If you're learning about daylighting systems for the first time, especially skylights, you may come across some new terminology. We've outlined these terms below.
Visible Light Transmittance (VLT) – The amount of light in the visible portion of the spectrum that passes through a glazing material.
- The higher the VLT = More light reaches the floor area
Solar Heat Gain Coefficient (SHGC) – Measures how well a skylight blocks heat from sunlight.
- SHGC is expressed as a number between 0 and 1
- The lower a skylight's SHGC, the less solar heat it transmits
U-Value – Measures the amount of heat transfer through a skylight and tells you how well the skylight insulates.
- The lower the u-value, the better the skylight insulates
Transmission Haze or Diffusion – Measures of the light scattering characteristics of a material.
- ASHRAE 90.1-2010 and IECC 2012 require ≥90% haze under ASTM D 1003
Effective Skylight to Floor Ratio (ESFR) or Skylight to Floor Ratio (SFR) - The percentage of the total square footage of floor area covered by the total square footage of skylights in the roof.
- With high VTL skylights (properly diffused) lower total ESFR is needed to properly daylight
- Implementing skylights in your design should not exceed 5% of the roof area (Code requires between 3-6%)