Can sheep's wool insulation be used in older homes?

The limits are just as clear. Wool is intended for internal environments and should not be used where permanent moisture build-up is expected³. It is not a cavity fill, and cavity wall insulation of any kind should not be installed where there are structural issues, damp, dirty cavities or exposed driving-rain locations⁴. Fire performance is the other constraint: wool is treated with borax rather than being inherently non-combustible, so mineral wool remains the material of choice where an A1 or A2 Euroclass rating is required⁵.

Why breathability matters in solid-walled and traditional buildings

A traditional building has no cavity to drain and no damp-proof course in the modern sense. Its walls take in moisture from driving rain, rising damp and condensation, and they have to let it back out again by evaporation. Official guidance is explicit that the fabric of a traditional building needs to "breathe", so that moisture within walls can be released through evaporation⁷. Independent retrofit guidance puts it more bluntly: in an older or solid-wall home, breathable materials and steady ventilation are essential⁸.

This is where the choice of insulation stops being a matter of thermal performance alone. The breathability of insulation materials is particularly important in older buildings with solid walls⁹, because a material that seals the wall from the inside traps moisture against timber and masonry. Wool's highly breathable properties allow moisture to move safely through the fabric of the building, reducing the risk of condensation, damp or mould¹⁰. The same principle governs the finishes: in older homes, breathable materials such as lime plaster let walls dry naturally⁸.

There is a summer benefit too. Researchers from Leeds Sustainability Institute found fabric insulation of walls and roofs to be one of the most effective ways to retrofit an older, solid walled home to prevent overheating in summer and prevent heat loss in winter¹¹. For a household, the independence argument is straightforward: a breathable fabric retrofit reduces reliance on heating and on mechanical cooling, and it does so without creating the damp problems that force a household back onto a dehumidifier or an extractor running continuously.

Where it cannot be used: cavity walls and other limits

The maker states that wool insulation can be used in floors, roofs, walls and lofts¹⁰. That describes internal construction, not a blown cavity fill. Wool is not a cavity wall insulation product, and the general rules on cavity filling are strict regardless of material. Cavity wall insulation should not be installed where there are poor maintenance or structural issues such as cracks, evidence of rising or penetrating damp, dirty cavities, wall ties or builder's rubble, exposed locations with extreme weather and driving rain, or overflowing and blocked guttering, where no remedial action is appropriate⁴.

Where a cavity is uninsulated or only partially insulated, official guidance for external wall insulation says it should be assumed not suitable unless expert advice is sought¹². That is a separate measure from cavity fill, but it shows how cautiously the fabric of an older wall is treated. For rented homes, the position is written into the exemption rules: cavity wall insulation, external wall insulation or internal wall insulation for external walls is not considered a relevant measure where written expert advice indicates it is not appropriate because of its potential negative impact on the fabric or structure of the property¹³.

Other natural materials carry their own exclusions. Hemp insulation is not suitable for use in potentially damp environments without a vapour barrier, such as cellars and basements⁶. Spray foam should not be used in listed buildings or houses with thatched roofs¹⁴. Wool's own exclusion is the moisture one: it is intended for internal environments and should not be used where permanent moisture build-up is expected³.

Fire performance: Euroclass E, borax treatment and where mineral fibre is still needed

Wool is treated, not inert. The maker states that under normal service conditions the borax remains within the wool fibres and provides protection for the entire life of the insulation³. The maker also states that sheep wool is not flammable and has the benefit of being far more breathable than synthetic loft insulation¹. Those are the maker's own claims about its own product, and they describe a treated natural fibre rather than a mineral one.

Mineral wool occupies a different category. It is non-combustible and A1/A2 Euroclass fire rated, and in separating floors that rating helps compliance with Part B⁵. The same non-combustible A1/A2 rating is cited for mineral wool in timber frame walls¹⁶ and for roofs¹⁷. Mineral wool is also widely used for closing the external wall cavity at the junction between separating walls and external walls, to minimise flanking sound transmission and fire spread¹⁸.

The practical consequence for an older home is that wool and mineral wool are not interchangeable in every position. In a loft floor, a breathable wool batt between joists is a normal specification. At a party wall junction, in a separating floor, or anywhere a building control body expects a non-combustible A1/A2 material, mineral wool is the material the guidance describes. A household retrofitting a period property will often end up with both: wool in the breathable parts of the fabric, mineral wool where fire separation or cavity closure is the governing requirement.

Pests, insects and rodents: what the evidence shows

The maker's position is that the borax treatment provides protection for the entire life of the insulation under normal service conditions³. Independent guidance does not single wool out as a pest problem. What it does list, among the reasons old loft insulation has to be removed, is rodents or other pests causing damage, alongside significantly squashed insulation, hazardous material such as asbestos, and any damage from water or fire¹⁵. Pest damage is a general risk in lofts, not a property of one fibre.

The conditions that invite rodents are structural and behavioural: gaps at eaves, unsealed service penetrations, stored food, and voids that give shelter. A tightly fitted wool batt with no gaps is no more attractive than a mineral wool roll in the same position, and both are less vulnerable than a loose fill that can be burrowed through. Where a thatched property is involved, the guidance on flues is separate and specific: a bird guard type cowl of an open-type mesh design may be required to avoid nesting within the flue¹⁹.

Ventilation is the other half of the pest and moisture picture. Pollutants including VOCs, humidity, CO2 and carpet fibres build up in sealed homes²⁰, and a breathable fabric only works if the building can still exchange air. For an older property, that means keeping the ventilation paths the building was designed with rather than sealing them as part of an insulation upgrade.

Cost, thickness and installing it yourself between joists

Wool is a DIY-friendly material in the same family as blanket insulation, which independent guidance describes as suitable for DIY install, inexpensive, and available in more sustainable and non-irritant options including recycled glass and sheep's wool²¹. The installation method for a loft floor is well established: one of the cheapest ways to do this is by installing mineral wool insulation between and on top of the joists²², and insulation can be placed between the ceiling joists with the thickness varying depending on the material chosen²³. The same placement applies to suspended timber floors, where insulation is placed between the joists with thickness depending on the product used²⁴, and to conversion projects where insulation can be placed between the ceiling joists²⁵.

On cost, the published comparisons are relative rather than absolute. Cellulose insulation is described as still comparatively expensive, at up to twice that of mineral wool or rigid foam⁶. Spray foam is much more expensive to install than standard mineral wool loft insulation laid on the loft floor²⁶. Wool sits in the natural-fibre bracket above mineral wool, and no published UK price for wool is given here, so figures are installer-quoted.

Cutting is straightforward. The maker's guidance for stone wool boards is that they can easily be cut to shape using a knife or cutting saw and preserve their stability²⁷, and wool batts behave in the same way. Cut slightly wide so the material friction-fits between joists with no gaps at the edges, and keep the full loft-floor coverage continuous over the joist tops where a second layer is added.

Thermafleece: composition, sourcing and UK availability

Thermafleece is a British-made, sustainable insulation rich in British sheep's wool, described by the maker as natural fibre insulation²⁸. Its composition is three-quarters British sheep's wool and a quarter recycled polyester¹. The wool comes from the fleece of British Hill Sheep, supporting British farming and local business¹⁰, and specifically from hardy hill and mountain sheep in England, Scotland and Wales, shorn annually in the spring²⁸.

The product range is built on British wool. Thermafleece CosyWool and UltraWool harness the unique properties of British wool to provide versatile, durable insulation²⁹, and the range is available in a wide range of sizes to suit any project, with the maker describing the wool blends as a sustainable way to save energy and reduce carbon²⁹. For a household in an older property, the significance of the sourcing is that the material is produced domestically from a fleece that would otherwise be a by-product, which is a different supply chain from imported petrochemical foam.

The independence picture is mixed and worth stating plainly. A wool-insulated loft reduces the household's demand on the gas grid and on imported insulation materials, and it does so with a fibre sourced from British farms. What it does not do is remove dependence on the grid, on a supplier, or on the building's own ventilation. Wool is one component of a fabric strategy, and the household still buys its heat and its electricity from the network. For the wider picture, see insulation and energy independence and retrofitting traditional and pre-1919 buildings.