Low-e glass reduces the heat loss of the building by:

1. Reflecting the energy emitted by room heaters and surfaces back into the room (long wave radiation)
2. Allowing high transmission of the solar radiation (short wave radiation) through the glass to benefit from passive solar heat gain through the glass.

During the winter, low-emissivity glass can reduce heat loss while allowing high levels of free solar gain (heat from the sun) to heat buildings with no significant loss in natural light.

Thermal Insulation

Heat loss is normally measured by the thermal transmittance or U-value, usually expressed in W/m²K. In its most basic terms, the lower the U-value, the greater the thermal insulation, however, the amount of free heat energy from the sun, the solar gain or g-value can also have a big impact.