izoGRASS® PIR Boards Without Facing — Insulation Core

Q: What izoGRASS® thickness should be selected for a flat roof per WT 2021?

To achieve U ≤ 0.15 W/m²K (WT 2021) with λD = 0.027 W/(m·K), approximately 180 mm of PIR insulation is needed. In practice, two-layer arrangements are used (e.g. 2×100 mm with offset joints), which eliminates thermal bridges at the joints. The final thickness depends on calculations that account for the other layers of the envelope and functional requirements.

izoGRASS® PIR Boards Without Facing — Characteristics, Applications, Limitations

Modern commercial and logistics construction calls for insulation solutions that combine low lambda, dimensional stability and a rational price point. izoGRASS® PIR insulation boards are a polyisocyanurate core without a factory facing, manufactured in Wola Batorska near Kraków and intended for incorporation into building envelope systems. In this article we show where izoGRASS® boards perform best, what limitations they have (fire reaction class E) and how they fit into WT 2021 (Polish Technical Conditions 2021) requirements.

Technical parameters — what the documentation says

izoGRASS® boards without facing are produced in accordance with EN 13165 (thermal insulation products for buildings — factory-made polyisocyanurate (PIR) foam products). Key declared parameters:

Parameter	Value	Standard
Thermal conductivity λ_D	0.027 W/(m·K)	EN 12667
Fire reaction class	E	EN 13501-1
Compressive strength (at 10% deformation)	≥ 150 kPa	EN 826
Dimensional stability (70 °C / 90% RH)	≤ 2%	EN 1604
Long-term water absorption	≤ 3%	EN 12087
Service temperature range	-50 to +110 °C	—

For comparison, faced PIR boards in a system reach significantly higher fire reaction classes — termPIR® AL with aluminium foil achieves class B-s2,d0 in the system, and termPIR® ETX with glass fleece holds ETA 17/0066 for ETICS.

Fire class E — what it means in design practice

This is an absolutely critical point when specifying izoGRASS® boards without facing. Fire reaction class E (EN 13501-1) denotes an easily ignitable material that does not meet the requirements for most fire-rated partitions. Design consequences:

Not suitable for ETICS — external wall insulation composite systems require PIR boards of class at least E, but in practice ETICS uses PIR boards with a glass fleece facing carrying a European Technical Assessment. termPIR® ETX (λ_D 0.025–0.027) with ETA 17/0066 is dedicated to such applications. izoGRASS® boards without facing are not suitable for ETICS.
Not for fire-rated partitions — any fire-separation walls, intermediate floor slabs and escape routes require materials of at least B-s1/s2,d0 (faced).
OK as a core within a system — when the board is built into the assembly beneath the flat-roof waterproofing (the fire class of the roof covering is determined by a BROOF (t1) test of the entire system) or in the core of a sandwich panel with steel facings.
OK as tapered insulation — izoGRASS® tapered insulation on flat roofs is always covered with a PVC/TPO/EPDM membrane or bitumen felt, which eliminates exposure of the core.

Suitable applications — where izoGRASS® without facing makes sense

Core of roof systems beneath waterproofing

On flat roofs where low lambda and optimised thickness are required, izoGRASS® can serve as the primary layer beneath the waterproofing. For system solutions with FM Approved certification (FM 4470/4471 guidelines), however, the standard is termPIR® Pro-F with a glass fleece facing — compatible with membranes and holding the appropriate system approvals (see FM Approved roof system).

Thermal insulation beneath screed / within the floor build-up

PIR boards with compressive strength ≥ 150 kPa are suitable as a thermal insulation layer beneath a cement screed. A separating foil and an appropriate thickness of the load-bearing layer are required in accordance with PN-EN 13813.

Thermal insulation of floor slabs over unheated cellars from below (with a finishing facing)

When the slab is subsequently finished with a GK board or a fire-resistant spray, izoGRASS® can serve as the core thermal insulation layer.

For most other envelope elements — choose the faced variant

For typical applications (ETICS, external wall insulation, over-rafter pitched roofs, foundations) we recommend boards with factory facings:

ETICS external walls → termPIR® ETX
Pitched roofs, floor slabs, three-layer walls → termPIR® AL
Foundations, plinths, ground-bearing floors → termPIR® WS

WT 2021 — izoGRASS® thicknesses for the required U-values

The Technical Conditions in force since 1 January 2021 introduced tightened thermal transmittance requirements for building envelopes. The table below shows the minimum izoGRASS® thicknesses (λ_D = 0.027) needed to reach the limit values (excluding thermal bridges and facings):

Element	U_max WT 2021	Min. izoGRASS® thickness
Roof / flat roof	0.15 W/m²K	~180 mm
External wall	0.20 W/m²K	~135 mm
Floor slab over cellar / garage	0.25 W/m²K	~110 mm
Ground-bearing floor	0.30 W/m²K	~90 mm

The values are indicative — the final thickness results from a full calculation of the envelope’s thermal resistance, including all layers and thermal bridges (PN-EN ISO 6946).

Dimensions, thicknesses, availability

izoGRASS® PIR insulation boards are produced at the Wola Batorska plant in standard formats of 1200×600 mm and 1200×2400 mm, in thicknesses ranging from 30 to 200 mm. Edge profiling (flat FIT, stepped LAP, tongue-and-groove TAG) is available to project order. For flat roofs a separate product range is offered — izoGRASS® tapered insulation — designed individually for a given roof layout (recommended slopes of 1.5–3%).

Installation accessories

For PIR boards without a factory steel facing, system metal flashings are not used. Instead we use:

Reinforced aluminium tape to seal joints (with AL boards).
Polyester (PES) gaskets at butt joints.
Low-expansion PUR foam to fill gaps.
Telescopic fasteners with plates for mechanical fixing to a concrete or trapezoidal steel sheet substrate (DIN 18234).

Frequently asked questions

Can izoGRASS® boards without facing be used in ETICS?

No. The external wall insulation composite system (ETICS) requires PIR boards with a vapour-permeable glass fleece facing and a European Technical Assessment. For ETICS the dedicated product is termPIR® ETX with ETA 17/0066 (λ_D 0.025–0.027). izoGRASS® boards without facing have fire reaction class E and are not compatible with the thin-coat renders of the system.

What is the difference between izoGRASS® and termPIR® AL?

izoGRASS® without facing is a PIR core intended for incorporation into systems (sandwich panel core, layer beneath waterproofing, tapered insulation). termPIR® AL is a board with a factory gas-tight aluminium/paper facing, λ_D 0.022 W/(m·K), in-system class B-s2,d0 — ready for installation on roofs, walls, floor slabs and foundations as a stand-alone insulation product.

Is izoGRASS® suitable for flat roofs?

Yes — as a core thermal insulation layer beneath the waterproofing (PVC/TPO/EPDM membrane or bitumen felt). The BROOF (t1) class applies to the entire roof system — the whole assembly is tested. For FM Approved certified solutions, the industry standard is termPIR® Pro-F with a glass fleece facing, compatible with most roofing membranes.

What izoGRASS® thickness should be selected for a flat roof per WT 2021?

To achieve U ≤ 0.15 W/m²K (WT 2021) with λ_D = 0.027 W/(m·K), approximately 180 mm of PIR insulation is needed. In practice, two-layer arrangements are used (e.g. 2×100 mm with offset joints), which eliminates thermal bridges at the joints. The final thickness depends on calculations that account for the other layers of the envelope and functional requirements.

Can izoGRASS® PIR boards be cut on site?

Yes. As standard, PIR boards are cut with a circular saw, a saw blade or a knife. The PIR core is dimensionally stable, does not crumble and does not produce dust. After cutting, the edges should be sealed (tape, gasket) at contact points to maintain the continuity of the insulation. On-site cutting is standard practice when fitting boards to non-standard geometries.