Everything You Need to Know about Natural Insulation

By John Ferguson, Founder & CTO • Published 01 Oct 2025 • Updated 11 Nov 2025

Why Choose Natural Insulation?

If you’re planning a build, renovation, or retrofit, natural insulation is one of the smartest choices you can make. Made from renewable plant and animal fibres like wool, hemp, sisal, and wood, it’s breathable, low-impact, and works with your building, not against it.

Natural insulation helps cut energy bills, prevents damp and mould, and keeps your home healthy without relying on petrochemical-based products. It’s especially effective in older or traditional homes where breathability matters.

In this guide, you’ll learn how to choose and install natural insulation, compare materials, understand costs and energy savings, and see real-world examples in action. You’ll also discover Sisalwool, a unique blend of sisal fibre and sheep’s wool that delivers performance, comfort, and sustainability in one.

What Are the Options?

Natural insulation materials are made from renewable sources that have been used for centuries to keep homes warm and dry. Modern manufacturing methods have improved their consistency and performance, allowing them to compete with or even outperform many synthetic alternatives. Below is an overview of the most common natural insulation types available today, including their origins, characteristics, and practical considerations.

Sheep’s Wool Origin: A by-product of the wool industry, sheep’s wool is washed, treated for pests, and formed into flexible batts or rolls. Eco credentials: 100% renewable and biodegradable. The production process has low embodied energy, and it can be recycled at the end of its life. Thermal performance: Typically 0.035–0.040 W/mK. Moisture handling: Excellent. Wool can absorb and release up to 30% of its weight in moisture without losing insulating properties, helping to regulate indoor humidity. Fire behaviour: Naturally self-extinguishing due to its high nitrogen content. Cost and availability: Readily available in the UK and mid-range in price compared with other natural options.

Hemp Origin: Derived from the stalks of the industrial hemp plant, which grows rapidly and requires few pesticides. Eco credentials: Very sustainable, as hemp absorbs large amounts of CO₂ during growth and can be cultivated locally. Thermal performance: Around 0.040–0.045 W/mK. Moisture handling: Breathable and vapour-permeable, making it suitable for traditional solid-wall buildings. Fire behaviour: Naturally resistant to ignition when combined with lime or other binding agents. Cost and availability: Moderate cost, though supply can vary depending on the region.

Flax (Linen Fibre) Origin: Made from the stem of the flax plant, which is also used to produce linen fabric. Eco credentials: Renewable, biodegradable, and grown widely in Europe. Thermal performance: Approximately 0.037–0.042 W/mK. Moisture handling: Naturally regulates humidity, helping to prevent condensation. Fire behaviour: Can be treated with natural fire retardants; performs comparably to hemp or wool. Cost and availability: Similar in price to hemp, though not as widely stocked in the UK.

Cotton / Denim Origin: Derived from recycled cotton garments and denim scraps, often post-industrial or post-consumer waste. The material is shredded and treated to create flexible insulation batts and rolls. Eco credentials: Excellent, as it diverts significant textile waste from landfills. It requires less water and energy to produce than virgin cotton insulation. Thermal performance: Typically 0.038–0.042 W/mK. Moisture handling: Good. It is breathable and can absorb some moisture without a major loss in performance. However, like most cellulose-based materials, it requires treatment to prevent mould and fungal growth in high-humidity areas. Fire behaviour: Must be treated with a fire retardant to meet building regulations, as untreated cotton is highly flammable. Cost and availability: Mid-to-high range in cost. It is an established product in sustainable building but may not be as widely stocked as conventional materials.

Sisal Fibres Origin: Derived from the leaves of the agave plant, sisal is strong, renewable, and widely grown in tropical climates. Eco credentials: Biodegradable, renewable, and requires little energy to process. Thermal performance: Around 0.040–0.045 W/mK, depending on density. Moisture handling: Performs well in breathable constructions when blended with other fibres. Fire behaviour: Naturally resistant to static and ignition when used in blends. Cost and availability: Becoming more common in eco-building products, often used as part of a composite material rather than on its own.

Cork Origin: Harvested from the bark of cork oak trees without harming the tree, mainly from Portugal and Spain. Eco credentials: Fully renewable and recyclable, with carbon-negative production. Thermal performance: Typically 0.037–0.040 W/mK. Moisture handling: Excellent resistance to rot, mould, and moisture absorption. Fire behaviour: Naturally slow to ignite and self-extinguishing. Cost and availability: More expensive but highly durable, making it ideal for floors and internal walls.

Cellulose (Recycled Paper) Origin: Made from shredded and treated recycled newspapers or cardboard. Eco credentials: High recycled content and very low embodied energy. Thermal performance: Around 0.035–0.040 W/mK. Moisture handling: Hygroscopic, meaning it can buffer moisture effectively, but requires proper ventilation to prevent damp issues. Fire behaviour: Treated with borate or ammonium phosphate to improve fire resistance. Cost and availability: Cost-effective and widely available, often used for loft or cavity wall applications.

Natural Fibre Blends Origin: Made by combining two or more natural fibres to balance different properties, such as strength, flexibility, and insulation. Eco credentials: Renewable and biodegradable, with improved performance and durability. Thermal performance: Varies between 0.035–0.040 W/mK. Moisture handling: Excellent moisture buffering and breathability. Fire behaviour: Dependent on the fibre mix but generally good when wool or treated plant fibres are included. Cost and availability: Mid-range and increasingly available from specialist suppliers.

Sisalwool is an example of a high-performance blend, made from sisal fibres and sheep’s wool. This combination brings together the structural strength of sisal and the moisture-regulating, thermal, and acoustic qualities of wool. It is durable, breathable, and particularly suited to older or natural buildings where vapour permeability is important. We will explore this blend in more detail later in the guide.

Natural insulation materials vary in form, performance, and price, but all share key advantages over synthetic alternatives: low embodied energy, healthy indoor air quality, and the ability to work in harmony with traditional building materials. The following sections will help you choose the most suitable option for your project and understand how to make the most of their natural performance characteristics.

Why Choose a Blend of Sisal Fibres and Sheep’s Wool?

Blending plant and animal fibres creates a balance of strength, performance, and sustainability that few single-material insulations can match. A natural insulation made from sisal fibres and sheep’s wool combines the durability and rigidity of a plant fibre with the flexibility and moisture-regulating qualities of wool. The result is an insulation that performs well thermally, helps maintain a healthy indoor environment, and remains fully recyclable at the end of its life.

Complementary performance

Sisal, a plant fibre derived from the agave plant, is strong, resilient, and naturally resistant to shrinkage and deformation. It adds structure and toughness to insulation, helping it hold its shape in walls, roofs, and floors over time. Sheep’s wool, on the other hand, offers exceptional thermal and acoustic properties. It can absorb and release up to 30 per cent of its weight in moisture without losing insulation performance, helping regulate indoor humidity and prevent condensation and mould. Together, these materials provide a stable, breathable layer that works well in both traditional and modern buildings.

Sustainability and safety

Both materials are renewable, biodegradable, and have significantly lower embodied carbon compared with synthetic insulations. The production process requires minimal energy and avoids petrochemical binders or formaldehyde-based treatments. The blend is also naturally non-toxic and contributes to better indoor air quality. Sisalwool is pleasant to work with — it can be handled without protective equipment and doesn't irritate skin.

Proven performance

Field studies and laboratory tests of wool-based insulation have shown consistent thermal performance (around 0.035–0.040 W/m·K) and excellent sound absorption. Wool’s natural keratin structure gives it resistance to mould and microbial growth, while sisal’s dense fibre composition provides longevity and structural stability. The combined material has a long service life and remains effective even under varying humidity levels, unlike some synthetic alternatives that degrade or lose efficiency over time.

Introducing Sisalwool

One example of this approach is Sisalwool, a blend that brings together carefully processed sisal fibres and British sheep’s wool. It is designed to deliver reliable, breathable insulation suitable for floors, walls, and roofs. Unlike some purely wool-based products that can compress over time, the inclusion of sisal fibres enhances shape retention and durability while maintaining excellent acoustic and thermal properties. Sisalwool offers a versatile and sustainable choice for builders and homeowners in both retrofit or new-build projects.

Addressing common concerns

Some people assume that natural insulation costs more than conventional alternatives, but the gap has narrowed considerably in recent years. When lifecycle performance, comfort, and reduced maintenance are considered, the long-term value becomes clear. Availability is also improving as demand grows across the UK, and installation methods are similar to those used for other flexible insulation batts or rolls.

Sisalwool is treated to be resistant to moths and will self extinguish if set alight.

How to Assess Your Home’s Insulation Needs (Beginner and Advanced Level)

Before choosing materials or starting work, it’s essential to understand what your home actually needs. Insulation should not only keep heat in but also help manage moisture, reduce noise, and work in harmony with the building’s structure and ventilation. Whether you are a homeowner, self-builder, or tradesperson, this section walks through the process step by step, from simple checks to advanced assessment methods.

Beginner: Spotting the Signs of Poor Insulation

If your home feels cold even when the heating is on, or certain rooms are noticeably draughtier than others, it’s a good sign that your insulation may need upgrading.

Checklist – Signs your home could benefit from better insulation:

◻ Cold internal walls, floors, or ceilings

◻ Condensation on windows or walls

◻ Uneven room temperatures

◻ Draughts near skirting boards, windows, or loft hatches

◻ High or rising energy bills

◻ Ice forming on roof edges in winter

Start by feeling walls and ceilings for cold spots and checking lofts or crawl spaces to see what insulation is already installed. Older homes often have minimal or poorly fitted insulation, leaving gaps that let heat escape.

Intermediate: Understanding How Insulation Works

Once you know there’s a problem, it helps to understand how insulation performance is measured and what can affect it.

Key concepts:

• U-value: Measures heat transfer through a material (lower is better).
• R-value: Measures resistance to heat flow (higher is better).
• Thermal bridging: Occurs where heat escapes through structural elements such as beams or metal fixings.
• Moisture and condensation: Poor insulation or ventilation can trap moisture, leading to damp and mould.
  
  

Checklist – How to evaluate your home’s insulation:

◻ Identify insulation in walls (cavity, solid, timber frame) and loft.

◻ Check insulation thickness and condition.

◻ Look for signs of moisture ingress or mould growth.

◻ Note ventilation points (trickle vents, extractor fans, roof vents).

◻ Record approximate thermal conductivity values from manufacturer data.

At this stage, a homeowner might start comparing materials. Natural options such as wool, hemp, or sisal blends not only insulate but also manage humidity, reducing condensation risk and improving air quality.

Advanced: Detailed Assessment and Long-Term Planning

For those planning a full retrofit or a new build, it’s worth carrying out a more detailed analysis to balance performance, cost, and sustainability.

Advanced considerations:

• Payback calculations: Estimate how long savings on energy bills will take to cover installation costs.
• Embodied carbon and lifecycle analysis: Assess the total environmental impact from production to disposal.
• Material selection: Choose insulation not just for thermal performance, but also for breathability, recyclability, and health impact.

Checklist of What to record before starting work:

◻ Building age and construction type

◻ Existing insulation type, condition, and thickness

◻ Presence of vapour barriers or membranes

◻ Ventilation system and airflow patterns

◻ Any previous damp or condensation issues

◻ Preferred material types for sustainability goals

Case Example: Mid-Terrace 1930s House in the UK

A typical mid-terrace property built in the 1930s may have solid brick walls, little or no loft insulation, and timber floors with air gaps underneath. These homes often lose heat through uninsulated walls and roof voids.

Recommended steps:

• Inspect loft insulation and top up to current standards (typically 270 mm or more).
• Use breathable insulation for walls and floors to manage moisture effectively.
• Seal draughts around floorboards and windows but maintain adequate ventilation.
• Consider natural fibre options like Sisalwool to improve comfort and air quality while supporting sustainability goals.

Over time, these upgrades can significantly reduce heating bills and make the property more comfortable throughout the year.

Installation Considerations for Natural Insulation

Best Practices

Natural insulation performs at its best when it is correctly installed. Because these materials are designed to work with the building fabric rather than against it, attention to detail during installation is essential. This section outlines the key stages, from preparation through to maintenance, with additional advice for those using Sisalwool.

Preparing for Installation

Start by assessing the condition of the surfaces where insulation will be installed. Walls, floors, and roof spaces should be dry, clean, and free from debris or mould. Identify any signs of water ingress or damp and fix these before proceeding.

Key preparation steps:

• Ensure all gaps, cracks, and joints are sealed to prevent uncontrolled air leakage.
• Check that the structure allows the insulation to fit snugly without being compressed.
• Confirm whether a vapour control layer or breathable membrane is needed, depending on the material and location.
• Plan around electrical wiring, plumbing, and ventilation ducts to maintain safe clearances and airflow.

Unlike rigid boards or foams, natural fibre insulation should not be over-compressed. It works best when the fibres remain slightly loose, allowing air to be trapped and moisture to move naturally through the material.

Installing Sisalwool

Sisalwool insulation combines the flexibility of wool with the strength of sisal fibres, making it easy to handle and fit into irregular spaces such as rafters or joist bays.

Handling and fitting tips:

• Cut using a sharp insulation saw or long-blade knife for clean edges.
• Fit insulation tightly between joists or studs, ensuring no gaps or sagging.
• Avoid compressing the material more than 5–10 per cent to maintain performance.
• Pair with breathable membranes rather than plastic vapour barriers to allow moisture to move freely and prevent trapped condensation.
• Works particularly well with timber framing and other natural building materials.

Because sisal fibres help the material retain shape, it tends to settle less over time than loose-fill products, offering long-term stability and consistent thermal performance.

Further information on installing Sisalwool.

Health and Safety Considerations

Natural insulation is generally safer to handle than synthetic alternatives, as it contains no petrochemical binders or harmful off-gassing agents, and dust levels are usually minimal. When installing it, it’s still best to follow a few basic safety precautions. Wear gloves and a dust mask if you’re cutting or handling large volumes, and use protective eyewear when working overhead. Make sure the area is well ventilated during installation, especially in lofts or other confined spaces, and always store the insulation in a dry place until you’re ready to use it.

DIY or Professional Installation

DIY installation can be suitable for smaller projects such as lofts or internal wall linings, provided you follow manufacturer guidance carefully. It offers cost savings and allows for flexible scheduling.

However, professional installation is recommended when:

• You are insulating external or cavity walls.
• Vapour control or airtightness layers must be integrated precisely.
• The project involves complex junctions or mixed materials.
• You require certification for energy efficiency schemes or grants.

A professional installer will also have the tools to cut, fit, and seal insulation to specification, reducing the risk of thermal bridging and ensuring long-term performance.

Maintenance and Longevity

One of the main benefits of natural insulation is its durability. With proper installation, it can last the lifetime of the building. However, occasional inspections help ensure ongoing performance.

Maintenance checklist:

◻ Check roof spaces and voids every few years for signs of moisture, pest activity, or compression.

◻ Ensure ventilation openings remain clear.

◻ Inspect for any damage following leaks or roof repairs.

◻ Replace or top up insulation only if it has been displaced or saturated.

Sisalwool is naturally resistant to pests such as moths and insects, and when installed correctly with good ventilation, it maintains both structure and insulating performance for decades.

Savings and Environmental Impact

Upgrading your home’s insulation is one of the most effective ways to reduce both energy costs and environmental impact. Natural insulation materials, such as blends of sisal fibres and sheep’s wool, deliver measurable performance benefits while also contributing to healthier indoor air and a lower carbon footprint. This section explores the potential savings, environmental advantages, and long-term value of choosing sustainable materials.

Energy Savings and Payback Period

Insulation prevents heat from escaping through walls, roofs, and floors. In the average UK home, up to 35% of heat is lost through walls and 25% through the roof. Improving insulation can cut heating energy use by 15–25%, depending on the building’s age and construction type.

Example:

• A typical semi-detached home with gas heating may spend around £1,500 per year on energy.
• A 20% reduction in heat loss from improved insulation could save £300 annually.
• If the insulation cost £2,000–£3,000 to install, the simple payback period would be around 7–10 years, not accounting for rising energy prices.

Natural insulations such as Sisalwool often perform on par with, or better than, conventional mineral wool. With a thermal conductivity of around 0.038 W/m·K, it provides similar or improved insulation while adding benefits such as moisture regulation and acoustic absorption.

Environmental Benefits

One of the strongest arguments for using natural insulation is its environmental profile. These materials are renewable, low in embodied energy, and contribute to a more circular building economy.

Key environmental advantages:

• Lower embodied carbon: Natural fibre insulations typically have up to 80% less embodied carbon than synthetic alternatives. Many materials even act as temporary carbon stores, locking away CO₂ absorbed during plant growth.
• Reduced VOCs: Unlike petrochemical foams, natural fibres emit minimal VOCs, improving indoor air quality.
• Biodegradable and recyclable: At the end of their life, natural insulations can often be composted or recycled rather than sent to landfill.
• Moisture regulation and longevity: Because natural materials handle humidity effectively, they reduce the risk of damp and mould, helping buildings last longer with fewer repairs.

Although some synthetic foams offer higher insulation per thickness, natural options like Sisalwool achieve excellent real-world performance while supporting healthier, longer-lasting buildings.

Cost Versus Benefit: What You're Paying For

While the initial cost of natural insulation may be slightly higher than conventional products, the long-term return is both financial and environmental. You’re not just buying insulation, you’re investing in a more resilient, comfortable, and sustainable home.

With Sisalwool, you are paying for:

• A renewable, non-toxic material with verified low embodied carbon.
• Consistent thermal performance without risk of trapped moisture or mould.
• Improved air quality and acoustic comfort.
• Long-term durability, reducing the need for replacement.
• A material that supports UK wool producers and sustainable fibre farming.

Over its lifetime, Sisalwool’s total cost of ownership is often lower than that of synthetic insulation when maintenance, comfort, and indoor air quality are factored in. It delivers savings not just on energy bills, but on health and environmental impact too.

Suggestions for further reading

“An Introduction to Natural Fibre Insulation” (ASBP guide)

“The Use of Natural Insulation Materials in Retrofit” (PDF)

“Embodied Carbon of Insulation Materials” (Ecohome)

“Sustainable Insulation Materials - Unity Lime”

“Insulation Materials and Their Thermal Properties” (GreenSpec)

“Understanding Insulation” (CAT)