In the context of energy performance improvement, a question often arises about the maintenance of historical rendered facades. Several thermal mortars have been developed with different lightweight fillers like EPS granules, hemp, or expanded mineral materials such as volcanic sand and clay. According to the standard EN 998-1, renovation and thermal mortars used in external elements must meet property values like the coefficient of water absorption due to capillary action (CWA). In the current study the influence of silane on water absorption properties was studied. Different levels of silane content were studied: 1, 10, 20, and 30 times the manufacturer recommendation and compared with a mixture without silane (0). During hardening, the specimens were kept in two different environments: in regular and forced carbonation conditions. Forced carbonation either increased (0, 20, 30 times of silane content) or had no influence on water absorption (1 and 10 times). In general, adding silane decreased water absorption by up to 19 times (from 2.84 to 0.15 kg/(m2min0.5)) for “carbonated” specimens). Silane contents of 20 and 30 times decreased (0.12–0.26 kg/(m2min0.5)) to meet the properties of thermal mortars according to EN 998-1 (Wc1 ≤ 0.4 kg/(m2min0.5)).