U-factor matters - even in hot climates

Twinsburg, Ohio

28 Mar 2019

by Helen Sanders, Ph.D

It's commonly thought that window u-factor doesn't influence building performance significantly in hot climates and that solar heat gain dominates fenestration energy performance. The impact of u-factors generally is discounted in hot climates because energy efficiency losses due to thermal conduction decrease as the difference between outside and inside temperatures decrease. For example, if the temperature is the same inside a building as it is outside, say 70ºF (21ºC), then there is no temperature difference, and the therefore no driving force for conductive heat flow either into, or out of, the building. In general, because the maximum temperature differences in hot climates are much lower, e.g. 110ºF (43ºC) outside vs. 70ºF (21ºC) inside, than the maximum difference in heating-dominated climates,-40ºF/C outside vs. 70ºF (21ºC) inside, the assumption misses a key energy transfer mechanism associated with the opaque elements of the window: solar absorption.

Dark vs. light

The opaque framing members on fenestration systems absorb solar heat from the sun, and as a result, can reach temperatures significantly higher than the outside air. The darker the frame, the more solar absorption, thus the hotter the frame becomes. Think of the difference you feel wearing a black shirt compared to a white one on a summer day. Solar absorption creates a much larger temperature difference between outside and inside than would be expected based on air temperature alone.
If the frame is not thermally broken, heat will flow unhindered from the outside (hot) frame elements to the room-side (cool) frame elements. Not only will that result in higher cooling loads, it also will be thermally uncomfortable to sit next to the window because of the hot interior surfaces. The measured room-side frame temperatures for windows with a range of thermal performance (non-thermally broken to high-performance thermally-broken) as installed in Singapore are shown in the graph below. The frames were light in color (so by no means a worst case for solar absorption), the outside temperature was 99ºF (37ºC) and the solar irradiance was 690W/m2. The room-side surface temperature of the non-thermally broken frame is an extremely high 118ºF (48ºC). A frame with a high-performance thermal break reduces the room-side surface temperature by more than 23ºF (-5ºC), which can make a huge difference in both thermal comfort and cooling loads.

Thermal breaks

This means that the lower the u-factor of the frame, the lower the transfer of absorbed solar energy from outside to inside, and thus the lower the solar heat gain of the frame therefore is proportional to its u-factor. Low solar hear gain windows can be achieved by having a low frame u-factor in combination with a low solar heat gain center of glass. Thermally breaking an aluminum frame can reduce the frame’s solar heat gain coefficient by a factor of three.
A similar rationale can be made for thermally breaking the edge of glass using warm-edge insulating glass spacer systems. Thermally breaking the frame, but not the edge of glass, will result in heat flowing throught he glass edge instead of the frame because heat finds the path of least resistance. This approach also will reduce the total soar heat gain performenca eof the window system.
Solar heat gain through the opaque elements fo the window can be significant and is actually dependent on its u-factor. The take-away from this is that thermally broken frames and warm edge of glass solutions are needed in the desert and tropics as much as they are needed in the wintry cold regions.