Certified performance

Thermal conductivity: λ (lambda)

The insulating properties of a material are measured in terms of what is called its thermal conductivity, represented by the Greek letter lambda: it refers to the intrinsic capability of the material to transmit heat through its thickness from a hot side to a cold side.

The lower the conductivity - i.e. the lower the lambda value - the less heat the product transmits/transfers, and therefore the greater its insulation value. The thermal conductivity coefficient is expressed in W/m.K (Watts per meter Kelvin).

Thermal Resistance: the R-value measures the resistance to heat flow

Thermal resistance expresses the ability of a material to resist the flow of heat (limiting heat losses in winter and excessive heating in summer. The higher the R-value, the more insulation the product provides.

Expressed in m².K/W (square meters Kelvin per Watt), the R-value is the ratio of material thickness (in meters) to its thermal conductivity. The R-value relates the conductivity of the material to the thickness of material installed.

R = thickness in meters/the thermal conductivity of the material (lambda).


  • In practice, it is not sufficient simply to compare the R-values of different materials, since R-values are specific to a given thickness: the thickness of the product. For example, a 30 cm thick brick wall has the same thermal resistance as 1 cm of glass wool.
  • The choice of insulation product is made on the basis of its thickness/performance/price balance and in relation to the space available and the cost of that space.

Product reaction to fire

At the request of the manufacturer, ACERMI can optionally certify the reaction to fire of a material in addition to its thermal conductivity. This certification expresses the flammability of the insulation product, and its propensity to contribute to the establishment and propagation of fire. The reaction to the fire of construction materials, decoration products, etc. is established by conducting tests that subject the products to thermal stresses. The way they behave when subjected to fire is evaluated against performance criteria designed to assess their flammability.

The ratings for construction products are:

  • A1, A2, B, C, D, E, F
  • s1, s2, s3 (for smoke) and
  • d0, d1, d2 (flaming droplets and debris)

Suitability characteristics

These characteristics complement product characterization by adding physical quantities that depend on the intended application (compression for insulation products used in floors or roof terraces, dimension or stability for internal and external insulation, etc.).

Acoustic performance

Some insulation products deliver good acoustic performance, as well as thermal performance. For most applications, the acoustic performances specified in the European standards applying to insulation products can be certified by ACERMI: acoustic absorption and air resistance.

The acoustic behavior of products used in under-layers beneath floor screed or tiles is characterized by their resistance to impact noise and the variance in dynamic stiffness after long-term compression (compressive creep). These aspects of performance are highlighted by the letter "A" in the floor rating of the certified product.

Mechanical performance

Insulation products are often subject to mechanical stresses in the walls they insulated: under a floor screed or slab, the products are compressed; used in external insulation, they are subject to tearing stresses.

Mechanical characteristics, such as compressive strength in service, shear strength, traction perpendicular to the surfaces (cohesion) and other characteristics defined in product standards can be certified.

Dimensional stability

Insulation products may be subject to extremes of temperature and humidity, depending on the wall in which they are installed, and the dimensions of the product (length, width and thickness) may vary as a function of these conditions. This is referred to as dimensional stability.

Some applications, particularly insulation in roof terraces and external insulation beneath the render finish, demand that the insulation product used remains extremely stable under a range of different temperature and humidity conditions to avoid causing any distortion to the roof or wall concerned (resulting in water ingress, cracking, etc.).

Dimensional stability is a characteristic specified by European standards and can be certified by ACERMI.

Behavior in the presence of water or water vapor

Insulation products may be sensitive to liquid water and/or water vapor. The relevant European standards define multiple characteristics for evaluating the behavior of these products in the presence of water and water vapor: absorption of water by short-term and long-term partial or total immersion (absorbency) and resistance to the spread of water vapor.

These characteristics can be certified by ACERMI.

ISOLE usage profile

The "ISOLE" usage profile is designed to make it easier to express of the suitability of insulation products relative to the needs of users, and as set out in a number of codified texts. They are based on the characteristics defined in European standards, except in specific cases where the criteria set out in the European standards are not sufficient in their current state to cover the needs of a particular application, or where the European standards do not express the relevant aspects of performance in a harmonized or practical way: for example, where one criterion has different meanings according to the reference standard.

  • I Mechanical properties under the effects of compression, with 5 levels: I1 to I5
  • S Resistance to differential movements, with 5 levels: S1 to S5
  • O Behavior in the presence of water, with 3 levels: O1 to O3
  • L Mechanical properties for cohesion and bending, with 4 levels: L1 to L4
  • E Water vapor permeation, with 5 levels: E1 to E5