So sustainable, so soft but is it for ‘ewe’?

Whether your aim is to save the planet or simply lower heating bills, it’s clear that insulating our homes is important. The question is which type of insulation material is best - and what are the benefits of using certain types over others?

The term ‘natural insulation’ covers sheep’s wool, flex and hemp as well as products that use a high proportion of recycled materials like cotton, paper pulp or wood pulp.

In recent years, the building industry has used mostly oil-based insulation materials in construction. Plastic materials, including polyurethane, phenolic foam and polystyrene insulation boards, outperform natural insulation in thermal resistance. Practically, this means natural materials often require a greater thickness of insulation to do the same job in a building. So why opt for sheep’s wool insulation?

Perhaps the answer is not everything about insulation is related to thermal resistance. Sheep’s wool insulation has other important benefits too.

What is sheep’s wool insulation?

Sheep have survived long, cold winters outside for thousands of years, thanks to their fluffy warm fleeces. No matter how harsh the environment, the wool on their backs keeps them warm and dry.  Due to its crimped nature, wool fibres form millions of tiny air pockets that trap the air and slow the transfer of heat. This is what makes wool a natural insulating material, keeping buildings warm in winter and cool in summer.

Sheep are no longer farmed for their wool, but they still need to be clipped annually for health reasons. Wool insulation is made from sheep wool fibres that are either mechanically held together or bonded to form insulating batts and rolls. Wool insulation can be cut to size and used in the same way as other forms of insulation. Some manufacturers offer 100 per cent sheep’s wool insulation while others provide wool-rich insulation (75 per cent wool) mixed with recycled polyester.

PROS

Natural, renewable and sustainable.

Sheep’s wool is among the top insulating materials in terms of sustainability. Sheep are shorn every year (some twice) and the wool just keeps growing back naturally. As a raw material, wool is abundant, continuously renewable and locally produced in Britain. It lasts for decades and at the end of its life can simply be composted. Unlike, oil-based insulation, there are no plastic fibres.  

There is growing awareness of the damaging effects of plastic pollution on our planet. While the amount of recycled plastic insulation is increasing, the majority still ends up in landfill.

Low embodied carbon

This attempts to balance the global warming gases used in producing materials with the amount conserved by the insulation. It takes into consideration the gases released from usually fossil fuel and energy spent extracting the raw material as well as the manufacturing process.

For example, to manufacture plastic insulation you need oil, chemicals and lots of energy. Meanwhile manufacturing rock wool involves re-melting lava from volcanic rocks in a furnace and spinning it into fibres.

Sheep’s wool uses a fraction of the energy used to produce man-made insulating materials. Sheep’s wool has a value of 6 MJ/kg compared to 101 for Polyurethane foam and 26 for glass mineral wool, according to an analysis by GreenSpec.

Protection against moisture

Manufacturers claim wool is one of the few insulation materials that is breathable - naturally absorbing and releasing moisture.

While the exterior layer of wool is hydrophobic (water resistant), its cortex or inner layer is hydrophilic (water loving). The cortex can absorb water without the wool feeling damp. This helps to protect the surrounding timbers from moisture and prevent mould.

Unlike many other materials, wool can absorb 30 per cent of its weight in moisture before it begins to affect its thermal properties. This makes it perfect for damp climates like the UK.

Help control temperature

When wool fibres absorb moisture, they release it back into the air as heat, raising the temperature of its surrounding area. Manufacturers claim that wool’s natural ability to absorb moisture in humid conditions and give it off when dry, can help maintain stable temperatures within a building - providing more comfort.

Ease of installation

Sheep’s wool is safe and easy to install. Unlike many man-made insulations, it won’t cause irritation to the skin (unless you have a wool allergy), eyes or lungs. Whereas a full-face mask, goggles and gloves are required when handling glass wool or rock wool insulation.

Sheep’s wool insulation is easy to cut and shape for an exact fix. It can be used throughout your home for heat and sound insulation, including attics, roofs, walls, floors, ceilings, insulating pipes, ducting and water tanks, say manufacturers.

Specific heat capacity

This is the amount of energy needed to raise the temperature of the material by one degree. Good insulators will have a high specific heat capacity as it takes time to absorb more heat before the temperature rises and it transfers the heat. According to an analysis by GreenSpec, sheep’s wool insulation outperforms plastic and mineral wool – 1800 J/Kg.K for sheep’s wool compared to only 1030 for glass mineral wool and 1300 for expanded polystyrene (EPS), for example. This means sheep’s wool can absorb more heat trying to get into your house, useful for the hot summer months.

Flame resistant

Sheep’s wool is naturally fire resistant. The material only chars as there is insufficient oxygen in the air to support combustion of wool. In the event of a blaze, it will not fuel the fire and produces no toxic gases.  

Sheep’s wool, like all insulating materials, must pass fire tests to ensure it is fit for purpose. Products are tested to Euro Class C or British Standards such as BS 476 or BS 5803. Inorganic mineral based fire retardants are added where necessary.

Soundproof

The acoustic properties of natural fibre insulation is another advantage. Sheep’s wool can significantly reduce noises that can be heard through a building. The high density, irregular shape and soft fibrous quality of wool means it can provide excellent sound proofing comparable to the best mineral fibres and outperforming plastic insulation materials. This makes it ideal for bedrooms, home offices and music rooms.

CONS

Lower thermal efficiency

A key feature of an insulation material is its performance. Thermal resistance (R) measures heat lost through a given thickness of a particular material expressed in resistance per unit area (m2K/W). The higher the figure, the better the performance.

According to GreenSpec, plastic insulation has about twice the thermal resistance of sheep’s wool insulation – an R value of 5 per 100mm m2K/W for phenolic foam and 4.5 for polyurethane foam, for example, compared to 2.5 per 100mm m2K/W for sheep wool. Rock mineral wool and glass mineral wool are also 0.3 points higher, so wool doesn’t offer the best thermal qualities overall.

Chemical treatments

A critical disadvantage of wool is its vulnerability to insects, especially moths. Raw wool needs to be treated with chemicals to make it insect resistant. Manufacturers, such as Thermafleece, treat the wool with borax to prevent insect attack.

There is an alternative. Manufacturer SheepWool alters the molecular structure of the wool fibre through a plasma-ion treatment, called ‘Ionic Protect’. This biocide-free process is said to permanently prevent the wool from being eaten by insects.

Expensive

Sheep wool can be more expensive compared to some other types of insulation. Expect to pay around £18 to £25/m2  at 100mm thick for 100% wool insulation. The wool content insulation combined with polyester is usually cheaper.

This compares to about £13 per metre for mineral or rock wool insulation. Spray foam is usually more pricey and costs upwards of £27 per metre.  Prices correct February 2021.

Sheep’s wool is a natural insulator, sustainable, locally grown, breathable, user-friendly, dampens sound and is fireproof. It can help to achieve an effective balance of temperature and humidity within your home. There are lots of benefits but it’s not for everyone. Some man-made materials have superior thermal performance and are cheaper but less environmentally friendly.

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