The current practice of landfilling fly ash generated by waste incineration is nonsustainable, so alternative ways of using this material are needed. Silanization effectively immobilizes the heavy metal contaminants in the incineration fly ash and enables its circular utilization because silanized fly ash (SFA) has market value as a low-cost filler for polymer composites. This study examines the ecoefficiency of a thermal insulation panel that consists of a polyurethane (PU) foam core sandwiched between two epoxy composite skins prepared by reinforcing glass fibers (GF) and SFA in epoxy resin. The ecoefficiency of such panels was evaluated by comparing their life cycle environmental externality costs (LCEE) to their life cycle costs (LCC). The LCEE was calculated by monetizing the panels’ environmental impacts, which were quantified by performing a life cycle assessment (LCA). The results revealed that the ecoefficiency of the composite panels is positive (47%) and superior to that of market incumbent alternatives with PU foam or rockwool cores and steel skins. The two market incumbents have negative ecoefficiencies, primarily due to their high LCEE. The environmental performance of the panel with SFA–GF epoxy composite skins can be further improved by using lignin-based epoxy resin or thermoplastic polypropylene as the polymer matrix of composite skins. Overall, application as a filler in fabricating polymer composite skins of sandwich panels is an upcycling pathway of SFA that combines circular economy prospects with sustainability benefits.