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I want to remake the insulation of my roof from the inside about 40m2 currently fibrocement ETERNIT amientee, rock wool + paneling the fibro is placed on failures of 10X20, there are 4 vents arranged in the middle of the roof on 2 slopes. I can only do an isolation between breakdowns, problem of height. I have a concern for condensation I thought to plug the vents (which are not effective) nailing 3X3 battens between the purlins, put a 10cm poluyrethane bouveté and fill the gap between the insulation and the fibro as and as a result of the installation with polyurethane foam and make a ceiling in BA13 in finish. Is this solution good or should I leave a layer of air between the fibro and the insulation, put a thin insulation below to remove the thermal bridges leaving the existing ventilation open although these vents are often obstructed by nest of hornbills and hornets. The best ventilation would be from sewer to fetiere but that would require me to remake the roof entirely. (I did not hold the mineral wool because difficult to ask and afraid that if there is still condensation it does not wet the vapor barrier)

The idea is good in principle but must be simplified. In the first place, it is important to keep the breakdowns. Then, maintain an air gap between the insulation and the cover. When there is a lack of ceiling height, the choice of polyurethane foam provides efficient insulation for a reduced thickness. In the present case, it is conceivable to lay polyurethane foam panels between the rafters (without touching the cement fibers), then to screw directly on the rafters thin doublers composed of a plasterboard and a foam insulation polyurethane. Attention nevertheless, this pose is not in conformity with the DTU, which recommends a laying on lines, which presents the disadvantage of being penalizing in thickness.

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