Emissivity ε

Emissivity is the ratio of the amount of energy radiated by a body to the amount of energy emitted by a black body under the same temperature conditions.

Thermal radiation is essential for the heat loss of glass, according to the emissivity of the glass surface. The emission is around Ɛ = 0.85 for uncoated glass. Simply stated: 85% of the heat absorbed by the glass is released on the glass surface.