If you’ve been looking at new windows, you’ve probably been told they are “low-e.”

Maybe your sales rep said it stands for “low emissivity” and that it’s a thin coat of metal on the glass that adds to the window’s insulation value.

I’ve looked into it (sorry, couldn’t help the pun), determined to find out exactly what it is and how it works. First, let’s get the nerdy stuff out of the way. The coating is an alloy of indium tin oxide (ITO) applied about 0.08 mm thick. Clearly, (yeah, again), it is transparent just like glass and plastic, and it has thermal properties.

Second, “emissivity” is an engineering term used to describe an object’s tendency to emit radiant heat. Take a room’s radiator, for example. Hot water flows into it where it sits for a bit, heating up the radiator to eventually “radiate” – or emit – into the surrounding air. Eventually, that heat warms up the window panes, too. In ordinary glass, the heat is emitted at a fast rate to the next pane, which in turn emits its absorbed heat to the outdoors.

Along came low-e coatings. In our predominantly cold climate, a “hard” coating is applied during manufacturing to the outside surface of the inner pane of an insulated dual-pane window to keep as much of your radiator’s heat from being emitted to the outdoors. On sunny days, the coating still allows for much of the sun’s heat to be emitted into your room while keeping out some of it in the summer. This is called HSG, or high-solar heat gain. In hotter climates, a “soft” coating is applied on the other inner pane to block more of the sun’s heat, referred to as LSG (low-solar heat gain).

If you’re like me, you might be wondering: In winter I want the sun’s heat to flood my room! And I want less solar radiant heat to enter my home in the summer. Well, we can’t have everything! While the low-e coating keeps out some of the summer sun’s heat, the best way is still to shade the windows from the outside with trees or ledges, followed by closing the blinds or drapes.

And as for losing some of that extra solar radiation in winter, a research project commissioned by the National Research Council of Canada showed that the savings in heating and cooling more than make up for it: “HSG glazing would be expected to produce savings of between 13% and 17% in combined heating and cooling costs [in a typical Canadian city] while the LSG glazing would be expected to produce savings of between 8% and 10% compared to the conventional glazing.”

Not bad for an invisible sheet of metal.