It can be hard to choose the best material for the insulation in your building. Even more care must be taken when considering the environmental impact and sustainability of these materials, as the commonly used plastics and fibreglass insulation are often unsustainable and take a lot of energy to produce.

At Cook Brown Energy, we pride ourselves on our experienced staff, our professional service, and our strong values. We have experience providing energy and sustainability consultancy for homeowners, commercial clients, and more.

In this article, we will go through what we think are some of the best sustainable insulation materials for use in your new or existing builds that can help improve energy efficiency and comfort.

If you’re in need assistance or advice with your energy efficiency, don’t hesitate to get in touch or get a quote for our energy consultation service.

The Top Three Sustainable Insulation Materials


Cork is an excellent sustainable insulation material. It has ideal thermal properties and resistance to dampness that enable it any number of applications in insulating your walls, floors, and ceilings. It is also both renewable and recyclable, making it great for the environment.

It has a thermal conductivity of around 40mW/mK, and a high specific heat capacity and thermal inertia. This means cork insulation performs well across a wide range of temperatures, while only being slightly less effective than unsustainable insulation made from plastic.

Cork is a great insulator for both heat and sound. Its unique structure and flexibility absorb most tones in the standard range of hearing, providing good acoustic insulation. It is ideal for urban areas with a range of constant noises.

Being naturally resistant to moisture is another of the inherent benefits of cork insulation. It requires less external treatment than other insulators, cutting down on the amount of chemicals and energy used in its production. This resistance to moisture helps to prevent common issues in building such as mould, mildew, and rot.


Wool can be used as a form of ‘natural insulation’. Other types of natural insulation can also be considered such as flex and hemp, or recycled insulation made from products like cotton, paper pulp, and wood pulp.

Wool insulation works well as it is filled with lots of tiny air pockets that help to slow down energy transfer. This is how sheep keep themselves warm during the winter with the very same substance. It works just as well during the summer, keeping the heat from reaching past the wool insulation.

The fibres of wool are held together through bonding or by mechanical means. It can then be cut to size on-site with ease and used in the same way as other insulating materials. Wool itself is sustainable, but it is sometimes mixed with recycled polyester to decrease its cost and increase its effectiveness, so keep an eye out for this if you’re trying to balance the sustainability and energy efficiency of your build.

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The inherent sustainability of wool is among the highest of any insulation material. It grows constantly on sheep, making it a highly abundant natural resource. It can be produced and procured locally and can be decomposed when it meets the end of its life.

Wool can help with moisture and dampness. It is externally hydrophobic but contains an inner layer that can absorb water. This inner layer helps to keep moisture away from the timbers of your house. The wool has breathable properties, this allows it to absorb around 30% of its weight in moisture before its thermal properties are affected.

A downside of using sustainable insulation such as wool is it can be more expensive than other options. Despite this increase in price, if sustainability is your goal, then there is no better option than wool.

Until now, oil-based insulation made from plastics has been the norm. To meet the same level of effectiveness as plastic insulation, wool needs to be of a greater thickness. However, it offers a sustainable alternative to those looking to lessen their ecological impact while also increasing the energy efficiency of their home.

For more news about energy and sustainability, check out our article on net zero new builds. >


Cellulose insulation is primarily a thermal insulation, made from up to 85% recycled paper or wood fibres. The processes it goes through leave it with a consistency similar to wool, after which it is treated with boric acid to improve its properties.

The production of cellulose is incredibly energy efficient, with the lowest amount of energy being used out of any leading insulation product.

It is great at filling cavities and spaces but can also be used to make insulation boards and mats.

Much like wool insulation, cellulose is easy to use in construction. It can be cut to size without losing any of its thermal resistance or insulate properties in the process.

Wood frames are typically not ideal in terms of their airtightness due to joints and cracks in the material. Cellulose insulation can improve these deficiencies much more easily and effectively than other insulations, eliminating gaps and air flow.

Heat recovery systems and air-to-air heat exchangers can be used to help circulate fresh air into a room sealed with cellulose insulation, improving the overall breathability. Between 60% and 70% of all ventilation losses can be saved through this method.

Due to the nature of how cellulose insulation is produced it can be made anywhere. This means it can be imported from local sources, cutting down on carbon footprint caused by transportation.

Look at some of our case studies to see how we’ve helped clients to improve their sustainability. >

Other Types of Sustainable Insulation

As well as the materials used, other methods can be applied to help provide sustainable insulation. The positioning of your house and its windows, as well as the solidity of the walls, are the limiting factors for some of these methods of insulation.

Thermal Curtains

Thermal curtains, or blackout curtains, are designed to control the amount of heat entering and retained in a room. They have two key benefits:

  • Reflecting light when closed to prevent as much heat and light transferring into the room through the window.
  • Increasing heat retention in a room so that less heat is lost during the colder seasons.

They provide an airgap between themselves and the window pane they cover, creating a layer of air to provide insulation much like that of cavity insulation.

Cavity insulation has better thermal insulation than solid walls due to the air it contains acting as a stopgap for heat transfer and sound transmission. As it has no material within it, it can be seen as sustainable as well.

Walls with cavity insulation are cheaper to build than solid walls, and less material is needed to achieve the same thickness.

However, other materials like cork and wool provide better insulation than having an empty cavity. Using a sustainable material such as these to fill any gaps in the walls of your structure will have a positive environmental impact while also increasing the overall energy efficiency of your build.

For more advice on how you can improve your sustainability, check out our top energy saving tips. >

Earth-Sheltered Homes and Mound Hill Insulation

There are two main types of house designs that feature mound hills – underground and bermed.

Underground structure.

An underground house or structure has been built below grade or completely underground. An atrium or courtyard design works well with an underground house while still providing an open feeling.

An underground house is hardly visible from the ground level, providing an outdoor space with complete privacy. However, passive solar gain will be limited due to the positioning of any windows, and extra considerations need to be made regarding drainage and snow removal.

Bermed house

This type of house can be built either above grade or partially below it, with one or more walls being covered with earth. Even the roof can be covered with earth, such as by building the house into a hillside or creating a mound hill around the house, to provide greater insulation.

The front of the house is typically exposed towards the south, allowing for passive solar heating through energy from sunlight. Ventilation and lighting throughout the rest of the house can be provided by active ventilation, exterior vents, and skylights.

Ins and Outs.

Some of the benefits of an earth-sheltered or mound hill home are:

  • They’re less susceptible to extreme temperatures than a normal house.
  • The earth can provide additional insulation and soundproofing when combined with other materials.
  • They blend into the environment.

Some of the downsides to a house built into the ground or a hill are:

  • The initial cost of construction will be higher than a conventional home of the same size.
  • They need extra care regarding moisture and humidity.

Factors to consider.

Earth-sheltered houses are more cost-effective in climates with extreme temperatures and low humidity.

The type of soil at the building site matters. Granular soils that contain sand or gravel are best for earth-sheltering as they are permeable, allowing water to be drained away easily, and compact well which helps to bear the weight of the building.

Waterproofing can prove a sizable challenge for a building using mound hill insulation. Substances used in waterproofing, such as rubberised asphalt or plastic sheets, can reduce the environmental benefits of an earth-sheltered build and lessen its overall sustainability.

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While a mound hill or earth-sheltering does provide insulation, it is often a good idea to supplement it with standard insulation materials. Some of the other examples on this list such as cork or cellulose can provide this extra insulation without a negative impact on sustainability.

Improve the Sustainability of Your Home with Cook Brown Energy

At Cook Brown Energy we pride ourselves on our dedicated energy assessment and consultancy services. It is our mission to ensure your home is as energy efficient as possible without compromising on its sustainability.

The materials listed in this guide are a great place to start when managing your energy efficiency, but why stop there? Get in touch to get started on receiving detailed analysis of the energy and sustainability of your property, alternatively, check out some of our other articles below.

Check out our top ten tips on improving household energy efficiency. >

Take a look at our case study on a unique and sustainable home between the trees. >