Building insulation materials play an important role in improving the energy efficiency of the construction and building sector. These insulation materials may also have environmental impacts attributed to the use of non-renewable raw materials and fossil-based energy consumption. Waste valorization provides an opportunity to use secondary materials for these building insulation materials. Thus, the selection of sustainable building insulation materials should not only consider the thermal properties such as heat and fire resistance but also environmental factors, among others. This paper proposes a decision modeling approach to optimally select the appropriate building insulation material even at the early design stage where the level of uncertainty will be high. The decision model integrates the Analytic Hierarchy Process (AHP) and Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) with spherical fuzzy sets to model ambiguous human opinion during the evaluation process. A case study is presented to illustrate the method in the prioritization of the insulation materials that include the use of waste materials and geopolymer. Indication suggests a foamed coal fly ash-based geopolymer is ranked 1st among the insulation materials being considered as it performs better in terms of thermal capacitance, embodied carbon and fire rating.