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I installed thin insulation in my attic and I regret it. I would like to put some glass wool. Should I remove this insulation. If I leave it how much should I leave between the two insulators. Is there a rule for the vapor barrier?
OTHER QUESTION I would like to make a wall of 1.20 m on a length of 3 m on an existing sidewalk. Do I still have to do a foundation?

In the absence of details on how to install what you call a thin "insulator", it seems prudent to deposit it. The tandem "thin insulation + conventional insulation" can work, but only in the context of a specific and precise implementation, considered as a global insulation the thin reflective product constituting additional insulation. In all cases, the vapor barrier must be implemented inside the house.
For the second question, a wall of a height of 1.20 m must lean on a foundation possibly slightly scrapped.

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