Question

a building design was complete and is under construction. You had selected a window with a U-value of 1.6 and a solar heat gain coefficient (SHGC) of 0.25. The window manufacturer informs you that they no longer carry that window model, but that they can give you a window with a U-value of 1.6 and SHGC of 0.4.

How would this new window impact your building's modeled heating and cooling energy consumption?

Answer 1

U indicates the overall heat transfer coefficient of window which will determine the heat lost or gained by thw window. Higher the value of U, more is the heat transfer. Higher value of U will lead to faster cooling of room if room is at higher temperature or faster heating if the room is at lower temperature with respect to surroundings. Here in this case, U remains same in both windows, so the heat transfer rate by conduction across the window will remain same.

SHGC indicates the fraction of total incident energy absorbed by the window.So, an increase in the value of SHGC indicates more heat transferred to the room through radiation.

Overall, we can see that in summer , if we want to maintain the room at lower temperature then- * since, more heat is transferred to room by radiation(SHGC 0.4) than we would have with SHGC of 0.25. Therefore we have to lose more heat heat from room and we would need more cooling requirements to maintain room temperature.

While in winter, more heat is already transferred to room with SHGC of 0.4, and losses are constant (U is same). So we will need less heating requirements than usual to maintain room temperature.