Thermal Performance and Energy Efficiency
Thermal questions test whether you can read a rating correctly, explain what drives it, and apply it to a code or above-code target. The exam expects fluency with U-factor, SHGC, visible transmittance and condensation resistance, and with the difference between centre-of-glass and whole-product values.
The rating vocabulary
- U-factor (Btu/h-ft2-F) is whole-assembly heat flow, including frame and edge; R-value = 1/U. Lower U is better.
- SHGC (0 to 1) is the fraction of incident solar heat admitted; VT is visible light transmitted; LSG = VT/SHGC.
- Air leakage affects real energy use but is reported separately from U-factor.
- Condensation resistance appears as CRF (AAMA 1503) or CR (NFRC 500); higher is better and depends on frame, spacer and interior humidity.
What drives performance
Frame conductivity (aluminium needs a thermal break), warm-edge spacers that reduce edge heat loss, low-E coatings, argon or krypton fill, triple glazing, and cavity width. NFRC 100 ratings come from simulation, NFRC 102 from hot-box testing; both are whole-product values. Code tables in IECC and ASHRAE 90.1 set maximum U-factor and, in many climate zones, maximum SHGC, sometimes by orientation. Compliance paths include prescriptive component tables, total UA trade-offs, and whole-building performance.
Worked example
Two options for 250 ft2 of windows in a 4,500 heating degree-day climate: U 0.38 versus U 0.29. Annual heat loss difference = (0.38 - 0.29) x 250 x 4,500 x 24 = about 2.43 million Btu, roughly 24 therms of site energy before equipment efficiency. That justifies the upgrade on heating alone. But if the building is cooling-dominated, the lower-SHGC option matters more, and a lower U-factor with high SHGC could raise cooling load. Always evaluate U and SHGC together for the climate zone.
Traps that cost marks
- Comparing a centre-of-glass U-factor to a code maximum written for whole products.
- Ignoring orientation-specific SHGC limits in cooling climates.
- Assuming a low U-factor guarantees condensation resistance; spacers, frames and humidity decide it.
- Forgetting that gas fill can be lost and that ratings assume the unit stays sealed.