U-factor & SHGC explained

U-factor: measuring heat loss

U-factor (or U-value) is simply how fast heat leaks through a material or assembly. In the US it is measured in BTU/(hr·ft²·°F), and in metric W/(m²·K). Lower is better: a low U-factor means less heat escaping, so a better insulator.

If R-value from wall insulation is more your language, U-factor is just its flip side: U-factor = 1 ÷ R-value. An R-20 wall works out to U-0.05. A decent double-pane window sits around U-0.30, roughly 6× leakier than that wall. That gap is exactly why windows are always the weak spot in the envelope.

Heat moves through a window assembly in three ways simultaneously:

• Conduction: Heat conducted directly through the glass and frame materials.
• Convection: Heat carried by air movement within the sealed gas space and on the glass surfaces.
• Radiation: Infrared energy radiated from the warmer surface to the cooler one across the gas space. This is what Low-E coatings primarily address.

U-factor is a composite measure of all three. Reducing any one of them, by adding a gas fill, using a Low-E coating, adding a second or third pane, lowers the U-factor and improves the window's insulating ability.

Center-of-glass vs whole-unit U-factor

A window is not uniformly insulating across its entire area. The glass at the center of a panel performs differently from the glass near the edge, the spacer, or the frame. For this reason, U-factor is reported at multiple scales:

• Center-of-glass (COG): The U-factor measured at the geometric center of the glass, far from edges and frame. This is the best-performing zone and is the value most commonly published in glass manufacturer datasheets. COG values are measured or calculated per NFRC 100.
• Edge-of-glass: The 2.5-inch perimeter band around the glass, where the spacer conducts heat more readily. A traditional aluminum spacer creates a significant thermal bridge here; warm-edge spacers reduce this.
• Frame: The U-factor of the window frame itself, aluminum frames conduct heat readily; fiberglass and wood frames are better insulators.
• Whole-unit (WU): An area-weighted average of COG, edge, and frame U-factors. This is the value shown on the NFRC label and the number that applies for energy code compliance. WU U-factor is always higher (worse) than COG U-factor.

U-factor scale for common assemblies

Solar Heat Gain Coefficient (SHGC)

Where U-factor measures heat loss, SHGC measures how much solar energy passes through the glass into the building. SHGC is expressed as a decimal between 0 and 1. An SHGC of 0.40 means 40% of incident solar energy, both direct transmission and absorbed-then-re-radiated heat, passes through.

SHGC has two components:

• Direct transmittance (T_sol): The fraction of solar energy that passes straight through the glass without being absorbed.
• Inward-flowing absorbed fraction: Solar energy absorbed by the glass that is then re-radiated or convected inward. This adds to the total SHGC and is the reason SHGC is always higher than solar transmittance alone.

High SHGC vs low SHGC, which is better?

Neither, it depends entirely on climate and building orientation. SHGC is a tradeoff, not a quality indicator:

• Cold climates, south-facing windows: High SHGC (0.40–0.60) is beneficial. You want passive solar gain to reduce heating loads in winter. This is the principle behind passive solar design and Passive House south glazing.
• Hot or mixed climates, east/west exposure: Low SHGC (0.20–0.35) is typically required. Blocking summer solar gain reduces cooling loads and peak demand on HVAC systems. ASHRAE 90.1 and IECC set prescriptive maximum SHGC values by climate zone.
• Large commercial buildings: Internal gains from people and equipment dominate the heating load, so cooling is the dominant concern year-round. Low SHGC is almost always preferred regardless of climate.

Energy code requirements by climate zone

The International Energy Conservation Code (IECC) and ASHRAE 90.1 set maximum U-factor and minimum/maximum SHGC values by climate zone. The US is divided into 8 climate zones ranging from 1 (very hot) to 7 (very cold) plus marine zone 4C.

Visible transmittance (VT) and the LSG ratio

A third metric closely related to SHGC is visible transmittance (VT), the fraction of visible light (380–780nm wavelength) that passes through the glass. Higher VT means brighter interiors and better daylighting.

The relationship between VT and SHGC is captured in the Light-to-Solar Gain (LSG) ratio:

LSG = VT ÷ SHGC

An LSG above 1.0 means the glass admits more visible light than solar heat, it "outperforms" clear glass, which has an LSG of about 1.0. Modern solar control Low-E coatings routinely achieve LSG of 1.5–2.0, meaning they selectively transmit daylight while blocking near-infrared heat. This is the core value proposition of spectrally selective coatings.