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I want to isolate the attic. I am on a hoard. The roof is not insulated, but it would be more convenient for me to unroll rockwool or other (does it make sense) by putting the vapor barrier side slab. In another part, the roof is isolated. Can I put in more rockwool too, and if so, how thick? What brand? Should ventilation be provided?

No there is no "sense" to spread your mineral wool on the floor, except that you must put the vapor barrier on the ground. Cross a second layer, the second without a vapor barrier. Two layers of 100 or better 150 mm thick are ideal. The good brands are numerous: ISOVER, ROCKWOOL, KNAUF, etc... Unlike the underlays, where an air space must be formed between the underside of the cover and the insulation, it is not necessary to create a ventilation between the soil and the insulation.

The ISOLATION section of the site is very rich. For the subject you are interested in, several cards concern you:

SHEET on the attic floor insulation (attic)

SHEET on attic floor insulation (convertible attic space)

SHEET on the insulation of floors and floors

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