 2021 for roofs = 0.15).
• Compatible with any waterproofing — two-layer torch-on bitumen membrane or synthetic membranes (PVC, EPDM, FPO, TPO). Selection per design and client requirements.
• Slope formed with izoGRASS® wedges — when the slab has no structural fall. Reduces roof mass by 80–90% vs a sloping concrete screed.

Waterproofing — technology choice

Waterproofing	Characteristics	Typical application
SBS torch-on bitumen two-layer (underlayer + cap sheet)	Classic. Torch-welded with gas burner. Overlap at each seam.	Multi-family buildings, public-use, standard industrial halls.
PVC membrane (PVC-P) single-layer	Lightweight (1.2–2.0 mm), hot-air welded. Colour-stable. Requires separation layer (geotextile) from PIR.	Visible roofs, residential, retail halls.
EPDM membrane (rubber) single-layer	Highest durability (40–50 years). Adhered or mechanically fixed. Very flexible.	Premium projects, active use.
FPO / TPO membrane	Thermoplastic polyolefins. Hot-air welded. High UV resistance without plasticisers.	Newer projects requiring chemical resistance.

Important: for PVC and FPO/TPO membranes a separation layer (300 g/m² synthetic geotextile) between PIR and membrane is required — protects against plasticiser migration or chemical incompatibility.

Roof slope — izoGRASS® wedges (add-on)

termPIR® does not produce tapered boards. When the reinforced concrete flat roof has no structural fall, we use izoGRASS® tapered wedges laid on top of termPIR® AL before waterproofing:

• 3 layout types: unidirectional, valley, hip (selection per roof geometry and drain locations).
• Slope 1.5–8% — dimensions and angle determined individually per project.
• Variable thickness 0–220 mm — from a thin wedge at the drain line to a “valley” at the outlets.
• Wedges are laid on termPIR® AL, joints sealed with aluminium tape, then waterproofing.

izoGRASS® wedges are a structural add-on providing slope for water drainage — they are NOT an insulation layer included in the system U-value calculation. The variable thickness of the wedges prevents unambiguous point U-value calculation. The designer calculates the system U-value using only termPIR® AL (constant-thickness layer); the wedges are a thermal bonus (adding thermal resistance especially in thicker zones — near the drains).

Required termPIR® AL thickness per WT 2021

WT 2021 for roofs over heated rooms: U ≤ 0.15 W/m²·K:

• 150 mm termPIR® AL → U = 0.14 W/m²·K ✓ minimum meeting WT
• 180–200 mm → U = 0.12–0.11 W/m²·K (energy-efficient standard)
• 220–250 mm → U = 0.10–0.08 W/m²·K (passive standard)

Installation variants

• Adhered — PU adhesive or bitumen adhesive for bitumen membrane. Full airtightness, no perforation of the vapour barrier. Requires even substrate and dry weather.
• Mechanically fixed — telescopic (tube-washer) fasteners through the insulation layers into the concrete slab. 4–6 pcs/m² in the interior zone, 6–8 pcs/m² in edge and corner zones (wind uplift). Standard: ETA-approved anchors for C20/25 concrete.
• Ballasted (rare) — loose-laid on the slab, weighted with washed gravel or concrete pavers (inverted roof system when ballasted).

System fire rating — REI 30 (ITB 01120.4/20/R97NZP)

The roof system with termPIR® AL insulation on reinforced concrete elements has been awarded fire resistance class REI 30 by the Building Research Institute (ITB Classification No. 01120.4/20/R97NZP, Warsaw, 2020-04-08):

Variant	Insulation configuration	Fire class of RC load-bearing part	Roof fire resistance class
1	1 layer termPIR® AL ≥ 70 mm	REI 30	REI 30
2	2 layers termPIR® AL totalling ≥ 100 mm (each ≥ 40 mm)	REI 30	REI 30

Classification scope of application:

• Roof pitch: 0°–15° acc. to PN-EN 1365-2.
• Load-bearing part: solid, hollow-core (multi-void), ribbed (trough, channel) reinforced concrete elements designed per Polish Standards and Eurocode 2 (PN-EN 1992-1-2:2008).
• Waterproofing: PVC/EPDM/TPO membrane ≥ 1.2 mm or bitumen membrane 1–2 layers or sheet metal (steel/aluminium/titanium-zinc).
• Insulation: termPIR® AL (or IzoProof), core density min. 30 kg/m³, tongue-and-groove min. 12 mm, facing AL / Agro AL / WS / PK.
• Vapour barrier: bitumen or PE film ≥ 0.2 mm or none.
• Tapered wedges: EPS, PIR, mineral wool or none (do not affect REI classification).

Meaning of the REI class:

• R = load-bearing capacity (Resistance).
• E = fire integrity (Etanchéité).
• I = fire insulation (Insulation) — temperature on the cold side does not rise by more than 140°C on average.

The final REI 30 class is valid provided all conditions of the classification scope are met (termPIR thickness, layer arrangement, certified facings, approved waterproofing systems). A higher class than REI 30 requires individual classification for the specific project.

Vapour barrier — critical

Below the termPIR® AL layer, an obligatory vapour barrier is required on the interior (warm) side:

• Vapour-tight bitumen sheet (with Al foil or polyester carrier) — most often in industrial halls.
• PE film with SD ≥ 100 m — in residential buildings.
• Joints with 100 mm overlap + butyl tape — airtightness is critical; without a vapour barrier interior water vapour condenses in the insulation = loss of efficiency within 2–5 years.

Technical documentation

System described in termPIR® Folder 2025 (Gór-Stal, 2025-04-23, p. 13). Holds CE certificates (EN 13165), PZH hygiene certificate for termPIR® AL board.

Waterproofing — choice of bitumen / membrane manufacturer remains with the designer and contractor. BOKKA supplies only PIR boards (termPIR® AL + izoGRASS® wedges); waterproofing is purchased from the membrane manufacturer or by the contractor.

Layer composition

U-value by insulation thickness

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