Mark Lynn from Eden Renewable Innovations explains how sheep’s wool insulation is made, outlines the pros and cons, and suggests how it is best specified and installed. He also shares some common traits of natural fibre insulation as a category.

Interview with Mark Lynn

In this episode Ben Adam‑Smith sits down with Mark Lynn, Managing Director of Eden Renewable Innovations and Chair of the Alliance for Sustainable Building Products (ASBP). Together they unpack what natural‑fibre insulation really means, why sheep’s wool insulation is gaining traction, and what homeowners and builders need to keep in mind when specifying and installing products such as Thermafleece.

Natural fibre insulation is any building insulation made from renewable plant or animal fibres

Mark defines natural‑fibre insulation as any product whose primary component comes from a biological source – sheep’s wool, wood fibre, hemp, jute, cotton, flax and cellulose are just some examples. These materials can be used alone or blended, and they are often processed into batts, rolls, rigid boards, or loose‑fill. Because the raw element is renewable, the embodied carbon of the product is typically far lower than that of conventional mineral or synthetic insulation.

“Natural fibre insulation is any building insulation that’s made from or using natural fibres.”

The variety of fibres gives designers flexibility: wood fibre offers rigidity, hemp provides high tensile strength, while wool excels at moisture regulation. Understanding the source material helps you match the product to the building’s performance goals.

Moisture‑buffering is a unique strength of natural fibres

At the molecular level, natural fibres carry electrical charges that attract water molecules. Rather than allowing water to pool as a liquid, the fibres hold it in a solid‑state “bound” condition. This limits the risk of mould and rot, because the water cannot act as a universal solvent that accelerates decay.

Sheep’s wool, composed of the protein keratin, is especially adept at this exchange, absorbing humidity when the indoor air is damp and releasing it when conditions dry out. The result is a more stable indoor environment, particularly in climates where indoor humidity fluctuates dramatically.

“When the water molecules adhere to the natural fibre the way they do, they can’t behave like a liquid.”

Breathability is not a substitute for ventilation

Mark cautions that a breathable fabric does not eliminate the need for a proper ventilation strategy. Inside our homes we generate moisture through our activities (showering, cooking, doing laundry, etc.) so there needs to be a constant exchange of stale air being replaced with fresh air. In Passivhaus homes, for example, mechanical ventilation with heat recovery (MVHR) fulfils this role. The insulation’s ability to buffer humidity is therefore an additional benefit rather than an excuse to forget about ventilation.

Good design requires adequate ventilation, sufficient vapour openness (the ability of water vapour to pass through the material), and the inherent moisture‑buffering capacity of the fibre. When these work together, the insulation remains effective without becoming saturated.

Condensation risks can be modelled

Tools such as WUFI can help model moisture, taking into account climate data, wall build-ups, and internal conditions. This will then identify potential condensation risks and allow them to be addressed at the design stage. Ignoring this step can lead to interstitial condensation and the knock-on effects of mould and damp, which in severe cases could even cause structural damage.

Three main forms of natural‑fibre insulation: flexible, rigid, and loose‑fill

Flexible insulation – often supplied in slabs, batts or rolls that can be cut to size.
Rigid wood‑fibre boards – medium‑density panels that can be fixed to walls or roofs, offering structural support.
Loose‑fill – shredded fibres (typically recycled newspaper or fine wood) blown into cavities, ideal for irregular spaces.

Sheep’s wool is available both as flexible batts and as rolls, giving installers the convenience of a rockwool‑style product while retaining the natural advantages of wool.

Sheep’s wool insulation is easy to install in a loft

We should be taking advantage of all natural fibre insulations

When asked about how we choose a natural fibre insulation, Mark suggests that it’s actually better to support the whole category. All natural fibre insulations will have their strengths. For example, when insulating a loft a roll of sheep’s wool may be the simplest and quickest to install.

“We’ve got to use the right type of the right resource in the right way, and that really involves a whole mix of materials.”

The ASBP’s Natural Fibre Insulation Group publishes a comparative guide that helps architects weigh factors such as thermal performance, acoustic qualities, fire resistance, and cost. Using that data ensures the selected material aligns with the building’s functional and regulatory needs.

The market for natural‑fibre insulation is growing

Historically, natural fibres accounted for roughly 1 % of the UK insulation market, compared with 10–15 % in France and Germany. Since the pandemic, consumer research into health‑focused homes has accelerated adoption, and manufacturers report strong year‑on‑year growth.

Mark predicts that reaching the 10–15 % threshold in the UK would make natural fibres a credible mainstream option, offering homeowners greater choice without sacrificing performance.

“These aren’t quirky eco products, they are part of the insulation mix.”

Shearing a sheep for its wool

Manufacturing sheep’s wool insulation

Sheep’s wool delivers acoustic performance comparable to high‑grade rockwool

Thermafleece UltraWool, a flagship wool product, achieves the same sound‑absorption ratings as a 45 kg/m³ rockwool product, despite being only 32 kg/m³. The lower density reduces dead‑load on the structure while still dampening noise, making it a compelling option for residential lofts and office partitions where acoustic comfort is a priority.

Fire safety is addressed through mineral fire retardants, not by claiming non‑combustibility

All natural‑fibre products, including wool, incorporate a mineral fire retardant that encourages charring rather than rapid flame spread. While they are not classified as “non‑combustible,” they meet Part B of the Building Regulations for the intended applications.

For projects that require a non‑combustible rating—such as certain high‑rise or public‑access buildings—additional layers or alternative materials may still be necessary.

Insulating a barn with sheep’s wool insulation

Cost is competitive when viewed against performance and lifespan

Mark notes that comparing wool to a cheap fiberglass roll can be misleading. When matched against high‑performance mineral products, the price differential narrows, especially when accounting for its long life (60 + years) and the reduced need for replacement.

A typical loft installation with Thermafleece can be completed for under £1,000, and the energy savings often recoup the investment within five years.

A thin layer of insulation can make a big impact

When dealing with existing buildings, for example a period property with a solid wall, a thin layer of internal insulation can make a big impact to comfort and keep costs down. It must however be a continuous layer, eliminate thermal bridges and be installed well. One advantage of this thin layer is that you’re far less likely to have moisture build-up in the fabric, but you’ll feel a noticeable benefit to the comfort of the property.

Environmental benefits extend beyond the insulation itself

Sheep’s wool used in insulation is often sourced from the coarse, dark fleece that has limited textile applications, turning an agricultural waste stream into a high‑value building product. The polyester binder incorporated into the rolls is made from recycled plastic bottles, further closing the material loop.

By sequestering carbon in the keratin fibres and reducing the demand for petrochemical‑based insulation, wool contributes to lower embodied emissions and supports rural economies.