Most of our homes by now are pretty energy savvy—with fiberglass sprayed in the walls and attic space, foam-core metal doors, and, of course, insulated dual-pane windows. All that stuff keeps the atmosphere inside our houses comfortable, keeps the cold outside in the winter and the heat outside in the summer. That’s where it should be. Door and insulation technology isn’t really too special. It’s been around for quite a while, and only the materials have changed. But windows are another story.
Dual-pane windows, or insulated glazing, might have originated in the late 19th century but became commercially useful around the 1940s. Libbey-Owens-Ford Glass Company (LOF) was the first to produce insulated windows in any quantity, marketing their Thermopanes as the height of thermally efficient construction. Dual-pane windows offered better thermal retention, sound dampening, and improved longevity compared to traditional sash or sash and storm window combinations.
But why are insulated windows so great? Many tend to assume it’s because there are two glass panes rather than one, but that’s not quite it. What makes insulated glass, or IGs, so much better is, well, nothing. That’s right, it’s the vacant spot between the panes that does most of the work.
Okay, it’s not totally vacant. Some manufacturers inject their IGs with inert gasses, like argon or krypton, which have much less conductivity than regular air. Others suck most the air out and leave a vacuum. This is called evacuated glazing, and while it virtually eliminates heat transfer, it’s incredibly expensive and pretty volatile, as the glass is always stressed by the surrounding atmosphere. These VIGs have to be reinforced by glass “pillars” from the inside to prevent collapse. Imagine what would happen if you sucked the air from an empty pop can, then imagine it’s the glass in your window on a hot day. No one wants that mess.
The thicker IGs are, the better thermal properties they have. Most of the windows in our homes range from ½” to 1” thick, and the glass panes vary between 1/16”, 1/8”, 3/16”, and ¼”. The glass can be annealed, which means it is not heat strengthened and will break into shards or larger plates, or it can be tempered, meaning it is exposed to heat treatment. Tempered glass is incredible strong on its surface, and will break into tiny little cubes, which is much safer.
Glass can also be coated with LOW-E (yep, that means low energy) films or liquids. These are typically UV resistant materials designed to deflect the sun rays bouncing around outside. Not only do these coating help further reduce heat transfer, but they also help protect you and your stuff. LOW-E glass prevents carpet, walls, or pretty much anything in the house—yourself included—from the damage caused by sun exposure. That means the carpet won’t fade as fast, and your skin stays younger longer.
To hold the two panes of glass apart, IGs usually have an aluminum spacer—called, well, a spacer—and it’s filled with desiccant to absorb any condensation. The spacer is sealed to the glass first by a strip of sticky butyl tape, then by a thick adhesive called polysulfide. Some IGs might have a nonconductive foam spacer or, for very thin applications, just a cord of butyl.
The process of making IGs is pretty interesting. First, the glass is batched to optimize waste. Then it’s loaded onto huge, air-operated tables—not unlike air-hockey tables—where an automated cutter scores all the cuts and automated breakers pop up from the table to snap the glass. Once cut, the smaller pieces pass vertically through washers and air dryers, while spacers are cut and lined with butyl before being smashed between their corresponding panes. Next it’s the polysulfide, which a worker squirts around the edges of the IG and smooths off with a spatula. The assembled units are laid flat to dry and then loaded up and shipped to the glass shop. Hey, that’s us!
Having been around since the 1950s, Block & Olson Glass has seen and experienced the commercial and residential progress of the IG—from converting older homes to replacing them in newer homes and businesses. We know how valuable efficiency is in every season, and we’ve had a ton of time to master the technology of dual-pane windows. Heck, some of us have been around long enough to remember when IG technology first arrived, and thick plates of glass were spaced with led. Those unwieldy windows weighed a ton sometimes.
Anyway, whether it’s to cut down the noise of a busy neighborhood, to keep your carpet and your walls looking new, or to keep your heating and cooling bills in check, insulated windows shatter single panes every time (sorry, most of us are dads, so we make jokes accordingly).