Abstract An experimental study was carried out on buildings constructed in the 1980s adopting vertical envelopes with a glass wool insulation layer in the cavity. The aims of this study are to evaluate the insulation conservation state after 25 years, to assess the actual indoor thermal comfort and to compare different retrofit interventions. The research included the following phases: laboratory tests on insulation samples to quantify any changes of morphological, chemical, physical and thermal proprieties of the material; detailed on-site monitoring in order to analyze the thermal comfort conditions; dynamic thermal simulations to assess the impact of different retrofit scenarios to satisfy the limits imposed by the current energy saving standard. The results show that the glass fibers and the binder were affected by a degradation process thus increasing both water absorption and thermal conductivity; thermal comfort and consumptions were not influenced by the insulation aging; the most effective retrofit is the introduction of an external insulation layer.