Here is some helpful information for you on various properties and the physical nature of glass and the elements that can effect them.
Optical Properties
  • Solar Transmission (Tsol) – Percentage of solar energy directly transmitted through glass
  • Ultraviolet Transmission (Tuv) – Percentage of UV energy (300-380 nm) directly transmitted through glass
  • Visible Transmission (Tvis) – Percentage of visible energy (380-760 nm) directly transmitted through glass
  • Infrared Transmission (Tir) – Percentage of IR energy (760-2500 nm) directly transmitted through glass
Forms of Heat
  • Conduction – Transfer of energy through a solid (spacer, frame, wall)
  • Convection – Transfer of energy through air or a gas (in IG airspace, room air)
  • Radiation – Transfer of energy through space/no physical medium (sun, any object)


U-value is a measure of heat transfer due to outdoor/indoor temperature difference. Used almost exclusively to describe the heat loss of material.

Q = U A (T high – T low)

Heat = (U value) (area) (Temp. Difference)


R = 1/U The higher the R-value, the less heat transfer

Shading Coefficient

Shading coefficient (SC) is a ratio of the Solar Factor of the glass in question to the Solar Factor of 1/8" clear glass (0.87).

Therefore, SC = Solar Factor/0.87

The lower the number, the less heat into the building.

Relative Heat Gain

A number that represents the amount of heat entering a building due to both solar radiation and conductive/convective heat gain. Calculated for a specific environmental condition.

RHG = Solar Transmission + Cond/Conv. Heat Gain

85 –90%,10 – 15%

RHG, SC, and SHGC are all ways of answering the same question – How much heat gets into the building?


            Lump of Coal ..................1.00

            Uncoated Glass ...............0.84

            Reflective Glass ...............0.40

            Pyrolitic Glass ..................0.15

            Soft Coat Low-e .............0.09

            Heat Mirror – 66 .............0.04

            Tin Foil ............................0.00

Emissivity: a measure of how readily a surface absorbs and emits infrared energy (the lower the emissivity, the less infrared the glazing absorbs. Emissivity is the heat-emitting (or radiating) propensity of a surface. The emissivity number compares a given surface with that of a perfect radiator, called a blackbody (emissivity of 1). Ordinary window glass has an emissivity of 0.84, meaning it absorbs and emits 84% of the radiant heat.

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