Pitched roof for converted attic using role or sheet products

Insulation between and/or over the roof structure behind a plasterboard finishing

Thermal insulation of pitched roof

This solution is widely used in both new-build and renovation projects. The principle consists of insulating the roof pitch by inserting thermal insulation between and/or over the roof structure (rafters, trusses, purlins, etc.). For a better thermal performance, it is recommended to apply two cross-layers (between and over the roof structure) and ensure a good air tightness of the roof through the systematic use - wherever technically possible - of continuous vapor barrier membranes on the inside, and high-permeability vapor membranes on the outside (unventilated beneath).

This construction technique is described in the DTU 25.41, DTU 31.2 and DTU série 40 French standards (roof coverings and flexible air barriers), in the ‘Cahier des Prescriptions Techniques’ CPT 3560 v2 for more traditional insulation materials and in the Technical Assessments and Technical Application Documents for non-traditional insulation materials.

ACERMI certification is evidence of conformity of the used insulation product with the requirements set out in the application documents listed above.

The insulation products covered by this factsheet do not contribute to the mechanical stability of the structure. However, the insulation must be able to withstand the mechanical stresses imposed during installation, and especially the traction loads due its own weight. Additionally, the dimension and weight variations of the product in response to temperature and humidity must not be such as to induce significant mechanical stresses on other adjacent roof components (internal vapor barrier, structural framework or internal finishing). Neither must they limit the width of the external ventilated air space. Lastly, given that the installation of an internal vapor barrier in combination with external natural ventilation of the roof is systematically recommended, the required level of resistance of insulation material to water vapor transfer does not usually impose a restriction.

French building regulation DTU 25.41 specifies the minimum ISOLE* level suitable for all applications as follows:

1 1 1 1 1

For plastic insulation foams materials, level 2 is required for usage profiles O and L (O2 and L2).

Once the insulation product has been correctly selected, all that remains is to follow the recommendations set out in DTU 25.41 or Technical Assessments / Technical Application Documents to achieve long-term high insulation performance.

(*) ISOLE usage profiles

  • I : Mechanical properties under the effects of compression,
  • S : Resistance to differential movements,
  • O : Behavior in the presence of water,
  • L : Mechanical properties for cohesion and bending,
  • E : Water vapor permeation,

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