It is not useful to remove wool under the pretext that it is old; its life is equivalent to that of the building, conventionally evaluated at 50 years. However, it must be ensured that it is positively contiguous and continuous.
A Complement Can Then Be Realized:
If it is accessible shale: by adding an additional mattress whose thermal resistance will complement the existing thermal resistance up to the target performance. Attention when implementing the second bed not to walk on the existing mattress not to crush it; lift it to create a passage to put the second bed and fold down the raised mattress to progress.
If it is an inaccessible waste: by mechanically blowing insulation in the form of flakes. The thickness to be expected will depend on the performance targeted there too.
One imperative: the wool laid over the old insulation must be bare (only one vapor barrier positioned on the heated side of the roof).
Be aware that mineral wools (glass wool and Rockwool) do not lose their insulating power over time. Also, by preserving the old insulation of the attic, you take advantage of the insulating power already in place and limit the thickness of complementary insulation to implement. It is therefore an economical solution.
Should a vapor barrier be added if the old insulation did not have one? Can we improve the air tightness?
A continuous vapor barrier must be provided for all applications that require it. For example, a vapor barrier will be installed in peripheral walls of damp rooms, in a cold area (mountain climate), or when the technical advice of the chosen solution so provides.
Moreover, the vapor barrier participates, in renovation as in new, in the air tightness of the envelope of your building. This seal must be associated with a suitable ventilation system. In fact, a building whose air tightness is not ensured increases its energy consumption: the infiltration of air into the walls adversely affects thermal comfort, acoustic comfort, and the proper functioning of the ventilation systems. Ultimately to health.
To remember: a housing whose air tightness is not assured sees its heating needs increase from 7 to 10%! A well-insulated building relies jointly on the air tightness performance of its walls, the thermal performance of its insulation systems and an efficient and adapted ventilation system.
In the case where there is no vapor barrier on the walls, the air tightness performance can be obtained by the implementation of an independent and continuous membrane, validated for its fitness for use for its intended application, between the insulation and the interior facing.
The air tightness of the walls must be guaranteed in renovation as new. The maximum level of air permeability to be respected is defined by the Thermal Regulation that applies in new or renovation, i.e. a value of less than 0.6 m3 / h.m² in single family house and 1.0 m3 / h.m² in collective housing (under a pressure of 4 Pa). Obtaining the Effinergie-Rénovation label for any BBC (low energy building) renovation requires compliance with the level of air permeability taken into account in the preliminary thermal study. Permeability that is controlled before receiving the building insulation.