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Thermafleece Cosywool Sheeps Wool Insulation FAQ’s

What is Cosywool sheep's wool insulation made of?

Cosywool is made of 75% British sheep’s wool, 15% recycled polyester loft agent and 10% polyester binder which contains at least 50% recycled material.

What does loft agent do?


Loft agent helps to maintain the fibre stability and structural integrity of the insulation, preventing it from going flat and making it durable and long-lasting.

Does Cosywool insulation smell ‘sheepy’?

Sometimes Cosywool has a faint odour and although it’s not unpleasant, it can be a little stronger if the insulation has been stored in a damp place. This will dissipate fairly quickly however, once the insulation has had time to air.

What is the difference between Thermafleece Cosywool and SupaSoft insulation?

Cosywool insulation is made from sheeps wool and is highly breathable, enabling it to absorb and release excess moisture from the air which in turn, helps prevent the build up of harmful water in the fabric of the building.

SupaSoft insulation is made from recycled Polyester (PET bottles) - like the fleecy fibres used in duvets and jackets but unlike Cosywool, is not breathable.

Both Cosywool and SupaSoft are pleasant to handle and easy to install.

Cosywool has a slightly lower Thermal Conductivity value (0.039 W/mK) compared to that of SupaSoft (0.040 W/mK). The lower the value the better the thermal performance of the material. To give you an idea of the difference this makes, lets take the example of insulating a loft from scratch: you would need to install a slightly greater depth of SupaSoft (10mm in fact) than you would of Cosywool, in order to achieve the same U-Value. This extra thickness balances out in the cost of materials however, which makes Cosywool and Supasoft both really great options for insulation.

Why would I choose Cosywool Flexible Slabs over Cosywool Rolls?

Cosywool Flexible Slabs are a denser version of Cosywool rolls. Their constituent parts are identical but the slabs dense structure delivers a tighter fit which gives more friction to hold them in places like walls and roofs. Their higher density also provides a slightly better thermal conductivity value of 0.038 vs 0.039 for their roll equivalents.

Is Thermafleece Cosywool protected against damage by moths and other insects?

Yes. Cosywool is treated with a borax based fire retardant which also provides really good protection against moths and other insects.

What are the environmental benefits of using Cosywool sheep wool insulation?

  • Cosywool is made from coarse dark sheeps wool which normally goes to waste.
  • All wool used in Cosywool insulation is sourced from local sheep.
  • The largest part of Cosywool (wool) is biodegradable. The polyester component is not biodegradable and sadly there is no natural material that does the same job currently but Thermafleece do make every effort to use as much recycled polyester as possible in their binder. This means that they are able to keep the amount of virgin polyester used in their rolls to a minimum.
  • Cosywool insulation helps remove carbon dioxide from the atmosphere. i.e. 1 kg of wool irreversibly locks up approximately 2kg of CO2 from the atmosphere.
  • Once installed, Cosywool will last the lifetime of the property, therefore removing the need for replacement or repair in the future.

How does Cosywool insulation compare to other types of insulation?

The following table provides a comparison of Thermal Conductivity values for some of the most common types of insulation…
The lower the value the better the thermal performance of the material. 

Products in bold are sold here at Celtic Sustainables
Thermal Conductivity
(K Value)
Cosywool rolls 0.039
Cosywool flexible slabs 0.038
Ultrawool 0.035
Supasoft  0.04
Natrahemp 0.04


Woodfibre 0.041-0.043
Cornerstone Insulating Render 0.12
Adaptavate Breathaplasta 0.27
PIR Board (Celotex/Kingspan) 0.022
Rockwool 0.044
Glasswool 0.044

Can I use Cosywool insulation in my Wall/ loft/ roof/ floor?

Yes – details of how to install Cosywool in various types of construction can be found in the Insulation Guides on our Cosywool webpage.

Can I use Thermaflece Cosywool in my Camper van conversion?

Yes. Many people use Cosywool insulation in their camper van conversions because it’s easy to work with and provides really good insulation. Check out our blog post Eco Campervan Conversion – Insulating you Campervan which contains some great tips from two of our previous customers, who used Cosywool in their camper conversions.

How much insulation do I need?

The amount of insulation you will need for a project depends on where you are fitting it and the size of the area you want to insulate. For example, if you’re insulating a loft from scratch you will need 2 layers of insulation, one layer which sits between the joists and a second layer placed at right angles over the joists. This combination ensures there are no air gaps and enables you to achieve the required UV rating to meet UK building regulations. For other applications such as insulating a wall, you will most likely only require 1 layer of insulation. For further guidance on how best to install Cosywool insulation in some of the most common places in your home, take a look at the Installation Guides on our Cosywool webpage.

How do I calculate how many rolls of insulation I need?

Calculating the number of rolls required for a job is relatively easy. To help you work this out we’ve listed all the rolls sizes on our website with their depth, width and length and also the total square meterage they cover (this is the value shown in brackets in m²).

To calculate the number of rolls required for a job, you need to start by measuring the total area of the space you want to insulate. Now divide this total area by the m² value of your chosen roll size.


To insulate an area of 10m² at a depth of 50mm with rolls which are 370mm wide, first find the total m² for a single roll of 50mmx370mm.


Next, divide your total 10m² by 4.81m² = 2.439 = 3 rolls of insulation required in total.

What is the best way to cut Cosywool insulation?

The best way to cut Cosywool insulation is with a serrated knife (an insulation knife/cutting tool works well). To get an even cut, keep the insulation rolled up tightly and cut across the roll to get a straight even edge. Cosywool can also be pulled apart easily or torn by hand when inserting into smaller or less regular shaped spaces too.

Why do Celtic Sustainables sell Cosywool in rolls and not packs?

Cosywool rolls are supplied by the manufacturer in packs of either 2 or 3 rolls, depending on the width of the rolls. This means that customers usually have to purchase insulation in multiples of 2 or 3 rolls from most other retailers. Here at Celtic Sustainables we recognise that our customers do not always need that many rolls for their project, so we sell our Cosywool as individual rolls. This enables our customers to purchase only the number of rolls they actually require.

How is Thermafleece Cosywool delivered?

We ship our Cosywool insulation orders by either courier (small quantities) or by pallet (larger quantities).

Why is Cosywool shipped in plastic packaging?

At this time, plastic is the only material available that’s capable of protecting natural insulation sufficiently during its transit and storage although this is constantly under review. On the plus side, the use of plastic enables Thermafleece rolls to be greatly compressed, which not only reduces the amount of packaging required but also reduces the carbon miles used during their transportation.

Why is Cosywool sometimes shipped in cardboard boxes too?

The courier companies that deliver our products have experienced difficulties in the past whilst transporting individual rolls of plastic wrapped insulation through their network (e.g, they frequently bounced off conveyor belts in the depots). Because of these issues, we are now required to ship individual rolls in cardboard boxes.

Sometimes we ship insulation orders of more than one roll in individual boxes too. This is determined by the cost of transporting the goods either by haulier (pallet) vs courier and enables us to achieve the most cost efficient means of shipping for our customers.

My rolls seem thinner than the ones I ordered – why is that?

Cosywool rolls are hugely compressed for transportation. Because of this, they appear thinner than their actual size when initially unpacked but given a day or two unrolled, they soon puff up to their true thickness.

Other questions about Cosywool or eco insulation for your project?

Please do contact our helpful team, we are always delighted to help make the world a greener place.