U-factor matters - even in hot climates
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.
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