Choosing a thermal insulation for walls requires comparing precise physical quantities: thermal conductivity, target thermal resistance, and the necessary thickness to achieve it. With the gradual implementation of RE 2020 (and the expected adjustments for RE 2026), the performance thresholds imposed on opaque walls are tightening. This article measures the actual differences between insulation families based on the criterion that conditions everything else: the thickness required to achieve the regulatory thermal resistance.
Thermal conductivity of wall insulations: comparative table
Thermal conductivity (lambda, expressed in W/m·K) determines the thickness of insulation needed to achieve a given thermal resistance R. The lower the lambda, the more effective the insulation is at the same thickness.
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| Insulation Family | Common Material | Indicative Lambda (W/m·K) | Thickness for R = 4 m²·K/W |
|---|---|---|---|
| Synthetic | Polyurethane (PUR) | Among the lowest on the market | Low (gain in living space) |
| Mineral | Glass wool / rock wool | Intermediate | Medium |
| Bio-based | Wood fiber | Higher than synthetics | More significant |
| Bio-based | Cellulose wadding | Comparable to mineral wools | Comparable to mineral wools |
This table highlights a first trade-off: at equal thermal resistance, polyurethane takes up significantly less space than wood fiber. On the other hand, wood fiber offers a much higher thermal phase shift, a parameter that impacts summer comfort.
To delve deeper into the calculation of the thermal resistance R suitable for your walls and regulatory requirements, you can learn more on D Kom Déco which details the step-by-step method.
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Thermal resistance R of walls: what RE 2020 imposes and what RE 2026 prepares
RE 2020, applicable to new constructions, is based on Bbio (bioclimatic needs). It does not directly impose a minimum R value per wall, but Bbio indirectly constrains the performance of walls: a building that does not meet the Bbio threshold cannot obtain its building permit.
In renovations, financial aids condition the minimum thermal resistance of installed insulations. For walls, the floor value is around R = 3.7 m²·K/W to benefit from certain aids, but aiming for R = 4 or more remains the common practice among thermal engineering firms.
What changes with RE 2026
The expected adjustments for RE 2026 should strengthen the requirements for summer comfort and potentially raise the Bbio thresholds. For walls, this means that the choice of insulation will no longer be limited to winter thermal resistance alone.
The thermal phase shift (the ability of a material to delay heat transfer) becomes a full-fledged selection criterion. An insulation with a high phase shift, such as wood fiber, delays the entry of summer heat by several hours, whereas polyurethane transmits it quickly.
Thin insulation versus thick insulation for walls: performance gap analysis
The debate between thin insulation (polyurethane, vacuum panels) and thick insulation (mineral wools, wood fiber) crystallizes around three variables.
- Thickness and living space: in a home where every centimeter counts (old apartment, narrow room), polyurethane allows for several centimeters of gain compared to rock wool, at equivalent thermal resistance.
- Thermal phase shift and summer comfort: dense materials (wood fiber, high-density rock wool) store heat longer before releasing it. This delay reduces indoor temperature peaks in summer.
- Water vapor permeability: mineral wools and wood fiber allow vapor to migrate, which limits the risk of condensation within the wall. Polyurethane, almost impermeable, requires strict management of air tightness and ventilation.
Conversely, bio-based insulations have a more significant thickness, which can pose problems in internal thermal insulation (ITI) when the available space is limited.
Thermal insulation of walls from the inside or outside: what impact on the choice of insulation
The installation technique modifies the list of compatible insulations. In ITI (internal thermal insulation), the plasterboard-insulation complex (glued or framed) remains the most common solution. The insulations selected are generally glass wool, rock wool, or polyurethane panels.
In ITE (external thermal insulation), expanded polystyrene (EPS) dominates the market due to cost and ease of implementation under rendering. Rigid wood fiber panels are gaining ground in this segment, driven by the demand for low environmental impact materials.
The thermal bridge trap in ITI
ITI does not address structural thermal bridges (wall-floor junctions, wall partitions). These thermal bridges can represent a significant portion of the total heat losses of a building insulated from the inside. ITE, by enveloping the structure, eliminates most of these thermal bridges.
For the renovation of an old house, ITE will therefore often be more effective overall, even if the chosen insulation has a slightly less favorable lambda than that used in ITI.
Insulating materials and carbon footprint: a criterion that weighs in RE 2026
RE 2020 introduced life cycle analysis (LCA) into the regulatory calculation. Bio-based materials (cellulose wadding, wood fiber, hemp) show a significantly more favorable carbon footprint than synthetic insulations derived from petrochemicals.
RE 2026 is expected to emphasize the weight of carbon in the overall evaluation, which could favor bio-based insulations despite their greater thickness. The choice of insulation for your walls is therefore no longer just a simple calculation of thickness and lambda.
Data converge towards the same conclusion: the selection of thermal insulation for walls now relies on a trade-off between thermal conductivity, summer phase shift, water vapor management, and carbon footprint. No material dominates across all these criteria simultaneously, making sizing by a thermal engineering firm even more relevant before any insulation project.