External insulation under wooden cladding: everything you need to know

External insulation is an increasingly popular solution for improving the energy performance of a house in summer as well as in winter. It can in particular be combined with wooden cladding (or another material) so as to have good energy performance together with a fine appearance.

This guide offers you a complete overview of external insulation under wooden cladding or cladding in another material, from its advantages to the installation techniques, by way of the materials to favour.

In brief: what to remember

• External insulation under a façade covering such as wooden cladding significantly improves the energy performance of a house while modernising the appearance of its façade.

• Even though it has insulating properties, cladding alone is not enough to insulate effectively: it is recommended that it be combined with a suitable thermal insulating material, placed under the cladding.

• Wood (natural or composite) is an excellent covering option, provided the species or the product is well chosen, and there is cladding in other materials (composite, PVC and so on) as an alternative.

• The installation must follow precise rules (breather membrane, secondary framework, ventilation) in order to guarantee the durability of the system.

What is wooden cladding? Why use it for external insulation?

Wooden cladding is an external façade covering made up of boards (or sometimes panels) in natural or composite wood, fixed to a framework. It serves to protect and beautify a façade, give it an elegant facelift, and it has insulating properties (thermal and acoustic).

Widely used in façade renovation, cladding can accommodate an insulating material so as to form a very high-performing external thermal insulation (ITE) system.

Wooden cladding has many advantages for the external insulation of your façade:

• It allows renovation with no loss of living space.

• It fits well into its surroundings, with a natural, warm and customisable appearance.

• Combined with good insulation, it reduces heat loss and improves comfort all year round.

• It protects the walls from the weather and extends their service life.

• It makes it possible to carry out the work without leaving the home.

Is cladding alone enough to insulate?

No. Even though it has insulating properties, cladding — whether wood, composite or any other material — is a finishing covering that provides mechanical and decorative protection, but it is not used as an insulating material. It has to be completed by a thermal insulating material laid between the masonry and the cladding (often on a secondary framework), which will neutralise the thermal bridges.

Which type of cladding should you choose for good insulation?

The choice of the type of cladding does not greatly affect external thermal insulation performance, because it is the insulating material (laid behind the cladding) that plays this essential role. Each type of cladding does have insulating properties, however, and above all it affects the appearance, the durability, the budget and the maintenance constraints of your façade.

Here is a summary table by type of cladding:

Even though they have insulating properties, all these types of cladding will require a layer of insulating material placed underneath, which will neutralise the thermal bridges. The final choice of your cladding will therefore depend above all on the budget, the appearance you want and the level of maintenance you are prepared to accept. The most used in detached houses are wooden cladding or composite cladding.

Which insulating material should you choose for external insulation under cladding?

The choice of the insulating material that will be placed under your cladding depends on your priorities: thermal performance, acoustic effectiveness, summer comfort, budget or environmental impact. Wood fibre is ideal for natural, effective insulation; rock wool offers good value for money and high fire resistance; polystyrene or polyurethane perform very well thermally but are less breathable. The essential thing is to match the insulating material well to the cladding and to your environment.

Sources: study by Matériaux Naturels and Kenzai

How do you put external thermal insulation (ITE) in place under cladding?

Here are the main stages in installing external insulation under cladding:

1. Preparing the façade: cleaning and checking that the bare wall is flat.

2. Fixing the brackets: fitting metal brackets fixed firmly to the wall to support the timber framework.

3. Fitting the uprights (primary framework) on the brackets: the uprights are arranged vertically, spaced according to the insulation and the cladding. This framework creates a support and a space for the insulating material.

4. Fitting the thermal insulating material: putting an insulating panel or rolls of insulating material (wood fibre, rock wool and so on) in place between or in front of the primary framework. The insulating material is fixed mechanically or glued, depending on the material.

5. Fitting the breather membrane on the framework: installing a breather membrane (HPV film) on the framework, which protects the insulating material from water while letting water vapour through. It is stapled or nailed to the framework.

6. Counter-battening (secondary framework): fitting vertical or horizontal battens on the breather membrane, nailed to the primary framework. This double framework creates a ventilated air gap between the cladding and the breather membrane, essential for removing moisture.

7. Fitting the wooden cladding: fixing the cladding boards to the battens, keeping a gap to allow the wood to swell and to ensure ventilation. The cladding can be laid horizontally, vertically or diagonally according to your aesthetic choice.

8. Finishes: putting the ventilation elements in place at the bottom and at the top (rodent guards), treating the corners, sills and joints so as to ensure weathertightness and durability.

Frequently asked questions about external insulation under cladding

Is the process of laying the insulation under cladding different according to the type of cladding?

The installation principle remains broadly the same, whatever cladding is chosen: the insulating material is fixed continuously to the external wall, protected by a breather membrane, then covered by a ventilated framework to which the cladding is fixed.

Certain adjustments may be necessary depending on the material, however: metal cladding (which is heavier), for example, often calls for a reinforced framework, whereas natural wood needs more rigorous ventilation to avoid moisture problems. The choice of fixings, the spacing of the battens or the treatment of the special points (corners, openings) can therefore vary.

Can you carry out external thermal insulation (ITE) under cladding yourself?

Installing it yourself is possible for experienced DIY enthusiasts, but poor installation can harm performance. Calling in a professional is recommended.

What is the average service life of the different insulating materials under wooden cladding?

The average service life of an insulating material under wooden cladding generally varies between 30 and 50 years, depending on the material and the quality of the installation. Insulating materials such as cork, hemp, polystyrene or wood fibre offer the best longevity (up to 75 years) when they are well ventilated and protected from moisture.
Materials sensitive to moisture such as glass wool or cellulose require a very rigorous installation so as to avoid premature ageing. A good breather membrane and a ventilated air gap are essential to guaranteeing the durability of the cladding as well as the insulation.

Is there financial support for an external insulation project?

Yes, several forms of financial support exist for an external thermal insulation (ITE) project: MaPrimeRénov’, the zero-interest eco-loan, reduced-rate VAT, and also local support.

These schemes are subject to conditions (income, performance of the insulating material and so on) and the work must be carried out by a certified RGE professional (Reconnu Garant de l’Environnement, the French label for environmentally qualified tradespeople).