Insulation: useful thickness or pointless waste of space
Hello, this is Claude. To avoid wasting space while ensuring effective thermal insulation, you need to choose an insulation thickness suited to the desired energy performance and the type of insulating materials. This doesn’t mean piling on the layers, but rather optimizing the usable thickness for maximum interior comfort and thermal efficiency. Here, we’ll see how to transform this thickness into real added value, not wasted space.
To do immediately
- Choose an insulator based on its lambda value to reduce the required thickness.
- Opt for a glued installation system rather than a frame system to optimize space.
- Always check the displayed thermal resistance (R), target at least R3 for interior walls.
- Allow a 10% margin on the thickness to compensate for settling and errors.
- Treat thermal bridges thoroughly to maintain energy efficiency.
To delve deeper into these practical tips, discover how to create insulation without losing too much space with these tried-and-tested tricks on interior insulation and the techniques adapted to your home at Ithaca Renovation.
Understanding the relationship between effective thickness and energy performance
The thickness of the insulation is equal to its thermal resistance multiplied by its thermal conductivity (λ). The more efficient the material, the lower its lambda value, and therefore the less thickness is required. For example, achieving an R-value of 4 with fiberglass (λ=0.035) requires approximately 14 cm, while the same R-value with polyurethane (λ=0.025) is achieved with 10 cm. This makes a real difference in the room.
Don’t fall into the trap of thinking “thicker is better” without evaluating specific performance. The goal is optimal interior comfort and a balance between sound and thermal insulation without excessively reducing usable floor space.
Insulating materials and their usual thicknesses in 2026
| Material | Usage area | Minimum thickness (target R) | Optimal thickness (high performance) |
|---|---|---|---|
| Glass wool | Attic, interior walls | 12–32 cm (R3–R7) | 15–40 cm |
| Cellulose wadding | Attics, walls | 16–36 cm (R3–R7) | 20–40 cm |
| Polyurethane (PU) | Walls, floors | 8–22 cm (R3–R7) | 10–16 cm |
| Wood wool / Hemp | Walls, Exterior Insulation | 16–22 cm (R3–R5) | 20–25 cm |
| Thin insulation | Complement | 3–6 cm (never alone) | 6–10 cm |
If it’s a small apartment…
Opt for low lambda insulation materials such as PU to limit space loss while maintaining good insulation.
If it’s an old house with thick walls…
Cellulose wadding or wood wool, with appropriate installation, guarantees good indoor comfort by limiting the risks of humidity.
How to limit the loss of space in interior insulation?
The secret lies in the installation method and the choice of insulation materials suitable for interior thermal insulation (ITI). Bonded linings are particularly advantageous for reducing thickness compared to stud walls, which require more space.
Furthermore, choosing ACERMI-certified materials, which guarantee their performance, avoids unpleasant surprises associated with overly compact or poorly insulating products. Also, be wary of thin insulation used alone: its thermal efficiency is low and often disappointing as a primary insulation layer.
A 3-step procedure for effective and space-saving interior insulation
- Step 1: Perform an accurate diagnosis to define the necessary thermal resistance (R) according to your area and your needs.
- Step 2: Choose a high-performance insulation with a low lambda value, such as polyurethane, if you want to save space.
- Step 3: Opt for a glued installation on a dry and flat wall, with a vapor barrier and treatment of thermal bridges.
Mistakes to avoid
- Do not compress the insulation, it reduces its performance.
- Do not neglect the vapor barrier, otherwise you risk humidity and mold.
- Don’t buy insulation at random without checking its ACERMI certification.
- Don’t choose something too thin to “save space” because it will be a double loss: comfort and energy bill.
- Don’t forget about thermal bridges on corners and around windows.
To delve deeper, you can also explore real-world examples of space and performance optimization in this well-documented article and the appropriate technical solutions on IPF Immo.
Optimize space and improve sound insulation
Good interior thermal insulation also creates better acoustic comfort, especially with materials like rock wool or wood fiber. By reducing the thickness without compromising quality, you avoid the feeling of cramped space while blocking out noise. As one expert says, “Insulation is a balance between usable thickness and living space.”
Is it possible to insulate an interior wall without losing too much space?
Yes, by choosing high-performance insulation with low lambda and a glued installation, the loss of space is limited to a few usable centimeters.
What thickness of insulation should I aim for to achieve good energy efficiency?
For an interior wall, a thermal resistance between 3.7 and 5 is recommended, which generally corresponds to 8-20 cm depending on the material.
Which insulating materials are best suited to limit space loss?
Polyurethane and bio-based insulation materials such as wood wool offer a good compromise between performance and thickness.
Should we prioritize insulation from the inside or from the outside?
Exterior insulation is often more efficient and less bulky for rooms, but interior insulation remains a very good alternative when the exterior is not accessible.
How to avoid common mistakes when insulating the interior?
Respecting the installation of the vapor barrier, not compressing the insulation, and treating thermal bridges are essential for a good result.
Thank you for your interest; Claude
