Title 24 requires fenestration products to meet specific performance criteria. What does the Solar Heat Gain Coefficient (SHGC) measure?
Correct Answer
A) The fraction of solar radiation transmitted through the window
SHGC measures the fraction of solar radiation admitted through a window, both directly transmitted and absorbed then released inward. Lower SHGC values indicate better solar heat control.
Why This Is the Correct Answer
SHGC (Solar Heat Gain Coefficient) measures the fraction of incident solar radiation that passes through a window into the building — including radiation that is directly transmitted through the glass plus radiation that is absorbed by the glass and subsequently re-emitted inward. It is expressed as a value between 0 and 1; lower SHGC means less solar heat enters, which is desirable in cooling-dominated climates.
Why the Other Options Are Wrong
Option B: The air leakage rate around the window perimeter
Air leakage rate around the window perimeter is measured by the air leakage (AL) rating, not SHGC. Air leakage describes how much air infiltrates around the window assembly, which is a separate performance metric.
Option C: The rate of heat loss through the window frame
The rate of heat loss through the window frame is related to the U-factor of the frame assembly, not SHGC. U-factor (or its inverse, R-value) measures conductive and convective heat transfer, not solar radiation.
Option D: The thermal resistance of the glazing system
Thermal resistance of the glazing system is described by the R-value (or U-factor = 1/R). SHGC specifically measures solar radiation transmission, not thermal resistance to conductive/convective heat flow.
Memory Technique
SHGC = Solar Heat Gain Coefficient = 'Sun coming IN.' If SHGC is low, less sun heat gets in — good for keeping cool. Think: 'S-H-G-C = Stop Heat from Getting in (when Cool is needed).'
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