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I wish to realize a double layer insulation in rockwool. But I have a problem with the first layer which is 60mm thick and my rafters which are also 60X60. So no more air space, knowing that there is no roof under roof, it is a house from the 60s. Can I fix half-rafters 38X60 on my rafters to leave a blade of air? Is there another possible solution? or do I abandon the double layer for a single layer?

Your solution of fixing half-rafters is clever and quite applicable. That being the case, there is glass wool or rock in 45 mm thick... You just need to increase the thickness of the second layer? You are not obliged to make a pose between rafters. For information on laying in double layers, refer to our DIY SHEET.

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