Simulations of coatings to avoid external condensation on low U-value windows

Well-insulated windows, windows with a low U-value, become more and more common. On well-insulated windows, in contrast to on traditional, poorly insulated windows, dew easily forms on the outer side of the outer pane. This external water condensation usually happens during clear and humid nights with no or little wind. It sometimes remains till the morning and obstructs the view to the outside for the house occupants.The characteristics of the outer surface of the window affects the annual number of hours with condensation and how good the visibility is through the water layer.The spectra of two commercial glass pane coatings which have proved to have a dampening effect on how much the view through the window is obstructed by external water condensation were simulated; a low-emissivity coating and a self-cleaning coating. Optical constants were used as input. Furthermore, a new glazing was simulated with a layer combination which consists of both a low-emissivity coating and a self-cleaning coating. The simulations gave spectra similar to measured ones on real coated glass samples of the three kinds.When designing the window, it should be optimized so that the visual transmittance is high, the thermal transmittance low and the occurrence of external water condensation rare. In this article, after a discussion on different window functionalities, it is suggested that 50–100 nm of the low-emissivity coating and 10 nm of the self-cleaning coating would give a surface emissivity of 0.3–0.5. If used as the external surface of a well-insulated window this would substantially decrease how often the view through the window is obstructed by external water condensation.Simulations were performed on two well-insulated windows with and without this external surface. The effects of the surface on the light transmittance, U-value and the total solar energy transmittance, the g-value, are discussed here.