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Natural insulation materials—hemp, cellulose, sheep’s wool and wood fibre—can all reduce heat loss, but none is the universal “best” choice. Their published conductivity ranges overlap, while density, treatment, product form, moisture exposure and installation method can change real performance. Choose the product designed for your wall, roof or floor build-up, then verify its current declared data, fire classification and moisture compatibility.
For a quick comparison, cellulose is commonly used as loose fill or batts, hemp as fibre batts or slabs, wool as rolls or boards, and wood fibre as boards or batts. The practical decision is therefore an assembly-design question rather than a simple ranking of plant or animal fibres.
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At a glance
| Name | Best for | Key spec | Checked |
|---|---|---|---|
| Hemp fibre | Batts or slabs in framed walls, roofs and floors where a vapour-open build-up is designed | BRE typical conductivity: 0.038–0.040 W/mK | November 2026 |
| Cellulose | Loose-fill roof spaces and suitable cavities; semi-rigid batts where specified | BRE typical conductivity: 0.035–0.040 W/mK | November 2026 |
| Sheep’s wool | Rolls or boards in moisture-buffering, vapour-permeable assemblies | BRE typical conductivity: 0.038–0.040 W/mK | November 2026 |
| Wood fibre | Rigid or semi-rigid boards where a designed board system is required | BRE typical conductivity: 0.038–0.050 W/mK | November 2026 |
Conductivity figures are historical typical ranges from the Building Research Establishment (BRE) 2011 overview, not current specifications for a particular product.
What is the best natural insulation?
There is no single winner. The best material is the one whose declared thermal value, density, moisture behaviour, fire evidence and installation method suit the complete construction at the required thickness.
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A lower thermal-conductivity number generally means less thickness is needed for the same thermal resistance. However, compare like with like: a declared value for a dry hemp-fibre batt is not interchangeable with a design value for a dense hemp-lime wall, a wood-fibre sarking board or blown cellulose. The resulting U-value also depends on studs, thermal bridges, air leakage, fixings and every other layer.
Use manufacturer or supplier declarations for the actual product. BRE’s ranges are useful for orientation, but they date from 2011 and should not be treated as a modern product specification. A 2026 Lewes Home Retrofit Guide lists indicative values of 0.038 W/mK for wood fibre and sheep’s wool and 0.06–0.07 W/mK for hempcrete. That difference illustrates why hemp fibre and hemp-lime must be compared separately.
How do the thermal conductivity values compare?
The following figures show the overlap reported by BRE, not a controlled head-to-head test:
| Material | BRE typical conductivity (W/mK) | Formats listed by BRE | What can change the result |
|---|---|---|---|
| Cellulose (paper) | 0.035–0.040 | Loose fill; semi-rigid batts | Blowing method, installed density, settlement provisions and moisture condition |
| Hemp fibre | 0.038–0.040 | Semi-rigid slabs; batts | Density, binder or treatment, compression and the surrounding frame |
| Sheep’s wool | 0.038–0.040 | Semi-rigid boards; rolls | Product density, treatment, compression and installation gaps |
| Wood fibre | 0.038–0.050 | Boards; semi-rigid boards; batts | Board density, thickness, orientation and intended application |
An experimental 2024 paper on selected natural-fibre specimens reported conductivities from 0.0333 to 0.0438 W/(m·K) and short-term water absorption of 0.5–8.5 kg/m². Those are results for the specimens studied, not universal ranges for every hemp, wool or cellulose product; read the study’s methods and material descriptions before applying them.
For design, ask for the declared lambda value, the test or declaration standard, the density used, and any design correction for moisture or ageing. Then calculate the complete assembly’s U-value and check condensation risk rather than selecting by a brochure number alone.
What does “breathable insulation” mean?
“Breathable” is not a promise that a wall cannot become damp. The Centre for Alternative Technology (CAT) describes breathability as “the ability to buffer moisture – to soak it up and then release it.” Vapour permeability, hygroscopicity and capillary transport are related but different properties.
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A safe build-up needs a continuous drying path, controlled rain exposure, airtightness, adequate ventilation and a sensible sequence of layers. CAT cautions that natural materials can be sealed behind finishes that trap moisture. A vapour-permeable insulation will not compensate for a leaking roof, rising damp, driving rain, plumbing failure or poor indoor ventilation.
Check the vapour-resistance data for each layer, the direction in which the assembly can dry, and whether the existing wall contains impermeable paint, cement render or membranes. For older masonry, a hygrothermal assessment may be needed before internal insulation is installed.
How does cellulose compare with hemp, wool and wood fibre?
Cellulose: efficient loose fill when the installation is controlled
Cellulose is paper-based insulation supplied mainly as loose fill or semi-rigid batts. Its BRE range, 0.035–0.040 W/mK, is competitive with the other materials in this comparison. Loose fill can suit an appropriate loft or cavity because it fills irregular spaces, but the installer must achieve the specified density and provide for settlement where the product requires it.
CAT notes that loose fill is quick to use in lofts but that DIY use is limited to flat locations. Sloping roofs, enclosed cavities and areas exposed to air movement need a product and installation system specifically approved for those conditions. Ask how the material is blown or placed, how it is retained, and how penetrations and ventilation paths are protected.
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Hemp-fibre batts and slabs are flexible insulation products with a BRE typical conductivity of 0.038–0.040 W/mK. They can be friction-fitted or cut into framed construction when the selected system permits it.
Hemp-lime (hempcrete) is a composite of hemp shiv and a lime-based binder, not a denser version of a hemp batt. CAT explains that its conductivity varies with composition and density; the Lewes guide gives an indicative 0.06–0.07 W/mK. Because hemp-lime is usually installed as a wall or infill system with different thicknesses and drying behaviour, compare it with similar assemblies, not with a factory-made fibre batt.
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Sheep’s wool: moisture buffering with treatment to verify
Wool is supplied as rolls or boards and has a BRE typical conductivity of 0.038–0.040 W/mK. Its hygroscopic fibres can take up and release some moisture, which can help moderate short-term humidity swings in a correctly designed assembly.
That does not remove the need for a rain screen, airtight layer or ventilation. CAT advises checking the documentation supplied with manufactured wool products for insulation value and standards compliance. Confirm the exact product’s pest treatment, fire treatment, durability information and certification; do not assume untreated wool has a universal fire or insect performance.
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Wood fibre is available as rigid boards, semi-rigid boards and batts. BRE reports a wider typical conductivity range, 0.038–0.050 W/mK, because density and product purpose vary substantially. A dense exterior board, a flexible cavity batt and a flooring product should not be treated as interchangeable.
Board systems can provide useful continuity across framing or a defined sarking layer, but they require correct joints, fixings, weather protection and edge detailing. Verify whether the board is intended for internal, external, roof or floor use and whether its declared value is measured at the installed orientation and moisture condition.
Which natural insulation is best for an old stone house?
For an old stone house, compatibility with moisture movement and rain exposure matters at least as much as conductivity. A vapour-open internal wall build-up may be appropriate, but the correct choice depends on the stone, pointing, render, exposure, existing damp and the proposed finish.
- Repair water entry first. Resolve gutters, roof leaks, plumbing leaks, defective pointing and ground-level damp before adding insulation.
- Survey the existing wall. Record thickness, construction, salts, finishes, orientation and exposure. Cement-rich or impermeable finishes can alter drying.
- Model the proposed layers. Check vapour resistance, capillary behaviour, interstitial condensation and the direction of drying. “Breathable” should describe the assembly, not just the insulation.
- Detail junctions. Window reveals, floors, ceilings, corners and service penetrations often create thermal bridges or air leaks that undermine the insulation field.
- Use a compatible finish. The inner and outer finishes must not trap moisture or prevent the wall from drying as the design assumes.
BRE advises that natural-fibre insulation is not suitable below damp-proof-course level unless it is fully protected by a damp-proof membrane. Treat that as technical guidance to verify against current local requirements and the specific product approval.
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What should I check for fire, moisture and safe installation?
Natural fibres are combustible materials and are commonly treated with fire retardants. Before specifying one, obtain the current reaction-to-fire classification for the exact product, thickness and facing, plus the tested build-up or approved application. A classification for a board behind plasterboard does not automatically apply to an exposed batt in a roof.
Also check water absorption, capillary behaviour, vapour resistance, dimensional stability and any limits on ground contact. Keep insulation dry during storage and construction, protect it from wind and rain where required, and do not close a wall until moisture readings and the design sequence allow it to dry.
Installation methods differ. Some products can be cut with a knife; denser boards may need a saw. CAT recommends wearing a face mask during installation because small fibres should not be inhaled. Follow the product’s handling guidance, use eye and skin protection where specified, and keep loose fill out of ventilation channels and electrical clearances.
Are natural insulation materials environmentally better?
Bio-based content does not produce a universal environmental ranking. A 2024 scenario-based life-cycle assessment modeled eleven bio-based insulation products, including sheep’s wool, cellulose, wood fibre and hemp fibre or hemp lime. It varied product properties, manufacturing location, transport and end-of-life assumptions, and used a common thermal-transmittance functional unit. Its results show that conclusions change with those assumptions.
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Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How should I compare quotes and product data?
Prices are not supplied by the cited research and vary by region, thickness, access, labour and whether a specialist installer is required. Obtain quotations for equivalent thermal resistance and a complete installed build-up, not simply a price per bag, roll or board.
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- Thermal: declared conductivity, design value, density, thickness and calculated assembly U-value.
- Moisture: vapour-resistance factor, hygroscopic or capillary data, rain exposure limits and drying strategy.
- Format: loose fill, batt, slab, roll or board; cutting, blowing, friction-fit and retention requirements.
- Fire: reaction-to-fire class, treatment, facings, tested thickness and approved end use.
- Durability: settlement, compression, pests, moisture limits and expected service conditions.
- Environmental: current EPD or LCA, functional unit, transport, manufacturing and end-of-life assumptions.
- Installation: installer qualifications, inspection points, weather protection, airtightness detailing and waste handling.
An insulation knife may be useful for cutting suitable batts, but the tool does not make a product appropriate for a particular location. The product instructions and the assembly design come first.
What is the practical decision?
Start with the part of the building and the moisture risk. Cellulose is often practical for controlled loose-fill roof or cavity work; hemp fibre suits many framed, vapour-open applications; wool offers hygroscopic buffering where its treatment and certification meet the project; and wood fibre provides a range of board and batt systems with density-specific uses. Those are application tendencies, not guarantees.
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Shortlist products with current declarations, design the whole wall, roof or floor, and have fire, moisture and building-control requirements checked for your jurisdiction. The material with the lowest quoted conductivity is not automatically the safest, thinnest, lowest-carbon or cheapest installed solution.
How we compared the materials
The comparison uses the BRE format and indicative conductivity data, CAT’s guidance on moisture buffering and installation, the 2026 Lewes retrofit guide’s contextual figures, the cited natural-fibre performance study and the 2024 scenario-based life-cycle assessment. The criteria were thermal performance at the required thickness, moisture and assembly compatibility, product format, fire and approval evidence, environmental assessment quality, and practical installation. No current price, manufacturer declaration or code-compliant project specification was inferred where the sources did not provide one.
Frequently Asked Questions
What documents should I request from an insulation supplier?
Ask for the current technical datasheet, declared thermal conductivity, density, vapour and moisture data, reaction-to-fire classification, installation instructions and any EPD. Check that the documents cover the exact thickness, facing and application you are buying, rather than a related product in the same range.
Can I install blown cellulose myself?
Only in locations and formats the product documentation permits. CAT notes that loose fill can be suitable for flat lofts but that DIY use is limited there; enclosed cavities and sloping roofs generally require controlled blowing, retention and settlement provisions by a competent installer.
Should a damp wall be insulated before the moisture problem is fixed?
No. Leaks, defective gutters, plumbing faults, ground moisture and unsuitable finishes should be investigated and corrected first. Adding insulation can hide a defect or reduce drying if the new layer and finish are not designed for the wall’s moisture path.
Does a lower lambda value always mean a thinner finished wall?
Not necessarily. Thickness also depends on framing, thermal bridges, fixings, air leakage and the performance of every layer in the assembly. Use the product’s declared value in a whole-assembly calculation and include any design adjustments for moisture or ageing.
