Summary

Natural insulation materials have grown significantly in market share in the UK since approximately 2010, driven by growing interest in sustainable construction, heritage building retrofit, and the Passivhaus movement. They differ from synthetic insulants (PIR, EPS, XPS) and mineral alternatives (glass wool, rock wool) primarily in their hygroscopic properties: natural materials can absorb and release significant amounts of moisture vapour without losing their insulating function, a property known as hygric buffering.

This hygric buffering is important in the context of solid masonry buildings — the pre-1919 housing stock of Victorian terraces, Georgian townhouses, and pre-war semis — where walls have traditionally been vapour-open and managed moisture by absorption and evaporation rather than by exclusion. Installing a vapour-closed insulation system (PIR with a VCL) in these buildings introduces a condensation risk that breathable natural materials avoid.

However, natural materials are not universally superior. For most modern cavity wall construction, timber frame, and new build, standard synthetic or mineral insulation with correct VCL placement is technically sound and more cost-effective. The choice between natural and conventional insulants should be driven by the specific application and building type, not by a general preference for natural products.

Key Facts

  • Sheepswool (Thermafleece, Black Mountain) — λ ≈ 0.036–0.039 W/m·K; hygroscopic (absorbs up to 35% of its own weight in moisture without losing function); excellent for cold loft, stud frame, and rafter insulation; soft and easy to handle; treated with borax fire retardant and insect repellent; higher cost than mineral wool but comparable performance
  • Hemp fibre (Hempflax, Isochanvre) — λ ≈ 0.040–0.044 W/m·K; batts for stud frame and rafter use; good hygric buffering; similar handling to mineral wool; less widely available; growing availability through UK merchants
  • Wood fibre board (Steico, Gutex) — λ ≈ 0.038–0.052 W/m·K depending on product; rigid boards and batts; excellent hygric buffering; can be used as sarking boards in pitched roofs (Steico Protect) combining structure, weather barrier, and insulation; compatible with lime plaster; good acoustic performance; Class E fire rating
  • Cellulose (blown, Warmcel, Excel) — λ ≈ 0.035–0.040 W/m·K; loose-fill for attics (dry blown) or dense-pack for stud frames and floor joists (machine-blown); high recycled content (90%+ recycled newspaper); excellent airtightness when dense-packed; treated with borate fire and insect retardant; requires specialist blown-in equipment
  • Cork (expanded cork board, Amorim, Cantalou) — λ ≈ 0.040–0.045 W/m·K; vapour-open; very durable; not subject to insect or fungal attack; used in IWI and EWI applications; higher cost; excellent compressive strength relative to lambda; suitable for below-slab in combination with tanking
  • Aerogel (Spacetherm, Cabot) — λ ≈ 0.013–0.016 W/m·K; silica gel with nanoporous structure; the highest-performing insulation material available for commercial use; extremely expensive (typically 5–10× PIR cost per m²); used where thickness is severely constrained — in listed buildings, at reveals, for heritage plaster panels; available as blanket (Spacetherm A2) or slurry (Spacetherm WB)
  • Fire performance — most natural insulation materials are classified Class E in the Euroclass system; wood fibre and hemp fibre are combustible; sheepswool chars slowly; aerogel is non-combustible (A2 class for Spacetherm)
  • PAS 2030 compatibility — cellulose and sheepswool are increasingly accepted under PAS 2030-certified installer schemes for ECO4 and funded work; check the specific scheme requirements
  • Passivhaus — wood fibre is widely used in certified Passivhaus buildings in the UK; cellulose dense-pack is used for airtightness in combination with wind-tight sheathing; the Passivhaus Institute provides product assessment data

Quick Reference Table

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Material Lambda (W/m·K) Fire Class Hygric Buffering Relative Cost Best Applications
Sheepswool 0.036–0.039 E Very high Medium-high Cold loft, rafter, stud frame
Hemp fibre 0.040–0.044 E High Medium-high Stud frame, rafter, floor joist
Wood fibre board 0.038–0.052 E High Medium-high IWI, sarking, ventilated facade
Cellulose (blown) 0.035–0.040 B (with retardant) High Medium Attic top-up, dense-pack wall
Cork board 0.040–0.045 E Moderate High IWI (heritage), below slab
Aerogel blanket 0.013–0.016 A2 Moderate Very high Listed buildings, reveals, constrained thickness

Detailed Guidance

Sheepswool Insulation

Sheepswool is produced in the UK by several manufacturers, including Thermafleece (Natural Insulations) and Black Mountain Insulation. It is available as batts in standard stud and rafter widths (400mm, 600mm) and as loose-fill for attic applications.

The key advantage of sheepswool over glass or rock mineral wool is its hygroscopic behaviour. Wool fibres can absorb up to 35% of their own dry weight in moisture vapour without losing their insulating properties, and they release moisture as conditions change. This buffering means that in a breathable wall or roof construction, sheepswool can accommodate seasonal and diurnal moisture fluctuations without a condensation plane forming at a single interface.

Sheepswool must be treated with borax or a similar borate compound for fire retardancy and to resist moths and other insects. All UK commercial sheepswool insulation products include this treatment; it is not something the installer needs to add. The borate treatment is low-toxicity and water-soluble, which means sheepswool must be kept dry during installation — prolonged wetness will wash out the fire retardant.

For cold loft insulation: install in two layers (100mm between joists, 170mm across joists perpendicular) for a total of 270mm to achieve approximately 0.16 W/m²K. For rafter insulation in a warm pitched roof: fill the full rafter depth (typically 150mm) and add a further layer over the rafters before boarding. Handling is more pleasant than glass wool — no respiratory irritation, no itch, no need for gloves unless cutting.

Wood Fibre Insulation

Wood fibre products are manufactured by several European producers, with Steico (Germany/Poland) and Gutex (Germany) being the most widely available in the UK. Products fall into two main categories: rigid boards (Steico Protect, Steico Universal, Gutex Thermowall) and flexible batts (Steico Flex, Gutex Multitherm).

Rigid wood fibre boards have a unique combination of properties that make them suitable for use as external sarking boards in warm pitched roof constructions. Steico Protect, for example, is dense enough to act as a structural racking element in some applications, provides a degree of weather protection during construction, and its vapour-open properties allow the roof to manage moisture without a separate breather membrane in some configurations (check manufacturer's guidance and BS 5250 requirements for the specific build-up).

Wood fibre is compatible with lime plaster systems, which is important in heritage applications. Standard gypsum plasters are not breathable; lime plasters are. A wood fibre board IWI system finished with lime plaster is a fully vapour-open assembly suitable for solid masonry buildings where breathable construction is required.

Wood fibre has excellent acoustic performance — better than mineral wool of equivalent thickness for airborne sound reduction — which is a useful co-benefit in terrace houses and conversions.

Cellulose (Blown) Insulation

Cellulose insulation (Warmcel 100, Excel Fibre Technologies products) is manufactured from recycled newspaper, treated with borate compounds for fire retardancy and pest resistance. It has the highest recycled content of any commercial insulation product (typically 90%+).

For loft insulation, cellulose is blown dry by a machine on to the attic floor between and over joists. It is cost-competitive with mineral wool when installed professionally, and its hygric properties are excellent. The loose-fill nature makes it ideal for topping up existing loft insulation to 270–300mm total depth.

Dense-pack cellulose is a different application: the material is blown under pressure into a sealed cavity (wall void, floor void, closed roof panel). The density achieved (50–80 kg/m³) gives it significant structural stiffness and excellent airtightness — making it effective as an air barrier as well as an insulator. Dense-pack cellulose is used by many Passivhaus-specification builders in the UK for stud-frame wall cavities and closed-panel timber construction.

Both applications require specialist equipment (a blowing machine and appropriate hose). This is not a DIY installation for wall applications; loft installation is more accessible for competent DIYers using hired equipment and appropriate PPE (dust mask is essential, as cellulose dust is a respiratory irritant even with borate treatment).

Cork

Cork as an insulation material has been used in Europe for centuries and is experiencing something of a revival in the UK heritage and sustainability markets. Expanded cork board (dark brown, granular appearance) is produced by steaming and compressing granules of recycled cork stoppers or bark cork. The expansion process bonds the granules together without added adhesives.

Expanded cork is vapour-open, highly durable, naturally resistant to fungi and insects (it contains suberin, a natural preservative), and has a compressive strength (typically 200–400 kPa at 10% compression) higher than EPS and approaching XPS. This makes it suitable for:

  • Below-slab floor insulation in waterproofed basements (compatible with BS 8102 Type C systems)
  • IWI on historic masonry where breathability and durability are priorities
  • Below-grade wall insulation where moisture resistance is needed

Cork boards are produced in standard sizes (typically 1000mm × 500mm) and thicknesses of 20–150mm. They are cut with a standard hand saw or band saw. No specialist PPE is needed beyond normal construction dust precautions.

The cost premium for cork over EPS or PIR is significant — roughly 3–4× per m² at equivalent performance. This is justified in specific applications (heritage IWI, waterproofed basements, Passivhaus-quality details) rather than for standard domestic insulation.

Aerogel

Aerogel is the highest-performance commercially available insulation, with a lambda value of approximately 0.013–0.016 W/m·K for current products. Spacetherm (manufactured by Cabot and distributed by A.Proctor Group in the UK) is the most commonly used product on UK construction projects. It is available as:

  • Spacetherm Blanket — aerogel particles bonded in a non-woven matrix; typically supplied as 5–10mm sheets bonded to an insulation carrier board (PIR, EPS)
  • Spacetherm WB — aerogel plaster slurry applied directly to the wall surface; provides approximately 5–10mm equivalent insulation with minimal thickness loss

Aerogel's extreme cost (typically £150–300/m² installed) limits it to applications where thickness is genuinely critical and no other product can meet the requirement:

  • Listed buildings where wall thickness cannot increase more than 5–10mm
  • Narrow window reveals in EWI and IWI installations
  • Underfloor heating systems where slab thickness is constrained
  • Conservation area properties where EWI is not permitted and IWI must be minimal

Spacetherm A2-rated products (Spacetherm A2 Blanket) are non-combustible and can be used on buildings over 18m where combustible materials are restricted.

When to Specify Natural vs Conventional Insulation

The decision matrix:

  • Solid masonry pre-1919 building, breathable construction required → wood fibre, cork, or sheepswool; avoid PIR with VCL
  • Heritage/listed building, minimal thickness → aerogel; sheepswool for flexible applications
  • New-build timber frame → sheepswool, hemp, or cellulose are technically appropriate but mineral wool is significantly cheaper for same performance; natural materials offer hygric buffering advantage but the difference in practice is modest in well-designed timber frame
  • Cold loft, standard cost → sheepswool or cellulose are competitive with mineral wool; decision based on sustainability preference and budget
  • Flat roof warm construction → wood fibre rigid boards can be used as warm roof insulant; PIR is more cost-effective per mm
  • Below-slab or high-moisture floor → cork (if budget allows) or EPS/XPS (conventional choice); cellulose and sheepswool are not appropriate in wet environments

Frequently Asked Questions

Are natural insulation materials significantly more sustainable than synthetic?

Broadly yes, but the comparison is nuanced. Natural materials have lower embodied carbon when measured over the full lifecycle, and many are carbon-storing (wood fibre and cellulose lock up carbon during the tree's growth). However, borate fire retardants, processing energy, and transport all contribute to embodied energy. The Building Research Establishment (BRE) Environmental Profiles database provides comparative data. Sheepswool and cellulose consistently score well; wood fibre boards have higher processing energy than loose fill materials.

Can I use sheepswool below a floor screed?

No. Sheepswool requires moisture to function properly as an insulator, but sustained compressive loading from a screed will compress it over time, significantly reducing its thickness and therefore its thermal resistance. For floor insulation below screed, use rigid boards (PIR, cork, or EPS floor grade) that have the compressive resistance to support the structural load. Sheepswool can be used in a suspended timber floor where it is draped between joists without compressive loading.

Do I need a VCL with wood fibre or sheepswool insulation?

Not in a vapour-open breathable construction — that is the point. However, for a standard UK domestic new-build timber frame, the building physics still supports using a VCL on the warm side of the insulation. Whether the insulation is sheepswool or mineral wool, vapour pressure differential in a UK winter drives vapour toward the cold OSB sheathing. The insulation type (natural vs synthetic) does not eliminate this; it reduces the risk because the hygric buffering of natural materials moderates moisture peaks. For heritage solid masonry, no VCL; for modern timber frame, a VCL (foil-faced plasterboard) is still good practice even with natural insulation.

Are natural insulation products accepted by building control?

Yes, provided they hold a BBA certificate or equivalent third-party assessment. Most mainstream natural insulation products (Thermafleece, Steico, Warmcel) are BBA-certified. Some less common products may not be, which can create difficulties with building control. Always check the BBA status before specifying for a project requiring Building Regulations compliance.

Regulations & Standards