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I am considering insulating walls with Professional Insulation Panels from ErfurtMav (https://www.erfurtmav.com/erfurt-mav-wallpaper-products/insulating-wall-coverings/professional-high-density-insulation-panels). They claim U value of 5, which is not great but may add a bit insulation to a cavity wall with no/deteriorated insulation (I am planning to replace it some time next summer). Walls are cold on touch so I am hoping adding even 8mm of high density polystyrene should help to keep heat inside. They claim it is comparable to: Approx. 140mm of solid brick Approx. 200mm of stone block Approx. 300mm of concrete Anyone has experience with it?