What Is the Best Thermal Insulation Material? Our Advice for Making the Right Choice

Are you considering insulating your home to improve comfort and reduce energy bills? Good insulation is the key to protecting your home from heat loss, keeping you warm in winter and cool in summer, while lowering your energy costs. But with so many insulation materials available on the market, which one is the right choice? How do you choose the best insulation material for your project? What criteria should you consider and at what price? ENGIE guides you step by step to help you understand everything!

What are the best insulation materials? Cellulose wadding comes out on top

To measure the insulating performance of a material, two criteria are essential: thermal resistance (represented by the R-value) and thermal phase shift.

Thermal resistance (R) measures how effectively a material resists heat transfer. It reflects the ratio between:

• Thermal conductivity (represented by the lambda coefficient λ): expressed in W/(m.K), it indicates whether a material encourages heat transfer (high λ value) and therefore conducts heat easily, or whether it insulates effectively (low λ value).

• The thickness of the material: in general, the thicker the material, the more effective the insulation.

To calculate thermal resistance (R), simply divide the thickness by the thermal conductivity coefficient (λ). Therefore, when comparing insulation materials, their R-value should be compared at equal thickness. In fact, this R-value is the benchmark used by organisations such as ADEME (the French Agency for Ecological Transition) to assess insulation performance.

In addition, thermal phase shift expressed in hours must also be considered. This refers to the time it takes for heat to pass through the insulation material. The longer the phase shift, the better the insulation protects your home from summer heat. This criterion is essential if you want to avoid overheating during hot weather.

Here is a comparison table of the performance of the main insulation materials:

Polyurethane stands out as one of the best thermal insulation materials in terms of thermal resistance. However, it is important to note that this material comes from the petrochemical industry, making it less suitable for eco-construction projects. Wood fibre, on the other hand, is an excellent solution for protecting against summer heat.

If you need to choose between thermal phase shift and thermal resistance, cellulose wadding may well be the best option. It combines low thermal conductivity with excellent thermal phase-shift performance.

What are the main insulation materials used in France?

When starting renovation work, it is essential to compare the insulation materials available on the market carefully. There are three main categories of insulation materials:

• natural or bio-based insulation materials (animal- or plant-based);

• mineral insulation materials;

• synthetic insulation materials.

Let us explore together the advantages, disadvantages and price per square metre of each type of insulation.

What is the best bio-based insulation material?

Also known as eco-friendly insulation materials, bio-based insulation products generally have a lower environmental impact during manufacturing. Most are also recyclable. However, they are often more expensive.

The main animal-based insulation materials include:

• sheep wool;

• duck feathers.

The most common plant-based insulation materials are:

• wood in wool or fibre form;

• expanded cork;

• flax;

• hemp;

• cellulose wadding made from recycled paper.

To help you compare them, here is a summary table of bio-based insulation materials.

Mineral wool insulation: which material offers the best insulation?

Mineral insulation materials are made from naturally occurring minerals such as sand or volcanic rock like basalt. This type of insulation is the most widely used in France because it is highly affordable.

The most common examples are mineral wool products such as glass wool and rock wool, as well as vermiculite and expanded perlite.

Which synthetic insulation material performs best?

Synthetic insulation materials can be suitable for many surfaces and are generally the cheapest on the market. However, they are not recyclable and have a negative environmental impact because they are derived from the petrochemical industry.

The most common examples are:

• polystyrene;

• polyurethane.

Please note that the prices listed above are provided for guidance only, based on market averages. Do not hesitate to request several quotes from different professionals to compare the total cost of insulating your home.

You should also know that financial support may be available to reduce the cost of your renovation work.

Several schemes exist in France to help finance insulation work:

• MaPrimeRénov’, the French government’s flagship renovation grant, can cover up to 90% of costs for very low-income households, with grants reaching up to €4,000 for external wall insulation or €2,500 for lost attic insulation;

• Energy Savings Certificates (“Certificats d’Économies d’Énergie” – CEE), including ENGIE’s Energy Savings Bonus (“Prime Économie d’Énergie”), also provide additional support, up to €11/m² for lower-income households;

• The eco-interest-free loan (“éco-PTZ”), which can finance up to €30,000 of work without interest;

• The reduced VAT rate of 5.5%;

• Local and regional financial aid and/or property tax exemptions.

To qualify for these financial support schemes, make sure you hire RGE-certified professionals (“Reconnu Garant de l’Environnement”) and comply with the required technical eligibility criteria.

How to choose the best insulation material for your home?

To choose the ideal insulation material, it is essential to weigh up the pros and cons of each product, taking into account its advantages, disadvantages and cost. But that is not all. Several technical criteria should also guide your decision:

• Thickness expressed in cm: the thicker the insulation material, the better its performance but be careful not to lose too much living space;

• Thermal resistance (R-value), expressed in m².K/W: this indicates the material’s ability to resist temperature variations. The higher the R-value, the better;

• Thermal phase shift expressed in hours: this measures the material’s ability to block heat transfer. A long phase shift is ideal for keeping your home cool in summer;

• Thermal conductivity (represented by the lambda coefficient λ) expressed in W/(m.K): a conductive material encourages heat transfer, while a material with low conductivity limits it. The lower the λ value, the more efficient the insulation;

• Installation format: rigid or semi-rigid panels, rolls, or loose-fill insulation for blown installation. Each format has advantages depending on the application and the area being insulated.

Depending on the areas you want to insulate, here are our recommendations to help you make the right choice.

Which material should you choose for attic insulation?

It all depends on whether your attic space is converted or unconverted: insulation methods differ for “combles perdus” (unconverted attic spaces) and “combles aménagés” (converted attic spaces).

Unconverted attic spaces

For insulating unconverted attic spaces, glass wool or rock wool are often preferred because of their excellent value for money. However, if your budget allows it, loose-fill wood fibre is an even better option. Easy to install, it reaches every corner and provides better protection against heat than mineral wool insulation.

Converted attic spaces

For insulating roof slopes in convertible attic spaces, choose thin insulation materials such as wood panels or cork. These solutions help save space in rooms with sloping ceilings while still delivering excellent insulation performance.

What is the best material for wall insulation?

Walls can be insulated in two ways: from the inside or from the outside. The insulation material you choose depends on the installation method.

The best insulation material for internal wall insulation

For internal wall insulation, choose a thin insulation material such as cork. It takes up very little space and preserves your room’s living area.

The best insulation material for external wall insulation

For external insulation, choose a material with high density for long-lasting stability and durability, such as expanded polystyrene. This is the most commonly used material because it is easy to install, affordable and highly efficient. Rock wool is also a good alternative because it is moisture-resistant and cost-effective.

Which material should you choose for floor insulation?

There are also two techniques for floor insulation:

• insulating lower floors;

• insulating the ceiling of the room below.

Depending on the method used, the most suitable material will vary.

Insulating floors from above

For this technique, choose a thin insulation material to avoid losing too much room height. Cork, hemp or wood panels are effective and suitable options.

Insulating the ceiling of a cellar or garage

This method requires a crawl space or an unheated room underneath. Glass wool or rock wool are highly recommended for this type of insulation. Affordable and resistant to humidity, they are ideal for cellars and garages.