It can sound like something from an eco-village. Probably expensive. Maybe the tradespeople do not know how to install it. And is it even approved?
Those concerns are reasonable. Bio-based materials have carried an image problem for years: too niche, too uncertain, too experimental. But the Danish market has moved. Research and demonstration projects now document where the materials work, what they cost, and what still needs care.
This guide is based on Danish source material including BUILD Report 2022:09 on biogenic materials in construction, Realdania’s research programme “Veje til Biobaseret Byggeri” and CFBB - Center for Biobaseret Byggeri. The figures should be read as orientation, not as a specification for a particular house.
The short answer
Bio-based materials are no longer only experimental. Wood fibre insulation, hemp insulation and cellulose insulation are documented products used in Danish construction. They cost more than mineral wool, typically 30-100% on material price, but they store carbon instead of only adding emissions.
They can also fit older Danish houses better because their moisture behaviour is closer to many original materials. That does not mean you should always choose them. It means you can consider them seriously, and the choice should be based on the house, the construction and the budget rather than on assumptions.
What are bio-based materials?
Bio-based building materials are made from biological raw materials: plants, animals or marine organisms. Unlike mineral wool and glass wool, which require energy-intensive melting of stone or glass, many bio-based materials begin with photosynthesis. The plant absorbs CO2 while growing, and the carbon remains stored in the material while it is part of the building.
It is not only insulation. Bio-based construction also includes timber structures such as CLT, straw-bale construction, cork boards and clay plaster systems. In a Danish renovation, however, insulation is usually where the choice becomes practical first.
The six most relevant options in Denmark
| Material | GWP, approx. | Price compared with mineral wool | Best suited to |
|---|---|---|---|
| Wood fibre insulation | 0.15-0.55 kg CO2-eq/kg | +30-60% | Attics, exterior walls, summer overheating |
| Hemp insulation | -1.5 to -1.8 kg CO2-eq/kg | +50-80% | Exterior walls, vapour-open build-ups |
| Sheep wool insulation | -3 to -4 kg CO2-eq/kg | +80-120% | Moisture-sensitive rooms, timber construction |
| Straw | -1.0 to -2.0 kg CO2-eq/kg | Varies | New build, larger renovations |
| Cork | approx. -2.0 kg CO2-eq/kg | +60-100% | Floors, facade layers, thermal breaks |
| Seaweed insulation | -0.5 to -1.0 kg CO2-eq/kg | Rare in Denmark | Attics and niche projects |
GWP data should be checked against product-specific EPDs when a project moves toward specification. As a rough comparison, mineral wool is often around 1.05 kg CO2-eq/kg and glass wool around 0.85 kg CO2-eq/kg.
When does it make sense?
Bio-based materials are not automatically the right choice. But there are situations where they can be the best choice.
Older houses with vapour-open construction. A brick villa from the 1920s, a half-timbered house or a lime-rendered wall needs a careful moisture strategy. Hemp insulation and wood fibre insulation can suit these structures because they buffer moisture and allow drying when the surrounding build-up is designed correctly.
Attic insulation. This is often the most accessible place to switch material. Loose-fill wood fibre can be blown onto an existing attic layer in the right build-up. It is closer to mineral wool in installed price than many expect, and the thermal mass can help reduce summer overheating.
When carbon matters. If the renovation has a clear climate ambition, insulation is one of the places where a material change can matter. A typical detached house can contain 10-15 m3 of insulation. The difference between mineral wool and hemp insulation can become several tonnes of CO2 over the building’s life, depending on quantities and accounting method.
Indoor climate and moisture buffering. Bio-based insulation can absorb and release moisture from the air. In the right construction this can support a more stable indoor climate, especially in rooms without mechanical ventilation.
When is it the wrong choice?
There are places where bio-based materials should not be used, and that boundary is important.
Basements and ground slabs. Direct ground contact and capillary moisture are serious risks. Cork is more moisture-resistant than most bio-based materials, but basement insulation will often need materials such as EPS or XPS, depending on the detail.
Constructions with a plastic vapour barrier. Hemp, sheep wool and wood fibre need a vapour-open strategy. If they are trapped behind a plastic vapour barrier in the wrong build-up, moisture can enter without a reliable drying path. That is one of the common failure modes in retrofit insulation.
Projects with no budget room. If the budget is tight, mineral wool can still be a sensible choice. Good detailing with a standard material is better than a poor bio-based solution chosen for the wrong reason.
Inverted roof build-ups. Roofs where insulation lies above the membrane are exposed to long-term moisture. Bio-based insulation is not suited to that condition.
What does it cost?
| Material | Material price, approx. | Installed price, approx. | Compared with installed mineral wool |
|---|---|---|---|
| Mineral wool reference | 150-250 DKK/m2 | 350-500 DKK/m2 | Reference |
| Wood fibre insulation | 150-400 DKK/m2 | 400-800 DKK/m2 | +30-60% |
| Hemp insulation | 200-350 DKK/m2 | 500-700 DKK/m2 | +40-80% |
| Sheep wool insulation | 250-450 DKK/m2 | 550-800 DKK/m2 | +60-120% |
| Cork | 200-600 DKK/m2 | 500-900 DKK/m2 | +50-100% |
| Straw | 80-150 DKK/m2 | 600-1,200 DKK/m2 | Varies |
The material price and the installed price are not the same thing. Straw can be cheap as a raw material but expensive to detail and install. Wood fibre loose fill can be surprisingly close to mineral wool when the whole installation is counted.
The price difference is usually largest in small projects. On larger renovations, scaffolding, site setup and labour coordination can matter more than the material price alone.
Compatibility matters more than the label
The most important word is not “natural”. It is compatibility. Bio-based insulation only works well when the whole construction can handle moisture, air movement, fire requirements and maintenance.
Good combinations often include:
- Hemp insulation with lime render and untreated timber in a vapour-open exterior wall.
- Wood fibre insulation with timber structure, wood fibre wind barrier and a suitable vapour retarder.
- Sheep wool with a paper-based variable vapour retarder in lightweight timber construction.
- Cork with masonry or concrete where moisture resistance and compression strength are needed.
- Straw with clay plaster and lime in construction systems designed for straw.
Problematic combinations include:
- Bio-based insulation behind a plastic vapour barrier in a build-up with no drying path.
- Mineral wool and wood fibre mixed in the same cavity without a moisture assessment.
- Hemp insulation directly against damp concrete.
- Cork combined with unsuitable chemical adhesives.
CFBB publishes guidance on bio-based construction in Denmark. Product manufacturers also provide build-up details, but these should still be checked against the specific house and the existing construction.
How to move forward
Start with the construction, not the material. Is the wall vapour-open? Is there existing moisture damage? Can the structure dry? Are there fire or heritage requirements? What is the budget difference after labour, scaffolding and detailing are included?
If the house is cold, the broader question may be retrofit insulation. If the problem is condensation or mould risk, start with moisture and the vapour barrier. If the goal is lower climate impact, look at GWP, EPDs and the full LCA, not only the marketing name of the product.
Bio-based materials can be a strong choice in Danish renovation. They just need to be chosen as part of a building detail, not as a slogan.
Sources: BUILD Report 2022:09, Biogene materialers anvendelse i byggeriet. Realdania, Inspirationskatalog for Biobaserede Konstruktioner and Veje til Biobaseret Byggeri. Realdania, Opskalering af biobaserede byggematerialer (2025). CFBB - Center for Biobaseret Byggeri.