Condensation effects on energy performance of building window systems

Low-e products have been accepted widely as an energy-efficient solution or energy retrofitting strategy for glazing systems, including single-pane windows, which may reduce the emission of radiant infrared energy and improve the insulating properties of overall windows. However, the addition of a low-e layer may increase the condensation risk, and in turn, significantly diminish the window’s thermal performance. To better understand the condensation effects of low-e windows on overall building energy use, we extended the classic Nusselt filmwise condensation model to establish and validate a simplified double-film-based analytical model, and then characterized the condensation-dependent thermal dynamics of low-e coated single-pane glazing. A series of whole building energy use comparisons between simulations ignoring and considering the condensation effect were conducted. The results indicate that if the condensation effect is taken into account, the low-e single-pane glazing may lead to adverse effects on the energy-saving purpose in certain climates relative to the original single-pane glazing. More careful and comprehensive energy analyses are needed to use low-e products for window energy retrofits.