The case against conventional mineral wool insulation has been building for some years. Glass fibre and rock wool perform adequately as thermal insulators — and they are cheap — but they are not hygroscopic, meaning they cannot absorb and release moisture from the surrounding air. In an increasingly warm and humid climate, this is not an incidental shortcoming: it is a fundamental mismatch with how buildings actually behave.
Hemp fibre insulation — along with wood fibre, its closest competitor — is hygroscopic. It can absorb up to 20% of its weight in moisture without losing insulating performance, and it releases that moisture back into the air as conditions change. This dynamic moisture buffering is what passive-house designers increasingly understand as the missing piece in their building envelope strategy.
The carbon case
Hemp grows extraordinarily fast — a full crop in 3–4 months — and sequesters approximately 1.63 tonnes of CO₂ per tonne of dry hemp fibre produced. When you manufacture insulation from it, that carbon stays locked in the building for its entire life. Knauf Insulation's NaturBoard range achieves a declared global warming potential of -1.5 kg CO₂ eq / kg — meaning it removes more carbon from the atmosphere than its production generates. This is not a rounding error. It is a genuinely carbon-negative product specification.
"The question is no longer whether bio-based insulation works. It does. The question is whether architects and engineers are ready to update their specification habits." — Dr. Anna Fischer, Passive House Institute Darmstadt
Knauf NaturBoard: the manufacturer's story
Knauf Insulation — part of the global Knauf Group — launched their NaturBoard range in 2018 after a decade of R&D into agricultural-fibre insulation systems. The product line consists of three formats:
- NaturBoard WOOD — Wood fibre panel for timber-frame wall and pitched roof applications. λ = 0.038 W/mK. Available in 40mm, 60mm, 80mm, 100mm thicknesses.
- NaturBoard HEMP — Hemp-fibre flexible batts for stud-frame cavities. λ = 0.039 W/mK. Friction-fit installation without adhesive or fixings.
- NaturBoard RENDER — Blown wood-fibre loose fill for attic floors. Excellent for retrofit insulation without structural intervention.
All NaturBoard products carry the Natureplus sustainability label — the strictest third-party certification for bio-based building products in Europe — and are available in Spain through Knauf's national distributor network.
Hemp vs wood fibre: the key differences
Both hemp and wood fibre insulations share the core benefits of hygroscopicity and carbon negativity, but there are meaningful differences worth understanding at specification stage:
- Thermal performance: Both are broadly equivalent (λ ≈ 0.038–0.042 W/mK). Neither matches aerogel or rigid PIR foam for thin-section performance, but in any application where thickness is not constrained, they are fully competitive.
- Moisture buffering: Hemp fibre has a higher moisture buffering value (MBV) than wood fibre — approximately 2.3 g/m²·%RH vs 1.8 — making it slightly superior in high-humidity applications such as bathrooms or swimming pool enclosures.
- Fire performance: Both require fire-retardant treatment to achieve Euroclass E. Knauf NaturBoard uses ammonium sulphate, a safe and widely approved treatment. Untreated bio-based insulation is not acceptable under most European building codes.
- Installation: Hemp batt is the most contractor-friendly bio-based insulation format — cut with a bread knife, friction-fit between studs, no protective gloves required. Wood fibre board requires standard saw cutting and mechanical fixings.
- Cost: Both are approximately 20–40% more expensive than equivalent glass-wool products at point-of-purchase. On lifecycle cost models including avoided condensation remediation, the gap narrows significantly.
Our recommendation
For new-build timber-frame residential projects and passive-house renovations in Spain, Knauf NaturBoard HEMP is our first-choice specification for stud-cavity insulation. The carbon sequestration story is compelling for client presentations, the installation is genuinely easier than mineral wool, and the hygroscopic performance is measurably superior for the Mediterranean climate.
For blown attic insulation — the single easiest sustainability upgrade in any Spanish residential renovation — Isover ISOFLOC wood fibre can be professionally installed in a day and will deliver both meaningful thermal improvement and carbon removal simultaneously.
Affiliate disclosure: This article contains affiliate links to Knauf NaturBoard and Isover. Soul Materials only recommends products independently evaluated by our editorial team. Commissions support our work at no additional cost to you.
