CSLB General Building (B)Finishes and Building EnvelopeMedium
A contractor is selecting replacement windows for a new home in a California climate zone with high summer cooling loads. To minimize solar heat gain through the glazing while meeting Title 24 prescriptive requirements, which window rating should the contractor prioritize being as LOW as possible?
- AVisible Transmittance (VT)
- BCondensation Resistance (CR)
- CSolar Heat Gain Coefficient (SHGC)
- DAir Infiltration Rating (AI)
Show answer & explanationAnswer & explanation
Correct answer: C. Solar Heat Gain Coefficient (SHGC)
SHGC measures the fraction of solar radiation admitted through a window that becomes heat inside a building; a lower SHGC blocks more solar heat gain, which is critical in hot climate zones to reduce cooling loads. Title 24 sets maximum SHGC values by climate zone for prescriptive compliance.
Why the other options are wrong
- A. Visible Transmittance affects daylighting, not directly the solar heat load.
- B. Condensation Resistance relates to moisture on glazing surfaces, not solar heat control.
- D. Air infiltration relates to air leakage, not solar heat gain.
Solar Heat Gain Coefficient (SHGC)
A rating from 0 to 1 indicating the fraction of solar radiation admitted through a window as heat; lower values block more solar heat.
- Title 24 sets maximum SHGC by climate zone for prescriptive compliance
- Lower SHGC is desirable in hot/cooling-dominated climates
- Different from U-factor, which measures conductive/convective heat loss
Memory trick: Low SHGC keeps the sun's heat outside the door.