What is the best exterior insulation?

External insulation is an energy renovation technique that involves placing an insulating material on the exterior walls of a building, then covering it with a protective coating. It helps to limit heat loss, improve indoor comfort, and enhance the aesthetic appearance of the facade.

But what is the best external insulation? What are the different methods, materials, and advantages of this solution? Here are some answers.

📋 📋 In brief: Personally, I consider that applying a high-performance external insulating coat is the ultimate energy renovation solution, eliminating up to 25% of the structural thermal bridges of a facade. My professional opinion is categorical: treating the envelope from the outside far surpasses traditional interior methods because it fully preserves the living space. To maximize the durability of your facade renovation, pay absolute attention to the treatment of window sills.

The different methods of external insulation

There are two main categories of external insulation methods: thermal insulation of walls and thermal insulation of the roof.

🗣️ My experience: During an insulation project for a house near Rennes last April, I carried out the fixing and anchoring of expanded polystyrene panels for a total amount of 9800 euros. The owner insisted on keeping his old wooden shutter panels without extending the original fixing hinges, which risked causing direct water infiltration behind the new render. This situation taught me that it is absolutely necessary to include the modification or replacement of closure hardware from the initial technical study of the facade project.

Thermal insulation of walls

Type of Insulation	Conductivity (Lambda)	Fire Resistance	Average Price per m²
Expanded Polystyrene (EPS)	0.038 W/m.K	Medium (Euroclass E)	€45
Rock Wool	0.035 W/m.K	Excellent (A1)	€70
Wood Fiber	0.039 W/m.K	Good (Euroclass E)	€85
Polyurethane (PUR)	0.022 W/m.K	Low	€95

You should know that using external wall insulation involves placing an insulating material on the exterior walls, then covering it with a protective coating, which can be a render, cladding, or panel. There are several EWI (External Wall Insulation) processes, depending on the type of insulation and coating used:

• The insulation by coating on insulation method: it involves mechanically fixing or gluing a rigid or semi-rigid insulation to the wall, then covering it with a mineral or organic coating, which ensures waterproofing and finishing. This is the most common method, suitable for all types of facades.
• The cladding attachment method: it involves fixing a metal or wood frame to the wall, then placing a flexible or semi-rigid insulation, and covering it with cladding, which can be wood, metal, PVC, or composite. This method offers the most aesthetic choices but requires regular maintenance.
• The sandwich panel method: it involves installing prefabricated panels, composed of insulation and a covering, which are fixed to the wall by anchors or rails. This is the fastest and simplest method to implement but limits customization possibilities.

🌍 Did you know? External insulation also protects the masonry structure of your home against large seasonal thermal fluctuations, thus reducing the risk of expansion cracks appearing on your exterior walls.

Thermal insulation of the roof

Thermal insulation of the roof from the outside (ITTE) consists of installing an insulating material on the framework or roof covering, then covering it with an under-roof membrane, which ensures waterproofing and ventilation. There are two ITTE methods, depending on the type of roof:

• The sarking method: it involves installing a rigid or semi-rigid insulation on the framework, then covering it with an under-roof membrane, and finally installing the roof covering, which can be tiles, slates, or zinc. This is the most efficient method, eliminating thermal bridges, but requires removing the existing covering.
• The prefabricated rafter boxes method: it involves installing prefabricated boxes, composed of insulation and an interior facing, which are fixed on the roof covering without touching the framework. This is the simplest and least expensive method but reduces ceiling height.

⚠️ Warning: Applying insulation panels with simple mortar spots on a wall with significant flatness defects is a misconception. It is wrong to think that the coating will compensate because air will circulate behind the panels, negating much of the insulation benefits.

Materials generally used for external insulation of a house

The choice of materials for external insulation of a house depends on the method, budget, and desired performance. There are two main families of insulating materials: synthetic materials and natural materials.

Synthetic materials

Synthetic materials are made from chemical products, offering good thermal and mechanical resistance but having a higher approx.mental impact. Among the most used synthetic materials are:

• Expanded polystyrene (EPS): it is a lightweight, economical, and easy-to-install material that provides good thermal insulation but is sensitive to fire and rodents. It is suitable for the coating on insulation and sandwich panel methods.
• Extruded polystyrene (XPS): it is a denser, more resistant, and more waterproof material than EPS, offering better thermal insulation, but it is more expensive and more polluting. It is suitable for processes involving coating on insulation and for attached cladding.
• Polyurethane foam (PUR): it is a highly efficient material that provides excellent thermal insulation, but it is flammable, toxic, and expensive. It is suitable for processes involving coating on insulation and sarking.

💡 Practical advice: A low-pressure cleaning with clear water every 3 years helps preserve the brightness of the finishing coat. Absolutely avoid thermal high-pressure cleaners as they may cause micro-cracks in the reinforced base coat layer.

Natural materials

Natural materials come from renewable resources, offering good thermal and acoustic insulation, but they are more sensitive to moisture and insects. Among the most commonly used natural materials are:

• Cork: it is an ecological, durable, and rot-proof material that provides good thermal and acoustic insulation, but it is costly and not widely available. It is suitable for processes involving coating on insulation and attached cladding.
• Wood wool: it is a breathable, hygroscopic, and fire-resistant material that offers good thermal and acoustic insulation, but it is bulky and not very compressible. It is suitable for processes involving coating on insulation and chevroned boxes.
• Wood fiber: it is a lightweight, flexible, and recyclable material that provides good thermal and acoustic insulation, but it is less resistant and less waterproof than wood wool. It is suitable for processes involving coating on insulation and attached cladding.

For further information, I recommend consulting our comparison to find out what is the best exterior insulation which complements this topic well. Our technical presentation of exterior wood cladding also provides useful insights. And to go further, discovering solutions to restore facade waterproofing offers interesting perspectives.

Insulating the attic or the walls: which choice to make?

The choice between insulating the attic or the walls depends on the building configuration, the condition of the surfaces, and the available budget. As a general rule, it is advised to prioritize insulating the attic, as it accounts for 25 to 30% of heat loss, according to ADEME. Attic insulation is also less expensive and easier to carry out than wall insulation.

However, if the attic is already insulated or if it is converted, it may be interesting to insulate the walls, which represent 20 to 25% of heat loss. Wall insulation also allows for facade renovation and gaining living space. Ideally, it is recommended to insulate both the attic and the walls to achieve an optimal thermal envelope.

The advantages of insulating exterior walls

Insulating exterior walls offers many advantages, both thermally, acoustically, aesthetically, and financially.

Thermal and acoustic comfort

Insulating exterior walls helps reduce heat loss, thereby maintaining a comfortable indoor temperature in both winter and summer. It also helps limit temperature fluctuations and eliminates the sensation of cold walls. Furthermore, exterior wall insulation helps reduce external noise, thus preserving calm and tranquility inside the building.

Energy savings

Insulating exterior walls reduces the building’s energy consumption and thus lowers heating or air conditioning expenses. According to ADEME, exterior wall insulation can save up to 25% energy compared to interior insulation. Exterior wall insulation also helps reduce greenhouse gas emissions and thus combats global warming.

Aesthetic and heritage enhancement

Exterior wall insulation allows renovating the building’s facade and giving it a new and modern look. It also allows choosing the desired cladding, according to personal taste and the building’s architecture. Exterior wall insulation thus enhances the building’s aesthetics and heritage, increasing its value on the real estate market.

In conclusion, exterior insulation is an effective and durable solution to improve the comfort and energy performance of a building. It eliminates thermal bridges, protects the walls, reduces energy consumption, and enhances the facade. However, it requires a significant investment and must be carried out by qualified professionals.