[Note that this article is a transcript of the video embedded above.] Have you ever thought about windows? Like really considered the idea? If we hadn’t been exposed to them our entire lives, it might seem like kind of a bizarre concept. On the face of it, windows are about light. But it’s more esoteric than that, especially now that we can make our own light. We want to be inside, but not all the way inside. So we put holes in the wall. It’s a heck of a trade-off: you get light and a view of the outdoors, but it comes at a cost. Not just the cost of the glass itself, but also the structural changes they require and their effects on ventilation, sound, cooling, heating, safety, and security inside a building. As simple as they seem, windows are a technology of sorts. And like a lot of building science, we’ve slowly been making progress, readjusting those tradeoffs and giving us the benefits without so many of the costs. I love to pick a relatively innocuous part of the built environment and really take a close look. So, I’ve built a few demonstrations in the garage to show you how far the science of glazing has come. I’m Grady, and this is Practical Engineering. Glass is not a good substitute for an exterior wall, in many respects. If you take some wall away and replace it with a window, you’re not trading like-for-like. For one, you never accidentally throw a baseball through a brick wall. But it’s especially true when it comes to heat. And this is a pretty big deal because we collectively spend vast sums of money, resources, effort, and energy on maintaining a comfortable thermal climate inside our buildings. Almost all the time, in almost all the places, the temperature outside of a building is different from the temperature we want it to be inside. But the laws of physics (also all the time in all the places) are constantly working to equalize those temperatures. We usually categorize the flow of heat into three separate modes. Conduction is heat moving through solid objects, atoms or molecules that are tightly packed, transmitting energy to their neighbors directly by vibrating against each other. Convection is heat transfer via the movement of a fluid like air or water. Radiation is heat transfer through electromagnetic waves. Since it is literally light, radiation doesn’t need a medium. All three play a role in the walls of buildings. And of course, it works both ways: sometimes it’s too hot outside and you want to keep that heat out. Sometimes it’s too cold and you want to keep the heat in. Any movement of heat from inside to out or vice versa is a flow of power, just like we measure with electricity. If you have 500 watts of heat flowing out of your house, you essentially need a 500 watt heater inside to make up for the loss and maintain a steady temperature. And just like voltage is a difference in potential that drives electrical current, heat flow is driven by a difference in temperature. The bigger that difference, the greater the flow of heat through a given wall. Continuing the electrical metaphor, insulation is the generic term we use to describe materials that resist the transmission of heat from one side to the other. It makes sense that we put it in our walls. In the US, we use R-value as a measure of this resistance. This is kind of a cursed unit of measurement, so I’m going to gloss over the scientific definition. But, a typical exterior wall assembly will usually have an R value between 10 and 25, depending on climate and how new the building is. A simple pane of glass has an R value of about 1. Let me show that to you. I built this box of foam insulation in my garage. Inside, I have a 25 watt ceramic heat emitter. I can close this up and wait for the temperature inside to stabilize. When that happens, we can assume that the heat going into the box is the same as the heat flowing out. Okay, this has pretty much settled out, and you can see that the difference between the interior and exterior probes is about 43 degrees Fahrenheit, I think that’s around 24 celsius. This is R-5 foam, soif you do the math for a perfectly sealed box with these dimensions that should be right around 50 F, 28 C, um, so accounting for the imperfections in my setup, like this, uh, terrible tape seal, that’s actually pretty close. Um, so now I’m going to swap this lid out with a pane of glass, and we’ll set this going again. Okay, this has settled out. And you can see that the temperature inside the shop hasn’t really changed, but the temperature inside the box has gone down a lot compared to the foam lid. The difference is about 22 F, which I think is around 13 C. So, just replacing one side of the box with glass instead of foam, it takes a much smaller difference in temperature to move that same 25 watts out of the box. I’m sure you see the issue here. If we want windows in our buildings, it comes at a cost of thermal efficiency. For every square foot or square meter of exterior wall that we swap with single pane windows, heat will move through that space roughly 10 to 25 times faster. So, we rarely use single-pane glazing anymore. Almost all modern windows use two, and sometimes even three panes of glass with a sealed air gap between them. Sometimes this gap is filled with a heavy gas like argon or krypton that has lower conduction and convection. There are even vacuum insulated panels that have no air inside, eliminating convection entirely. It seems like a simple change, but it makes a pretty big difference. I happen to have a double-pane glass unit, so we can try the experiment again. Okay, this demo has settled out now, and you can see that the temperature difference is right at 30 F, 17 C. Remember with the foam lid it was about 43 degrees and with the single pane it was around 23 degrees. So this is basically right in the middle. It’s not as good as when there was no window, but it’s a lot better than the single pane of glass. This varies a lot with details of the frame and coatings. But very generally, modern double-pane windows can double to quadruple the insulative value of a single pane of glass, and that can make a huge difference in a building with a lot of them. This idea of a sealed air gap between panes comes with one problem. If you’ve ever brought a bag of chips on an airplane, you know what I’m talking about. If the glass manufacturer is at a low elevation and the window’s going to be installed somewhere much higher, you end up with an overpressure situation between the panes compared to the barometric pressure. That can cause them to bow out and even fracture. In some cases, manufacturers will actually dial in the pressure for the altitude of the final destination. Another option is to install a capillary tube that equalizes the pressure inside and out. These are very narrow diameter and long tubes meant to reduce the amount of moisture that diffuses between the panes, which would otherwise lead to condensation inside the unit. Insulating glass panels help reduce heat conduction and convection, but that’s not the only way heat gets into and out of a building. The whole point of a window is to let in light, and we already learned that heat can move as electromagnetic waves. Of course, if we block visible light through a window to avoid heat transfer, that kind of defeats the purpose. But radiation also happens at other wavelengths on the electromagnetic spectrum. Specifically, for the temperatures we deal with on a day-to-day basis, most of the thermal radiation happens in infrared: wavelengths longer than visible light but shorter than microwaves. Even within that range, there are big differences in how the waves interact with matter. That’s particularly true for windows. Typical stuff at ambient temperatures like a sun-baked driveway or human body, emit thermal radiation mostly in the far-infrared spectrum. This is the stuff you can see on a normal infrared camera. And even though glass is designed to let visible light through, it is almost completely opaque to longwave infrared light. Try looking through a window with an infrared camera and you just see the window itself, not anything beyond. It’s cool to think that if our eyes worked a bit differently - if they were tuned to a slightly different part of the electromagnetic spectrum - a glass window would be entirely useless. And it seems like it would be a good thing for our thermal problem, preventing heat from transferring through. Unfortunately it’s not that simple. Glass isn’t just opaque to far-IR; it’s also highly emissive. That means it mostly absorbs thermal radiation rather than reflecting it. So, the radiation doesn’t come directly through, but it does warm the glass, which can then just conduct through the pane and reradiate it where you don’t want it to go, whether inside or out. It’s basically radiation with extra steps. On the shorter wavelength part of the spectrum we call near IR, it’s a little different. The main source of near IR radiation is the sun, and like visible light, that radiation can pass directly through most glass. This is how greenhouses work. The shortwave radiation passes right through where it can warm the stuff inside. But the stuff inside radiates longwave IR, which can’t pass back out through the glass. So, windows are kind of a two-way half-door for far infrared (since they absorb and reradiate it in both directions) and a one-way door for near infrared, but we can do better than that, mostly through low-emissivity or low-e coatings. These coatings essentially convert a pane of glass into a mirror for IR. And you can see the difference really well with an IR camera. You still get some reflections with an uncoated pane, but the difference is obvious when you put a coated pane next to an uncoated one. But this gets a little tricky, especially when it comes to near IR. If you live in a cold climate and your main use of energy is heating, you’re happy to invite as much sunlight as you can get inside. You don’t want to reflect the near IR. The opposite is true in warm climates. Fortunately, manufacturers can tune the coatings according to the climate. You can reflect shortwave radiation if you’re worried most about cooling, or you can let it in to gather more heat inside. Window manufacturers also sometimes vary which face is coated. In warmer climates, you can coat the inside of the outer pane, called “surface 2,” reflecting the IR before it crosses the gap to keep it from warming the air between the panes. In cooler climates, you can put it on “surface 3” to reflect the long-waveshort-wave IR back into your house without it having to cross the air gap. Making windows more thermally efficient isn’t the only use for glass coatings. If you’re like me, there’s a good chance you have the Merlin app installed on your phone. I think it’s a bit of an inevitability that once you reach a certain age, you start getting interested in birds. Unfortunately, one of the largest human-caused sources of bird mortality is actually windows. At certain angles, windows work essentially like mirrors, reflecting the trees and sky, and birds (bless their hearts) can’t always tell the difference. It can happen the other way too. Sometimes a bird will see something inside a building or all the way through two windows that they want to fly toward, not recognizing that there’s solid glass in the way. For you and me, walking into a glass pane like a sliding door is embarrassing, but for a bird flying at much higher speeds with tiny, hollow bones, it is often fatal. McCormick Place Convention Center on the lakeshore in Chicago may be the most famous case study. In an average year, monitors would find around 1,000 dead birds around the building from collisions with the windows. In October 2023, nearly 1,000 birds died in a single night. The owner finally decided to address the problem head on, installing a pattern of dots on all the glass surrounding the building. The solution cut the number of bird strikes by 95 percent. Awareness about bird safe glass is continuing to grow, and so are the number of solutions. Many glass manufacturers have bird-safe products that work in a similar way. The goal is to break the illusion of glass being a mirror or imperceptible as a solid surface, so most of the fixes involve patterns or designs on the glass. I think it looks pretty cool, actually. And because many bird species can see into the ultraviolet spectrum, you can even get dots and patterns that are practically invisible to humans, but still perceptible to birds to keep them from crashing into windows. Some cities are already including requirements for bird safe glass into codes, particularly in major migration corridors and for buildings that use large, uninterrupted facades of glass. This is a really common design for tall buildings and skyscrapers. Ever since I was a kid, I have always been impressed with this kind of facade. From the outside, it looks like a sleek, continuous column of glass. And it’s pretty interesting how it’s actually done. These are called walls, specifically curtain walls. But they’re also not walls in the traditional sense, at least when it comes to bearing loads. We typically think of a building’s exterior walls as the primary load path - the way forces from the building are transferred to the foundation. With curtain walls, that’s not true, and that name kind of gives it away. In buildings with curtain walls, the load bearing happens through interior columns. The exterior walls just hang there. Not that this needs another analogy, but it’s pretty much like the building is wearing a coat. This has a lot of advantages. A curtain wall is relatively lightweight, reducing structural loads on a building. It’s also thin, maximizing space in a building where it comes at a premium. They’re also unitized: you get very large pieces assembled in a factory so they go up quickly on a construction site. And, of course, you get a lot of light into the building when the entire exterior wall is a window. But how do you hold the glass in place? The process of installing glass, called glazing, is not like installing most other materials. You can’t use traditional fasteners like nails or screws. Glass is brittle, and the sharp point forces would induce too much stress. If you look at the windows of your house, the glass panes are usually sandwiched inside a frame with softer material that both seals against the glass to keep out the weather and spreads out the holding forces across a larger area along the edges of the glass. That’s sometimes called captured glazing. It’s done on large buildings with expansive glass facades too, but it’s not the only way. When you really want the outside of a building to look like only glass with no mullions or framing elements protruding beyond the panes, you can use structural silicone glazing, where the sealant does some or all the work to hold the glass in place. In 4-sided structural glazing, the glass panels are adhesively bonded on all four edges. You have nothing on the exterior except smooth glass. If you’re used to the silicone sealants sold at the hardware store, this might seem absolutely crazy to you. I definitely wouldn’t trust a tube of bathroom caulk to hold thousands of pounds of glass hundreds of feet in the air above sidewalks and roads full of people. To be fair, these systems do usually have some kind of mechanical clips or toggles that surreptitiously prevent panes from coming entirely loose if the sealant fails, but also, structural silicone is very different from caulk. Just to demonstrate this, I have a glass block. First I’m going to attach this to a metal plate using typical silicone caulk you might use in the bathroom. I’m starting with a 45-pound weight on the hook, which is probably a bit ambitious. And of course, the camera died right when that dropped, but it held 45 lbs for just about a minute before it dropped. And part of that’s cause, even after 2 weeks of curing, you can see, only the caulk around the edges actually cured. This stuff is just not designed for heavy weights like this. Now I’ll try it with structural silicone. This stuff comes from a few manufacturers in a variety of strengths, curing systems, and material compatibilities. After I let this cure for a week, which is when the manufacturer says it reaches 50 percent of its strength, I tried my little demo again. Wow. It’s holding 45 pounds just fine. So I added this small metal anvil just for good measure, and it’s still holding fine. Just a silly little experiment like this can give you a lot more confidence that those big insulated glass units on tall buildings aren’t going anywhere. But the engineering of windows doesn’t stop at holding them in place. We also have to think about what happens if they break. The most typical glass we think about with picture frames and windows is actually annealed glass. The manufacturing process is pretty impressive. The molten material is allowed to spread out and float atop a bed of molten tin, making it extremely flat and very consistent in thickness. Then the glass is allowed to cool slowly, which relieves internal stresses. That makes it possible to cut along clean lines and even drill holes in the material. But it also means that when annealed glass breaks, it usually fractures into large, razor-sharp shards that can be extremely dangerous. I think most people have enough experience with the issue that there’s hardly a need at all to say that broken glass is a serious hazard. And in many cases, it’s a simply unacceptable hazard: a slip in the shower that breaks the door, a toddler on a glass tabletop, even pushing too hard on a glass door. There are plenty of documented cases where a simple accident became fatal simply because of how annealed glass becomes jagged and razor-sharp when it fractures. One of the main solutions to the problem is tempering the glass. If you heat glass and cool it down quickly, often using a high-pressure blast of air, you can get the outside to cool a little quicker than the inside of the glass. The outside faces shrink. Then when the inside cools, it shrinks as well, pulling the outside faces of the glass into compression. If you use a polarizing filter, sometimes you can even see the stresses in tempered glass. This is a tricky process because if the cooling happens unevenly, the glass will crack or break. I’ve discovered this myself by accidentally setting a glass dish on the stove burner. But if you do it right, tempering glass makes it much stronger than typical annealed glass. Since the outer faces are in compression, cracks are less able to propagate. But even if it does break, tempered glass has a big advantage. Instead of the large shards, you tiny, relatively harmless pieces. These can still have sharp edges, but just being so much smaller means they’re much less able to injure someone. They have a lot less mass when they fall, and the edges are shorter, so it’s more like angry gravel and less like deadly knives and spears. That’s why tempered glass is often called safety glass, and lots of building codes require it in situations where broken glass can pose a safety hazard like glass doors, large windows close to the ground, and in bathrooms and showers. Most vehicles also use tempered glass on the side windows to reduce the risk of injury during a crash. One disadvantage of tempered glass is that it can’t be modified after the fact. All that built-up stress in the material makes sure it shatters into tiny pieces, but it also prevents you from cutting or drilling holes. Tempering has to be the final step. The other disadvantage also comes from its main advantage. Because tempered glass breaks in a relatively safe way, it doesn’t offer much security. And really it’s true for annealed glass too. Windows aren’t just a thermal weak point in buildings; they are also a security weak point. Why would a criminal struggle to break through a reinforced door if they can just quickly smash a window? That’s where laminated glass comes in. You can glue individual panes of glass together where they still retain almost perfect optical clarity. But even better than that, you can sandwich a tough plastic interlayer between those panes of glass, creating a composite material. And it behaves a lot differently when it breaks. Now you don’t have shards separating at all. Even completely cracked apart, the glass stays adhered to the plastic instead of flying apart. And more importantly, this is still a barrier even after it breaks. I’m not getting my hand through there. Laminated glass is ideal for overhead applications, where a break could otherwise rain down shards on people below. And it’s perfect for commercial buildings, especially on ground floors, to make it more challenging for someone to break in through a window or glass door. Laminated glass is also standard on vehicle windshields to offer more protection from a road hazard that could otherwise break through the glass and injure the driver or passengers. What’s kind of crazy is that I’m just scratching the surface of modern window technology, not to mention the glass itself, which can get a lot more complicated for applications like phone screens. But these are all innovations that are kind of hiding in plain sight, and to me at least, they make the view through a window just a little bit sweeter.

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