Window Performance for Human Thermal Comfort

A method based on the ASHRAE two-node comfort model has been developed for predicting the effect of windows on thermal comfort. The method embodies separate analyses for long-wave (thermal infrared) radiation, induced drafts, and solar load effects. Of these three impacts, modeling results demonstrate that long-wave exchange between the body and the window is the most significant except for the case where the body is in direct sun, in which case the impact solar load can be more significant. For most residential-size windows, draft effects exist but are typically small.

Generally, windows are not the primary element affecting the comfort of a buildings occupants. However when a window is very hot or cold, the occupant is very close to the window, or other factors result in thermal conditions near the edge of the comfort zone, windows can become quite influential. Furthermore it is believed that current methods may under-predict discomfort caused by windows.

We discuss potential refinements to the method that might address this inaccuracy by accounting for asymmetries in radiant temperature. In the near term, the model could be used to create a simplified window comfort index. To accompany the index, we envision educational material that would educate designers and consumers on the comfort implications of glazing selection.