As power equipment insulation materials, epoxy resin and phenolic resin are inevitably exposed to damp-heat environments during long-term operation, leading to ageing degradation. This study systematically investigates the effects of damp-heat ageing (85 °C, 90% RH, 0–56 days) on the flame retardancy of both resins through oxygen index tests, vertical combustion experiments, and microstructural analysis (SEM/EDS). Results indicate that ageing unexpectedly enhances flame retardancy: Epoxy resin: Oxygen index rapidly increased from 64.4% (0 days) to 76.5% (21 days), then stabilised at 75–76%. Afterflame time decreased from 143 s to 109 s after 56 days. Phenolic resin: Oxygen index rose continuously with ageing; afterflame time dropped sharply from 211 s (0 days) to 0 s (56 days). Mechanistic analysis reveals that ageing promotes surface enrichment of flame-retardant elements (Ca, Mg, Si in epoxy; Al, Mg in phenolic) and structural changes (e.g., porous carbonisation), which facilitate barrier effects against heat/oxygen diffusion. This work challenges conventional views on ageing-induced degradation, providing new insights for evaluating insulation safety in humid environments.