Climate-Dependent Effects of Environmental Longwave Radiation on the Hygrothermal Performance and Mold Risk of Building Envelopes

Environmental longwave radiation (ELR) can alter exterior-surface temperature and thereby modify moisture transport in building envelopes, yet its coupled hygrothermal effects are often simplified and remain insufficiently quantified across different climates. A transient coupled heat and moisture transfer (HAMT) model was developed using temperature and capillary pressure as the primary variables, with wind-driven rain (WDR), solar shortwave radiation, and dynamic sky longwave radiation incorporated into the exterior boundary. Validation against the HAMSTAD benchmark and long-term field measurements demonstrated that the model reproduces both near-saturated moisture behavior and the dominant transient hygrothermal response of multilayer walls. The capillary-pressure formulation was therefore adopted for subsequent ELR analyses in Harbin and Guangzhou, representing severe-cold and hot-summer/warm-winter climates in China, respectively. In Harbin, ELR-induced radiative cooling increased mean exterior-surface relative humidity by 9.1 percentage points and raised its peak from 91.4% to 99.7%; annual sensible and latent heat transfer increased by 7.88% and 3.33%, respectively. In Guangzhou, transient surface responses were much weaker, yet annual sensible heat transfer decreased by 11.99% while latent heat transfer increased by 7.54%, indicating a climate-dependent redistribution of heat and moisture transport. The maximum exterior-surface mold index increased from 2.20 to 3.89 in Harbin and from 3.69 to 4.27 in Guangzhou. These results identify a coupled pathway through which ELR-induced radiative cooling modifies surface humidity, internal moisture redistribution, energy partitioning, and long-term mold risk. Explicit representation of dynamic ELR is therefore important for climate-specific assessments of envelope hygrothermal performance and moisture durability.