Three Numbers Drive Every Window Specification
Specifying windows is not about aesthetics and frame color - it is about three performance metrics that determine how much heat, light, and solar radiation the fenestration allows through the building envelope. U-factor, SHGC, and VT are the numbers ASHRAE 90.1 enforces, the numbers energy modelers use, and the numbers ARE PPD questions ask about. Understanding what each measures and how they interact is essential for both the exam and for practicing energy-conscious architecture.
U-Factor: Resistance to Conductive Heat Transfer
U-factor measures the rate of heat transfer through the window assembly per degree of temperature difference between inside and outside. It is the inverse of R-value: a lower U-factor means better insulating performance. A single-pane clear glass window has a U-factor around 1.0. A double-pane low-e unit might be 0.30. A triple-pane passive house unit can reach 0.15 or lower. ASHRAE 90.1 sets maximum U-factor requirements by climate zone - colder climates have more stringent (lower) U-factor limits. U-factor applies to the whole window assembly including the frame, not just the glazing.
Solar Heat Gain Coefficient: Solar Energy Through Glass
SHGC measures the fraction of solar radiation that passes through the window as heat. SHGC ranges from 0 (no solar heat gain) to 1.0 (all solar radiation becomes heat inside). The appropriate SHGC depends on the building location, orientation, and whether passive solar heating is a goal. In cooling-dominated climates, a low SHGC (0.25 or lower) reduces summer cooling loads. In heating-dominated climates, high SHGC on south-facing glass can reduce heating loads through passive solar strategies. ASHRAE 90.1 sets maximum SHGC by climate zone and orientation.
Visible Transmittance: Daylight Without Heat
VT measures the fraction of visible light transmitted through the glass. High VT (0.50 to 0.70) brings in more daylight; low VT results in dark-tinted glass that reduces glare but also reduces useful daylighting. The light-to-solar-gain ratio (VT divided by SHGC) is a quality metric for glazing: a high ratio means the glass admits lots of light relative to the heat it lets in. Spectrally selective coatings are used to achieve high VT with low SHGC by transmitting visible wavelengths while reflecting infrared wavelengths.
Low-E Coatings and Their Effect on Performance
Low-emissivity (low-e) coatings are microscopically thin metallic layers applied to one or more glass surfaces in an insulating glass unit. Hard-coat (pyrolytic) low-e is more durable and admits more solar heat (higher SHGC). Soft-coat (sputter) low-e offers better thermal performance (lower U-factor) and can be tuned for different SHGC values. The position of the low-e coating within the IGU (which surface it is applied to) affects both thermal performance and appearance.
Key ARE PPD Exam Points
- U-factor measures conductive heat loss - lower is better; ASHRAE 90.1 sets maximums by climate zone.
- SHGC measures solar heat gain through glass - appropriate value depends on climate and orientation.
- VT measures visible light transmission - light-to-solar-gain ratio indicates spectral selectivity.
- Low-e coatings reduce U-factor and can be tuned to control SHGC separately from VT.
- Passive solar design uses high-SHGC south glass; cooling-dominated climates use low-SHGC glazing everywhere.
Study PPD on AREprep
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