Insulation of low floors: mistakes to avoid, sources of pathologies

Batirama.com 21/10/20210

After walls and attics, low floors are the subject of an AQC study. This essential insulation project is often problematic, but mistakes can be avoided.

Correctly insulating the undersides of low floors after a building renovation requires an almost exhaustive knowledge of the techniques to be applied.

This is the conclusion that can be drawn from the twelve sheets written by the Construction Quality Agency (AQC) and the Envirobat Occitanie agency and presented in detail in its recent feedback. The Construction Quality Agency (AQC) publishes, as part of its REX High-performance Buildings (REX BP®) system, the report "insulation on the underside of low floors under renovation". This work covers site visits over the past 10 years

The intervention and the implementation of insulation in these precise places of the constructions, whether they are cellars, car parks, at worst in crawl spaces, makes it possible to reduce a good part of the thermal losses of the works ; traditionally, we quote the ratio of 7 to 15% depending on the type of building.

Easy operations for operators thanks to ceiling heights

Operations are all the easier, the authors of the report indicate, when the ceiling height is sufficient to carry out the work , these can even be carried out on an occupied site "without any particular inconvenience for the inhabitants", otherwise having to free up the space.

When reading these pages, seasoned practitioners will certainly be surprised by the cases encountered to illustrate the faults put forward. This consultation of the sheets essentially underlines that the rules relating to the continuity of insulation and the control of thermal bridges are, in these cases just as much as elsewhere, essential.

Insulation: paying attention to details and adapting to the presence of equipment

Thus, the sheets recommend paying attention to details as essential as the junctions between insulating panels, the quality of the connections between the vertical exterior insulation of the walls and that of the underside of the slab, the overhangs of the beams or the shear walls.

Above all, this document points out the singular cases linked to the presence of a staircase, the opening of a door just the height of the floor and hampered by the thickness of the insulation... It details and also gives solutions to adapt to the presence of equipment as different as pipes (plumbing, sanitation, transfer of granules, etc.), rails for automated car park doors, hydraulic network valves, etc.

Cellars and damp places: avoid vapor barriers that are too tight

The cure can sometimes turn out to be worse than the disease. Clearly, isolating can produce pathologies on the structures, and the AQC devotes a few sheets to this. Among the problems posed, that of the migration of water vapor through the walls.

The installation of insulation with a vapor barrier in damp cellars is typical of this scenario. Turned towards the cold zone, this membrane will block vapor transfer, concentrate humidity and sometimes show condensation on the surface.

The damage incurred is easily imaginable: mould, rotting, degradation of the building... To remedy this without undermining the insulation work too much, two responses are provided: install bare insulation or with a very open type membrane at vapor diffusion; and install mechanical insulation of the volume, at the very least, recreate natural ventilation.

Another case to consider with precision: the crossing of these volumes in the basement by a flue. The installation of the insulation must obviously take into account the fire protection and the ventilation of the fireproof sheath. Sprains in the continuity of insulation here totally justifiable.

Source: batirama.com/ B. Reinteau