Tilt-angle (°)

Angle of the glass relative to the horizon

• Typical skylights with a slight pitch: 3-5°
• Typical windows in vertical walls: 90°

Ug (W/m².K)

Heat-transfer coefficient (center-of-glass)

• Heat transfer (W) per square meter (m²) of glass for each degree (C) of temperature difference
• Lower implies better insulation

.w

NFRC 100-2010 Winter Condition

• The standard environmental condition for Ug quoted in the Window industry
• Useful as a reference for performance comparison between different glazing systems but can lead to inaccuracies if used to estimate energy consumption
• Inside Condition:
  • Air and effective room temperature: 21 deg C
  • Room emissivity: 1
• Outside Condition:
  • Air and effective sky temperature: -18 deg C
  • Effective sky emissivity: 1
  • Convection coefficient: 26 W/m².K
  • Wind speed: 5.5m/s windward

Ug (W/m².K)

Heat-transfer coefficient (center-of-glass)

• Heat transfer (W) per square meter (m²) of glass for each degree (C) of temperature difference
• Lower implies better insulation

.s

NFRC 100-2010 Summer Condition

• The condition used to calculate SHGC
• Ug.s better represents performance in warm environmental conditions
• Inside Condition:
  • Air and effective room temperature: 24 deg C
  • Room emissivity: 1
• Outside Condition:
  • Air and effective sky temperature: 32 deg C
  • Effective sky emissivity: 1
  • Convection coefficient: 15 W/m².K
  • Wind speed: 2.75m/s windward
  • Direct solar radiation: 783W/m²

SHGC (Range: 0 – 1)

Solar heat gain coefficient

• Proportion of solar heat radiation transmitted
• Generally lower SHGC is desirable for north and west facing windows for reducing air-conditioning cost in summer
• However, higher SHGC for north and west facing windows would help reduce heating cost in winter

VLT (Range: 0 – 1)

Visible light transmittance

• Proportion of visible light (380 - 720nm) transmitted
• Generally higher VLT is more desirable

VLRf (Range: 0 – 1)

Front Visible light reflectance

• Proportion of visible light reflected on the external side of the IGU

VLRb (Range: 0 – 1)

Back Visible light reflectance

• Proportion of visible light reflected on the room side of the IGU

Tdw-ISO (Range: 0 – 1)

Damage weighted transmittance

• Proportion of colour fading radiation transmitted
• Lower Tdw-ISO means lower risk of colour fading
• Accounts for the colour fading potential of both UV and visible spectrum

Tuv (Range: 0 – 1)

UV transmittance

• Proportion of UV radiation (280 - 380nm) transmitted (Range: 0 – 1)
• Not recommended as a metric for assessing colour fading potential
• Visible spectrum could account for as much as 40% of the fading from solar radiation

Selectivity (VLT/SHGC)

Ratio of visible light transmittance over solar heat gain coefficient

• Useful for comparing spectral selectiveness of the low-E coating
• Higher selectivity means less visible light is sacrificed to lower solar heat gain

Example:

• Sel. in the 0.5 - 0.8 range results from the use of Grey glass; less visible light is transmitted vs infrared radiation
• Sel. in the 1.9 - 2.2 range results from the use of LightTech LI Max; less infrared radiation is transmitted vs visible light

Nominal thicknesses (mm)

List of nominal thicknesses of the IGU components separated by the '+' symbol

Example:

17.52:

• 17.52mm single-pane laminated glass

6+12+6:

• DGU made up of:
  • 6mm glass
  • 12mm spacer
  • 6mm glass

Total thickness (mm)

Total thickness of the glazing system; sum of component thicknesses