Wood Fibre Insulation Boards: Technical Guide
STEICO flex wood fibre insulation. Source: Wikimedia Commons / thingermejig (CC BY-SA 2.0)
What wood fibre boards are made from
Wood fibre insulation boards are manufactured from mechanical or thermo-mechanical pulp of softwood, primarily spruce and pine. The fibres are bound either with natural wood resins released during wet process manufacturing, or with a small percentage of polyolefin binder in dry process boards. The resulting panel is compressed to a defined density and dried.
This process produces two main product families: flexible batts (low density, used as cavity fill) and rigid boards (higher density, used as external insulation, sarking, or structural sheathing). The two differ substantially in compressive strength, thermal conductivity, and handling characteristics.
Density and thermal conductivity
Thermal conductivity (λ) is the single most referenced property when comparing insulation materials. For wood fibre, the relationship between density and λ is non-linear. Both very low-density and very high-density products can have elevated λ values compared to the mid-range.
Declared λ values are determined at 10°C mean temperature per EN 12667. Boards sold in the EU must include the declared value on the CE marking label and in the declaration of performance (DoP) issued by the manufacturer.
CE marking under EN 13171
Since the Construction Products Regulation (CPR, EU 305/2011) took full effect, wood-based factory-made thermal insulation products intended for use in buildings must carry CE marking under EN 13171. This standard specifies the required performance characteristics, test methods, and how results must be declared.
The declaration of performance must state:
- Declared thermal conductivity (λD) and corresponding thermal resistance (RD) for the stated thickness
- Compressive stress at 10% deformation (CS(10)) for rigid boards
- Reaction to fire (Euroclass A1 to F)
- Short-term water absorption by partial immersion (WS) or long-term water absorption by total immersion (WL)
- Dimensional tolerances (thickness, length, width, squareness)
Products without CE marking cannot be legally placed on the Polish market for use in regulated building applications.
Reaction to fire classification
Untreated wood fibre boards typically achieve Euroclass E (combustible, no additional classification). Boards incorporating mineral additives — commonly ammonium polyphosphate or boric acid — can achieve Euroclass B-s1,d0 or B-s2,d0 depending on test results.
The fire classification relevant for a specific application depends on the building use category and wall assembly configuration under Polish building regulation Rozporządzenie w sprawie warunków technicznych (WT). External wall systems and roof assemblies have minimum reaction-to-fire requirements that must be confirmed for the complete assembly, not just the insulation board alone.
Moisture behaviour and vapour diffusion
Wood fibre is hygroscopic: it exchanges moisture with the surrounding air. The vapour diffusion resistance factor (μ) for wood fibre boards ranges from approximately 1 to 5, depending on density and any surface treatment. This is substantially lower than mineral wool (μ ~1) or EPS (μ 20–100).
The low μ value means wood fibre boards are vapour-open. In a correctly designed wall assembly, this allows moisture that diffuses into the structure during winter to escape to the outside during warmer periods. Condensation risk must be assessed using the Glaser method (EN ISO 13788) or a dynamic hygrothermal simulation. The boundary between the insulation and structural layer, and the vapour control layer position, are critical inputs for this assessment.
Compressive strength
Flexible batts have negligible compressive resistance and are not load-bearing. Rigid boards used as sarking or external insulation must carry compressive loads from fixings, render systems, or roofing membranes. EN 13171 requires that compressive stress at 10% deformation (CS(10)) be declared; common values for rigid facade boards are between 60 and 200 kPa.
For use in inverted or warm flat roofs, the compressive strength must be sufficient to resist loading from substrate, ballast, and traffic loads without exceeding the deformation limit for the specific application.
Thickness and thermal resistance
Required insulation thickness is calculated from the U-value target for the building element divided by the declared λ of the product:
U = 1 / (Rsi + R + Rse) [W/(m²·K)]
Where Rsi = 0.13 m²K/W (internal surface resistance) and Rse = 0.04 m²K/W (external surface resistance) for a standard wall with horizontal heat flow per EN ISO 6946. Additional layers (structural timber, sheathing, render) contribute their own thermal resistances and must be included in the total assembly calculation.