A poorly insulated attic floor is like a radiator heating the sky. This is evident in every thermal diagnosis: the roof is the primary source of heat loss in a house, far ahead of the walls or windows. Insulating your attic is not a matter of optional comfort; it is the project that has the most impact on the heating bill.
Vapor barrier and air tightness: the point that quotes often overlook
There is much talk about the choice of insulation, but rarely about what happens underneath. A poorly installed vapor barrier ruins insulation in just a few winters. Indoor humidity migrates to the attic, condenses upon contact with cold air, and gradually degrades the insulating material.
The vapor barrier is placed on the warm side, meaning under the insulation when working from the inside. The strips must overlap and be taped with appropriate adhesive (not standard construction tape). The singular points (duct passages, access hatch, chimney flue) are critical areas: this is where air leaks concentrate the problems.
In a convertible attic project, air tightness becomes even more complicated. The hangers pass through the membrane, and each perforation must be sealed. When comparing two houses insulated with the same material, the difference in performance often comes from the quality of this implementation, not from the thickness of the chosen insulation.
To delve deeper into this topic, the solutions proposed by Maisons Euro France detail the technical configurations suitable for each type of attic.
Glass wool, cellulose wadding, wood wool: which insulation for which attics
The choice of insulation primarily depends on the configuration. Lost attics or convertible attics are not the same project, nor do they have the same constraints.
Lost attics: blowing dominates
For non-habitable attics, blown insulation remains the most effective method. Insulation is projected in bulk (cellulose wadding, glass wool, or rock wool) onto the floor. The layer is homogeneous, with no thermal bridges at the junctions, and the project lasts a few hours.
Cellulose wadding offers good thermal lag, which limits summer overheating. Blown glass wool is cheaper to purchase. Rock wool adds superior fire resistance, a criterion to consider if the floor is wooden.
Convertible attics: insulation under slopes
When wanting to inhabit the attic, the insulation is placed between and under the rafters of the framework. Two approaches coexist:
- Single-layer installation between rafters, which is faster but limited by the available height of the rafter. Space loss is minimal, but thermal resistance remains modest.
- Double-layer installation (one between rafters, one on the underside with a metal frame) which allows achieving high thermal resistance without modifying the framework.
- External insulation (sarking), which involves placing rigid panels of wood wool or polyurethane on the rafters before the roofing. This eliminates thermal bridges but requires removing the roof.
Wood wool in semi-rigid panels is increasingly being used on convertible attic projects. Its thermal lag, significantly superior to that of glass wool, protects against heat spikes under south-facing roofs.
Reform of MaPrimeRénov’ and CEE bonuses: what changes for attic insulation
Most online guides still present attic insulation as an easily subsidized gesture by MaPrimeRénov’. The situation is evolving. A draft decree presented in July 2026 plans that attic insulation will no longer be funded as a standalone gesture by MaPrimeRénov’ starting September 1, 2026. To benefit from this aid, insulation work must be included in a comprehensive renovation combining several areas (insulation, heating, ventilation).
In practice, insulating only your attic will no longer be enough to trigger MaPrimeRénov’. This refocusing pushes households towards more ambitious work bundles, which changes the game regarding budget and timeline.
In return, bonuses related to energy savings certificates (CEE) will take over for isolated gestures. Analyses published in the summer of 2026 indicate that the volume of certificates issued for roof insulation benefits from a bonus intended to compensate for the loss of MaPrimeRénov’. Feedback on this point varies among energy suppliers, but the trend is towards maintaining financial support through this channel.
Insulation thickness and thermal resistance: aiming right without oversizing
We are often asked what thickness to install. The answer lies in the thermal resistance R, expressed in m².K/W. The higher R is, the more the insulation slows down the flow of heat.
For lost attics, thermal regulations require a minimum R, but the recommended levels to access aids are higher. A common pitfall: oversizing the thickness without checking that the vapor barrier and ventilation are adequate. An overly thick layer in a poorly ventilated attic accumulates moisture and loses performance over the years.
Another point not to be overlooked: the settling of blown insulation over time. Cellulose wadding and blown glass wool settle after installation. Serious professionals increase the initial thickness to compensate for this settling. If the quote does not mention this additional thickness, it is a warning sign.
In convertible attics, the available thickness between rafters constrains the choice of material. An insulation with low thermal conductivity (polyurethane, for example) allows achieving a good R with less thickness, but its environmental impact and fire behavior raise questions. Wood wool requires more space, but it better regulates humidity and summer heat.
The final choice always depends on the existing building, the use of the attic, and the available budget. A prior diagnosis by an RGE professional remains the best starting point to correctly size the insulation and create a coherent aid file with the new regulatory framework.