FAQs

Everything You Need to Know About Thermafleece UltraWool Sheep’s Wool Insulation

Whether you’re renovating a period property, building a timber frame extension or improving the thermal performance of your loft, choosing the right insulation is one of the most important decisions you’ll make. While reducing heat loss is always a priority, modern insulation also needs to help manage moisture safely, provide good acoustic performance and contribute towards a healthier indoor environment.

Thermafleece UltraWool has been developed to meet these demands. Manufactured in the UK using British sheep’s wool and recycled polyester fibres, it combines excellent thermal performance with the natural moisture buffering properties of wool, making it particularly well suited to breathable construction. With a thermal conductivity of 0.035 W/mK and a density of 31 kg/m³, it is one of the highest-performing flexible natural insulation products currently available.

Below you’ll find answers to the questions our technical team are asked most often.

Quick Facts

What is Thermafleece UltraWool?

Thermafleece UltraWool is a high-performance natural insulation slab designed for projects where thermal efficiency, moisture management and acoustic comfort are equally important. Manufactured from 75% British sheep’s wool with recycled polyester fibres, it delivers excellent thermal performance while remaining vapour open, making it an ideal choice for breathable construction.

Supplied as semi-rigid slabs, UltraWool friction-fits neatly between timber studs, rafters and floor joists, helping to reduce gaps during installation. Its higher density also improves acoustic performance, making it suitable for walls, roofs, lofts and suspended timber floors in both new-build and refurbishment projects.

What is Thermafleece UltraWool made from?

UltraWool is manufactured from a carefully engineered blend of 75% British sheep’s wool together with recycled polyester fibres and a small proportion of polyester binder fibres. Each material has been chosen for a specific purpose, combining the natural performance of wool with the durability required for modern construction.

The wool provides excellent thermal insulation and natural moisture buffering, while the recycled fibres help the insulation retain its shape, resist settlement and continue performing throughout the lifetime of the building.

Why does UltraWool contain recycled polyester fibres?

The recycled fibres are included to improve long-term performance. They help the insulation maintain its thickness, improve resilience during installation and reduce the risk of slumping over time.

Where can Thermafleece UltraWool be used?

UltraWool is suitable for most timber construction applications where a flexible insulation slab is required. Typical uses include external timber frame walls, internal partitions, suspended timber floors, loft floors, warm pitched roofs and loft conversions.

Because it combines excellent thermal and acoustic performance with vapour permeability, it is equally suitable for modern low-energy homes and traditional buildings that rely on breathable construction methods. As with any insulation, the best results are achieved when it forms part of a complete system that includes appropriate airtightness and moisture management.

How well does Thermafleece UltraWool insulate?

UltraWool has a declared thermal conductivity of 0.035 W/mK, making it one of the best-performing flexible natural fibre insulation products currently available. This allows excellent U-values to be achieved while keeping wall and roof build-ups relatively slim, making it particularly useful for refurbishment projects where insulation depth is limited.

Thermal conductivity is only part of the story, however. UltraWool also combines good thermal performance with excellent acoustic properties, moisture regulation and low embodied carbon, providing a balanced solution that supports both energy efficiency and long-term building health.

Why is UltraWool described as a higher-density sheep’s wool insulation?

With a density of 31 kg/m³, UltraWool is denser than many traditional sheep’s wool insulation products. This higher density improves the slab’s stability, helping it friction-fit securely between timber framing members while also enhancing acoustic performance.

A denser insulation also helps slow the transfer of heat through the building fabric, contributing to improved comfort during warmer weather. Combined with the natural properties of sheep’s wool, this makes UltraWool a well-rounded insulation for year-round performance.

Does UltraWool help reduce summer overheating?

Yes. Insulation slows the movement of heat in both directions, so while it helps keep warmth inside during winter, it also helps reduce the rate at which external heat enters the building during summer.

UltraWool’s density and high specific heat capacity allow it to absorb and store more heat before passing it through the construction. When used alongside good ventilation and effective solar shading, this can help maintain more stable indoor temperatures during hot weather. Although no insulation can prevent overheating on its own, UltraWool forms an important part of a well-designed low-energy building.

Is Thermafleece UltraWool good for soundproofing?

Yes. Thanks to its higher density and open fibre structure, UltraWool provides excellent sound absorption and is widely used to improve acoustic performance within timber floors, internal walls and roof constructions.

It achieves a Noise Reduction Coefficient (NRC) of up to 1.10 at 90mm thickness and can contribute towards airborne sound reductions of Rw 41 dB, depending on the complete wall or floor build-up. This makes it particularly useful for bedrooms, home offices and separating floors where reducing noise transfer is just as important as improving thermal performance.

Is Thermafleece UltraWool breathable?

Yes. UltraWool is a vapour-open insulation, allowing water vapour to move naturally through the building fabric rather than becoming trapped within walls or roofs. This makes it particularly suitable for breathable construction systems where moisture needs to disperse safely over time.

It’s important to understand that breathability doesn’t mean allowing draughts through the building. An effective insulation system should still be airtight, using suitable membranes and tapes to control air leakage while allowing moisture vapour to diffuse through the construction where appropriate. When specified correctly, UltraWool helps create building envelopes that remain both energy efficient and resilient to moisture.

Can sheep’s wool insulation absorb and release moisture?

One of the biggest advantages of sheep’s wool is its ability to regulate moisture naturally. The fibres can absorb moisture vapour when humidity levels are high and release it again as the surrounding air becomes drier, helping to moderate changes in humidity within the building fabric.

This moisture buffering can reduce the risk of condensation forming within breathable constructions and contributes to a healthier indoor environment. Unlike many conventional insulation materials, sheep’s wool can temporarily store moisture without significantly affecting its thermal performance, provided it is allowed to dry naturally afterwards.

Do I need a vapour control layer with UltraWool?

That depends on the construction. Although UltraWool itself is vapour open, many modern wall and roof build-ups still require a vapour control layer or intelligent airtight membrane to control the amount of moisture entering the construction from inside the building.

The aim isn’t to stop the building from breathing, but to limit excessive moisture movement while maintaining airtightness. Intelligent vapour control layers are often the preferred solution within breathable construction because they can adapt to seasonal changes, helping the structure dry when conditions allow. If you’re unsure which membrane is appropriate, our technical team can advise on complete system build-ups and condensation risk.

Is Thermafleece UltraWool easy to install?

Yes. UltraWool has been designed to be straightforward to install using standard woodworking tools. The slabs are flexible enough to compress slightly before expanding to create a secure friction fit between timber framing members, helping minimise gaps without requiring specialist fixings.

Unlike many mineral fibre products, sheep’s wool is pleasant to handle and doesn’t produce the same skin irritation associated with glass or stone wool. A dust mask is still recommended when working in enclosed spaces such as lofts, but most installers find UltraWool significantly more comfortable to work with.

Can UltraWool be compressed?

Like most flexible insulation products, UltraWool is designed to achieve its declared thermal performance at its manufactured thickness. Minor compression during installation is perfectly normal and helps create a secure friction fit, but excessive compression should be avoided as it reduces the amount of trapped air within the insulation, affecting its thermal performance.

Where deeper insulation is required, it’s generally better to install additional layers rather than compress thicker slabs into a smaller cavity.

What happens if UltraWool gets wet?

UltraWool is capable of absorbing moisture vapour as part of its normal performance, but it should not remain saturated with liquid water. If the insulation becomes wet due to flooding or prolonged water ingress, it should be removed, allowed to dry if appropriate, or replaced where necessary.

During storage and installation, packs should be kept under cover, protected from prolonged exposure to rain and stored clear of the ground. As with any insulation product, preventing water ingress into the building fabric remains essential for long-term performance.

Is Thermafleece UltraWool fire resistant?

UltraWool achieves a Euroclass E reaction to fire classification and has been tested for flammability and smoulder resistance. While sheep’s wool naturally has a relatively high ignition temperature compared with many synthetic fibres, it is still a combustible insulation and should always be used as part of a compliant wall, floor or roof system.

It should not be used as a cavity barrier, and where cavity barriers are required by Building Regulations they should be installed separately as part of the overall construction.

How long does sheep’s wool insulation last?

When correctly installed within a dry building envelope, sheep’s wool insulation is expected to last for the lifetime of the building. The addition of recycled polyester fibres helps UltraWool retain its loft and structural integrity over many decades, reducing the likelihood of settlement or loss of performance.

As with any insulation, longevity depends on good building design. Preventing persistent water ingress and ensuring the construction can manage moisture effectively are key to maintaining long-term performance.

Is Thermafleece UltraWool environmentally friendly?

Yes. UltraWool combines renewable British sheep’s wool with recycled polyester fibres to create an insulation with a relatively low embodied carbon while helping reduce operational carbon emissions through improved energy efficiency.

The product is manufactured in the UK, contains recycled content and is recyclable at the end of its service life. It is also supported by an independently verified Environmental Product Declaration (EPD), providing transparent environmental data for designers and specifiers.

Does sheep’s wool insulation improve indoor air quality?

Sheep’s wool is recognised for its ability to help regulate humidity and has also been shown to absorb certain airborne pollutants, contributing to healthier indoor environments. Unlike some construction materials, UltraWool has been tested for VOC emissions and formaldehyde, demonstrating extremely low emissions that support good indoor air quality.

Combined with good ventilation, breathable construction and appropriate moisture management, sheep’s wool insulation can play a positive role in creating comfortable, healthy living spaces.

Is UltraWool better than mineral wool insulation?

Both materials provide effective thermal insulation, but they offer different advantages depending on the project. Mineral wool is widely used because it is cost-effective and provides excellent fire performance, while UltraWool offers the additional benefits of natural moisture buffering, improved handling during installation and a lower reliance on non-renewable raw materials.

For breathable construction, heritage buildings and projects prioritising natural materials, UltraWool is often the preferred choice. For applications where fire resistance is the primary consideration, mineral wool may remain more appropriate. The best solution depends on the performance requirements of the building rather than a single technical property.

Is Thermafleece UltraWool the right insulation for my project?

If you’re looking for a high-performance natural insulation that combines excellent thermal efficiency with breathability, moisture management and acoustic performance, UltraWool is an excellent choice. It is particularly well suited to timber frame construction, loft conversions, suspended timber floors and renovation projects where maintaining a breathable building fabric is important.

Every building is different, however, and selecting the right insulation should always take account of the overall construction, target U-values, condensation risk and intended use of the building. At Unity Lime, we can help you specify complete breathable build-ups, calculate U-values and advise on suitable membranes and complementary products, ensuring your insulation performs exactly as intended for many years to come.