Two windows are being compared. Window A has a U-factor of 0.30 and window B has a U-factor of 0.45. Which window loses less heat?
- Window B, because a higher U-factor means more insulation
- Neither; U-factor rates only solar heat gain in summer
- Neither; U-factor rates only air leakage around the sash
- Window A, because a lower U-factor means less heat flow
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D. Window A, because a lower U-factor means less heat flow
U-factor is the rate of heat transfer through an assembly, so lower is better. It is the inverse of R-value. Treating U-factor like R-value, where higher is better, is the common mistake.
Reference: IRC 2021, Chapter 11 (Energy Efficiency): fenestration U-factor
A wall assembly has a total thermal resistance of R-20. What is its U-factor?
Answer in Btu/h-sq ft-deg F.
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0.05 Btu/h-sq ft-deg F
U-factor is the reciprocal of total R-value: U = 1 / R = 1 / 20 = 0.05.
Reference: IRC 2021, Chapter 11 (Energy Efficiency): U-factor and R-value relationship
What does the solar heat gain coefficient (SHGC) of a window describe?
- The amount of visible light that passes through the window
- The share of the sun's heat that passes through the window
- The rate of heat loss through the window on a winter night
- The amount of air that leaks through the window when closed
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B. The share of the sun's heat that passes through the window
SHGC is the fraction of incident solar radiation admitted through a window as heat. A lower SHGC means less solar heat gain, which is why maximum SHGC values are set for warm climate zones. Winter heat loss is rated by U-factor.
Reference: IRC 2021, Chapter 11 (Energy Efficiency): fenestration SHGC