Energy glass is ordinary, heat-treated glass with a selective low-emission coating on one side. The coating is extremely thin and transparent, causing the glass to reflect heat radiation back into the room. At the same time, it lets in solar energy and visible light.

When the room heat is reflected back, thermal insulation improves considerably and the inner glass gets warmer. The high surface temperature reduces or even eliminates cold down-draughts and radiation from the window surface, which helps improve the indoor climate. In terms of light inflow, the properties of energy glass are almost identical to those of ordinary glass.

In connection with energy labelling, the Department of Civil Engineering at the Technical University of Denmark (DTU) performed heat transmission and energy calculations for pine windows fitted with Optoglas. According to DTU Civil Engineering, the total U-value of frame, casing and pane is 1.7 W/m2K for a pinewood window with a 4 mm single pane in the outside frame and an Optoglas 4 mm heat-treated energy glass pane fitted on the inside. The total U-value of frame, casing and pane is 1.39 W/m2K for an equivalent window with double-glazing on the outside.