When the thermal insulation is placed above the waterproofing of the flat roof (outside). Straight layer order; when waterproofing is above (outside) the thermal insulation. Flat roof with DUO or double thermal insulation, when there is thermal insulation both above and below the waterproofing. Any type of material can be used as thermal insulation below the waterproofing layer, but only a closed cell structured material can be used above it. (e.g. XPS)
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