Glass block windows have been popular for over 20 years and are often installed for either their art deco appeal or the privacy they offer while still allowing in light. However, many have not considered the dramatic energy efficiency of glass block. The windows originated before carbon footprints and going "green" were major construction or design topics. Standard glass block windows have equivalent energy efficiency to thermal pane windows and are airtight, so heat and cold will not draft in or out of the house.
Glass block windows are fabricated to have thick-walled, often textural, exteriors with an approximate 3-inch gap of air in the middle. This large pocket of air is the secret to their energy efficiency. Though the air was initially created to make the block cheaper and less dense for structural matters, it is also an insulating factor. The interior dead space traps air, while the light shines onto the thick glass, creating a pocket of trapped warm air. As temperatures decrease this warm air stays insulated within the block. Since many structures have a large wall or panel of the glass blocks, the entire wall becomes a low-level heating element that disperses warmth even when temperatures decrease.