Insulation on the inside is not recommended. The rule of thumb is "always insulate on the cold side." Otherwise, you risk moisture condensation inside the wall. Although it's not a panic situation when you have lightweight concrete blocks, it still feels completely wrong. Instead, place the insulation on the outside. Use mineral wool boards with plaster mesh and a thick plaster system, for example, from Maxit (Serporoc), and you'll get a good product.
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