Energy retrofitting of existing building stocks is essential to reduce building-related energy consumption. Cavity wall insulations are commonly applied, however, their hygrothermal performance is not well established. This study focuses on the hygrothermal performance of rehabilitated cavity walls in the Netherlands. A state-of-the-art aerogel composite developed for cavity wall retrofitting using the blown-in method is presented. The aerogel composite has a dry thermal conductivity of 22.5 mW⋅m −1 ⋅K −1 and low sorption isotherms. A retrofitted masonry wall with a 6 cm cavity using the aerogel composite can achieve thermal transmittance (U-value) of 0.32 W⋅m −2 ⋅K −1 , well below the 0.71 W⋅m −2 ⋅K −1 required in the Netherlands. It can reduce the annual heating and cooling demand by up to 72% in a simulated building, and also provides better thermal comfort to the occupants, lowering the percentage of thermally dissatisfied occupants from 51% to 18%. The tested aerogel composite outperforms conventional insulation materials in the market, without sacrificing the thermal comfort of its occupants.