Choosing a bio-based insulation material according to the substrate and expected performance
Lambda, time lag, density: how to read technical data sheets without getting it wrong
When comparing bio-based insulation products, start with three lines. Lambda (W/m.K) determines the thickness needed for a given R value. Density (kg/m³) affects mechanical strength and thermal mass. Time lag only makes sense if the data sheet states the wall build-up assumption (thickness, linings, ventilation).
Price per m², thickness, labour: comparing like with like
In a quotation, align your comparisons on the same thermal resistance (m².K/W) and the same substrate (roof slope, wall, floor). Separate supply, accessories (vapour retarder, adhesives, hangers), and installation time.
Installation rules and site watchpoints (moisture, vapour control layer, settlement)
Treat moisture as a performance requirement. Check airtightness continuity, choose the right vapour retarder for the build-up, and anticipate settlement in lofts by controlling installed density and blown thickness. To size the thickness correctly, use a thermal resistance calculator for building elements so you can hit the target R without over- or under-sizing.
Wood fibre: the right choice for sarking and internal lining
Roof sarking: rigid boards, lambda, time lag and budget in 2026
| Item | Lambda (W/m.K) | Time lag for 200 mm (h) | Installed cost 2026 (€/m² excl. VAT) |
|---|---|---|---|
| Rigid sarking board | 0,040 to 0,046 | 8 to 12 | 140 to 220 |
For sarking, use rigid boards that are compatible with the underlay membrane. Check the permissible additional load and the load transfer of the fixings. Time lag remains an order of magnitude, and will vary with density and thickness.
Internal wall lining: semi-rigid boards, vapour management and performance
| Item | Lambda (W/m.K) | Interior-side Sd (m) | Site target |
|---|---|---|---|
| Semi-rigid board | 0,036 to 0,042 | Depending on the membrane | Consistent installation |
| Variable vapour retarder | 0,25 to 25 | Continuity, bonding, junctions |
For internal lining, wood fibre is one of the bio-based insulation products that is easy to lay out. Check the substrate moisture content and size the membrane accordingly. Continuous airtightness is better than an extra centimetre of insulation.
When to avoid wood fibre: weight, cost and persistent moisture constraints
| Decision point | Value (unit) | Consequence |
|---|---|---|
| Density | 110 to 250 (kg/m³) | Extra load and handling |
| Surface weight at 200 mm | 22 to 50 (kg/m²) | Fixing verification |
Avoid it if the substrate remains damp, if the roof is close to its load limit, or if the quote cannot stand up to a lower bid. In those cases, first secure the drying and ventilation protocol, and check the dew point to avoid condensation.
Hemp insulation: a robust solution for walls and lime-hemp build-ups
Internal walls: hemp wool/boards, lambda, time lag and price
For internal lining, hemp wool and hemp boards fit well into a bio-based insulation strategy when you are looking for a vapour-open wall that can tolerate minor moisture incidents. Check density, mechanical strength and vapour retarder compatibility according to the build-up.
| Product | Lambda (W/m.K) | Indicative time lag at 140 mm (h) | Supply (€/m² excl. VAT) |
|---|---|---|---|
| Hemp wool roll | 0,040 to 0,045 | 7 to 12 | 18 to 35 |
| Semi-rigid hemp board | 0,039 to 0,044 | 8 to 13 | 22 to 45 |
External walls: where it makes sense, limits and possible combinations (renders, framing)
In ETI, hemp is especially relevant in a timber frame added system with board insulation and compatible render finishes, or in dedicated systems. Watchpoints: fire classification, render adhesion, support and overhang details, and protection from driving rain.
Hemp-lime: use cases, drying time and realistic performance expectations
Hemp-lime is mainly used for thermal and moisture buffering correction, as frame infill, sprayed lining or shuttered construction. Allow for a long drying time before closing up, often several weeks to a few months depending on thickness, humidity and ventilation. In terms of performance, expect moderate thermal resistance: the aim is first regulation and continuity, not record-breaking R values. To go further into installation and the logic of vapour permeability in hemp-lime assemblies, refer to our dedicated guide.
Cork insulation: best suited to singular details and high-risk areas
Thermal bridges, plinth zones, reveals: where cork makes the difference
At singular details, cork is a strong choice when you need an insulation material that is tolerant of accidental moisture and impacts. Typical examples: slab-edge thermal breaks, plinth zones exposed to splashback, and window reveals and supports in external wall insulation. It is also a good option in a bio-based insulation strategy when wood fibre or cellulose would be more exposed.
Expanded cork: lambda, time lag, moisture behaviour and price range
| Product | Lambda (W/m.K) | Time lag at 100 mm (h) | Moisture behaviour | Approx. supply price at 100 mm (€/m²) |
|---|---|---|---|---|
| Expanded cork board | 0,037 to 0,040 | 8 to 12 | low capillarity, rot-proof, suitable for plinths if properly protected | 35 to 70 |
On façades, remember that thermal resistance mainly comes from thickness. Cork is sold first and foremost for its robustness, not for beating the lowest lambda values.
Balancing cost: reserve cork for the places where it prevents defects
In a quote, justify the extra cost by the risk covered: splashback at the base of walls, condensation in reveals, continuity of insulation around a penetration. Reserve cork for areas where it avoids a render repair, skirting repair or anti-corrosion treatment. The rest of the run can use a more economical insulation material with equivalent performance.
Practical benchmarks for quick decisions by application (comparison tables)
Table by application: sarking, internal lining, wall, singular detail (lambda, time lag, price)
| Use | Typical bio-based insulation | Lambda (W/m.K) | Indicative time lag (h) | Supply price excl. VAT (€/m²) |
|---|---|---|---|---|
| Sarking | Rigid high-density wood fibre boards | 0,038 to 0,048 | 10 to 14 | 35 to 70 |
| Internal lining | Hemp (boards) or wood fibre (semi-rigid) | 0,039 to 0,045 | 6 to 10 | 20 to 45 |
| Wall (internal or external insulation) | Wood fibre (rigid) or hempcrete (depending on systems) | 0,038 to 0,055 | 7 to 12 | 25 to 60 |
| Singular detail | Expanded cork (boards) | 0,037 to 0,040 | 2 to 5 | 15 to 35 |
These ranges give you an order of magnitude for quickly comparing bio-based insulation options. Time lag depends mainly on thickness, density and build-up (underlay membrane, vapour retarder, air gap).
Special cases: lofts with lost attic space and roof slopes, link to the dedicated cellulose wadding page
For lofts with lost attic space and roof slopes, cellulose wadding should be treated separately (blown installation, dense-pack installation, installation density, settlement, vapour management). Direct link to cellulose wadding is treated separately.
Questions to ask the client and the auditor in 2026 to secure the choice
- Primary objective of the works package. Winter, summer, acoustics, or thickness constraint.
- Target R value per element in the audit. Assumptions for thermal bridges and airtightness.
- Documentation requirements. Marking, ACERMI, Technical Assessment, and traceability for inspections and grants.
- Site constraints. Load in sarking, substrate and flatness, membrane continuity, singular details.
