Thin insulation:
what it is really worth.

Thin multilayer insulation, thin reflective product, thermo-reflective insulation: behind these names lies a single family. One or more aluminium sheets a few micrometres thick, assembled around intermediate layers — flexible foam, felt, bubble polyethylene. The whole is a few millimetres to a few centimetres thick and is sold in rolls. This is the description given by CSTB specialist group no. 20, the committee that examines the Technical Assessments for these processes, in its information note of December 2007.

Thick insulation slows heat by conduction, thanks to the still air trapped in its fibres or cells. Thin insulation works mainly on radiation: its aluminium face generally has an emissivity below 0.1, whereas most building materials are around 0.9. It therefore reflects thermal radiation, provided that a still air gap separates it from the adjacent wall. Without this air gap, only its own resistance remains, which is low.

Every insulation material is compared by its thermal resistance R, in m²·K/W: the higher it is, the less heat the wall lets through. The CSTB compiled laboratory tests carried out in the United States, Canada, Germany, England, Belgium and France. The verdict: a thin product on its own generally offers an R of 0.1 to 1. With the airtight air gaps created during installation, the total resistance rises to between 0.5 and 2, hardly more.

Compared with a conventional insulation material with a conductivity of λ = 0.035 W/m·K, an R of 2 corresponds to 7 cm of thickness, since thickness equals R × λ. The product on its own, between 0.1 and 1, is equivalent to a thickness of between a few millimetres and 3.5 cm.

The air gaps still have to genuinely exist. According to the same note, a low-emissivity face only adds 0.15 to 0.50 m²·K/W to the air gap it touches if that gap is truly airtight, and creating unventilated, non-communicating air gaps "requires great care". A badly jointed batten, an open joint, and the gain disappears.

Packaging often highlights an equivalence in centimetres of conventional insulation. This is not a standardised quantity: it does not replace a thermal resistance measured to the standard, and it cannot be used to justify any grant.

The measurement method does exist, however. Standard NF EN ISO 22097, published in June 2023, sets out how to determine the thermal performance of reflective insulation products; it replaced NF EN 16012. These products can also be ACERMI-certified, a possibility open since February 2007, and some processes have a CSTB Technical Assessment that gives the effective resistance of the wall according to the method of installation. ADEME sets the same requirement, as we point out in our guide to insulating a roof from the inside: a resistance established by a technical assessment, and CE marking if the film serves as an underlay screen beneath the roof.

The reflex to adopt is simple: ask for the product's certified R and the R of the complete wall, never an "equivalent". Without an ACERMI certificate or a Technical Assessment, the comparison stops there.

Energy savings certificates (CEE) impose a minimum thermal resistance on the installed insulation, which differs by wall: R ≥ 7 in unconverted lofts, R ≥ 6 under rafters, R ≥ 3.7 for a wall, R ≥ 3 for a ground floor. Our guides cover each case in detail, from unconverted lofts to walls insulated from the inside.

Since 1^st May 2022, this resistance must be proven by ACERMI or QB23 certification, or failing that by a document from the manufacturer or an accredited body attesting to a calculation compliant with annex 7 of the order of 22 December 2014 (order of 10 December 2021).

The conclusion needs no calculator: even in the best conditions, thin insulation tops out at around R = 2, below the lowest of these thresholds. Installed on its own, it therefore qualifies for none of these grants. As for MaPrimeRénov', since 1^st September 2026 it only funds insulation as part of a major renovation (order of 25 August 2026), with at least two insulation measures: our overview of MaPrimeRénov' in 2026 sets out the conditions.

The CSTB does not condemn these products: its note treats them as thermal insulation complements and credits them with three qualities, when validated. Adding the resistance of one or two airtight air gaps to that of a conventional insulation material. Acting as a vapour barrier on the inside, since most films are highly impermeable to water vapour. Improving the airtightness of a wall, if the installation is perfect.

In practice, it belongs on the inner face of a thick insulation layer, behind an internal lining, where an air gap of a few centimetres can stay closed. Our guide dedicated to the vapour barrier and the vapour control layer details this role as a water-vapour barrier.

It also has its pitfalls. A vapour-tight film goes on the inside, on the heated side: fitted on the cold side, it traps moisture in the wall. The CSTB warns that an inappropriate use or poor installation can lead to defects, for example poor ventilation of roof structures or timber frames.

To reach the grant thresholds with an insulation material of λ = 0.035, you need roughly 10.5 cm under a ground floor (R = 3), 13 cm against a wall (R = 3.7), 21 cm under rafters (R = 6) and 24.5 cm over an unconverted loft (R = 7). No thin product, even with its air gaps, replaces these thicknesses.

When space is genuinely short, the serious option remains a thick insulation with a lower conductivity: at equal R, a λ of 0.022 needs about 37% less thickness than a λ of 0.035. Our guide to choosing your thermal insulation compares the families wall by wall.

And the price? We do not publish one: none of our 2024-2026 quotes in Savoie and Haute-Savoie includes thin insulation, and we do not quote prices we have not recorded. Above all, remember that a price per square metre can only be compared at equal thermal resistance: a roll that provides R = 1 does not do the job of an insulation that provides R = 3.7, even if it costs less per square metre.