Look a little closer
See the mechanism
Multiple barriers reduce different heat paths
Read the diagram
Two panes enclose a sealed gas gap that adds resistance to heat transfer. A low-emissivity coating separately reduces radiant exchange across the gap. Solid spacers at the edge connect the panes and provide another thermal path. The gas space is not a vacuum flask.
Original qualitative section, not a U-factor calculation or solar-ray model. The navy arrow is net warm-to-cool heat transfer by combined mechanisms, not a claim that all heat is radiation. Dashed wave indicates reduced radiant exchange across the cavity; low-e position is one example. No zero heat transfer, optimal spacing or product savings value.
Diagram basis: DOE: insulating glass, low-e, gas fills and spacers.
Sources: US Department of Energy · Pilkington / NSG
The second pane makes room for an insulating gap
An insulating glass unit holds its panes apart with a perimeter spacer and seals the space between them. That gap can contain air or a selected gas such as argon. It isn't an open channel through which indoor air is meant to travel to the outdoors.
The gas space adds thermal resistance between the warm and cold sides. It is still a material-filled gap, with heat transfer through it, rather than a perfect barrier. A vacuum flask uses an evacuated space to reduce gas-mediated heat transfer; ordinary double-pane glazing usually retains gas between its panes.
Sources: US Department of Energy · Pilkington / NSG
A low-e surface changes the radiant exchange
Warm surfaces also transfer energy by thermal radiation. Pilkington's explanation distinguishes the long-wave infrared emitted by room-temperature surroundings from sunlight. Ordinary glass absorbs and emits strongly in that long-wave range.
A low-emissivity, or low-e, coating changes that exchange. Depending on its position and type, it reduces emission or reflects more of the long-wave energy. The coating and the gas gap therefore don't perform the same task. Adding a coating is not the same physical change as simply moving two clear panes farther apart.
Sources: Pilkington / NSG
Heat can also pass around the gap
The spacer supports the pane separation and works with the seals to keep the unit intact. It also makes a solid connection at the edge. The surrounding frame provides another route between inside and outside. Those paths remain even when the center of the glazing performs well.
The US Department of Energy describes warm-edge spacers and more thermally resistant frames as additional design choices. This is why a whole-window result can't be read from the number of panes alone. Glass, gas, coatings, edges and frame all contribute to the final heat path.
Sources: US Department of Energy
The thing to remember
The gap slows heat transfer, a low-e surface changes thermal radiation, and the edge still provides solid routes for heat. A window combines all three.
What this example leaves out
This concerns ordinary sealed insulating glass units. It doesn't claim every unit uses argon or low-e glass, that a wider gap always performs better, or that one glazing choice guarantees a fixed energy-bill saving.
Sources & further reading
Source access dates are listed below. Manufacturer descriptions and patents support the specific constructions described, not every product.
- Window Types and Technologies ↗
US Department of Energy · Retrieved 2026-10-16 · Hermetically sealed spaced panes; gas fill; separate low-e, spacer and frame contributions; warm-edge spacer reduces edge transfer.
Insulated, Low-Emissivity Coatings and Gas Fills and Spacers passages read through live extracted primary result during same-day source-gap research. Repeated direct web fetch intermittently failed. No entire-document/graphic inspection or savings percentages claimed. - ATS-138 — How Pilkington Energy Advantage Low-E Glass Works ↗
Pilkington / NSG · Retrieved 2026-10-16 · Distinguishes sunlight and room-temperature long-wave infrared; uncoated glass absorbs/emits far IR; low-e reduces emission/reflection exchange according to surface location.
Three-page technical PDF fetched; p1 radiation and surface2/surface3 explanation read with pypdf. No universal coating chemistry, exact emittance/savings or identical solar behavior claimed. Original figure must show radiant exchange separately from gas-gap transfer.
