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2026-11-04 03:31:01 GMT

Windows Are Absurd
[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
https://practical.engineering/blog/2026/11/3/windows-are-absurd
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2026-10-21 03:22:59 GMT

Whatever Happened to Fire Escapes?
[Note that this article is a transcript of the video embedded above.]
The idea is simple: If there’s a fire in a tall building that blocks the main stairwell, people need an alternative way to evacuate, preferably one that is somewhat protected or at least far away from where the fire is happening. If you’re in a one or two story building, it’s pretty simple: just get out of any window or door. You may get hurt a little, but you’ll still be away from the fire. In the denser cities of the early United States, though, as the buildings grew taller, the options for evacuation became more complicated. High-profile, horrific building fires were a nearly constant plague in factories, tenements, office buildings, and theaters. The external fire escape seemed obvious to many: Bolt a metal staircase to the outside of the building with a landing below a window on each floor.
Fire escapes became an element of American culture in many ways beyond simple safety equipment. They serve as dramatic devices in films and plays, provide a scenic place for urbanites to get some fresh air, and they impart a quintessential character on the skylines of our oldest cities. But it turns out that we’ve pretty much never agreed on how well they actually work (if at all) or whether they were ever the right solution to the problem. And while many older buildings still have the original ones installed and maintained, they’ve slowly started disappearing. The story of their decline is a fascinating case study in how building codes actually work. I’m Grady and this is Practical Engineering.
An important thing to keep in mind about any safety devices in buildings, including elaborate metal contraptions that serve as fire escapes: owners don’t just install this stuff out of the goodness of their hearts. And I don’t mean that as a dig on the owners. It’s just very difficult to weigh the costs and benefits of safety devices that, honestly, might never even be called on to function over the lifetime of the building. The economics of real estate just don’t incentivize going above and beyond, or really much consideration at all of low-probability, dangerous situations. Building safety is rarely a primary factor in purchasing decisions for tenants, so it doesn’t hit the bottom line on a landlord’s balance sheet. It sounds like cold-hearted capitalism, and in many ways it is, but of course, there’s another side to this coin. You could cram so many sprinklers and fire alarms into a building that there’s no actual space to live or work in, and nobody could afford it even if they could squeeze in.
So obviously, there is some reasonable line between no safety at all and overkill. And it doesn’t make sense to put the burden of drawing that line on every individual building owner or even every individual architect and engineer who designs them. This is the entire purpose of building codes: they’re an (ideally) well-informed consensus around what’s quote-unquote “worth it.” It’s society’s way of taking responsibility for the safety of our constructed environment.
That’s exactly how fire escapes came into being in the first place. In major American cities like New York, Boston, and Chicago, we began building tall, multi-occupancy residential buildings around the mid-1800s. And they weren’t like the apartments and flats that are the baseline standard today. Multi-family housing, called tenements, were almost exclusively built for poor immigrants and laborers. They were designed for efficiency, cramming as many units as possible into a single building. And as early as the 1860s, cities started recognizing the issue of fire safety. Smoke and flames seek open vertical shafts where hot air can rise quickly, and the stairways in tenement buildings provided the perfect conduit. With lots of people, combustible materials, and often a single open stairway, these buildings were dangerous fire traps.
In February 1860, a bakery in the basement of a six-story New York City tenement caught fire. Flames quickly spread upward, but the fire department’s ladders could only reach up to the fourth floor. Ten people lost their lives, because there was no way to escape the building. The press coverage that followed catalyzed a public outcry for reform. The fire forced the city into action. Working with the state, they passed “An Act to Provide Against Unsafe Buildings in the City of New York,” which required a fireproof stairway or some means of exterior escape on all tenement buildings. The law was retroactive, meaning even existing buildings were required to comply. And that decision shaped the design of fire escapes perhaps more than anything else in their history.
Reconfiguring a stairway inside an existing building is an enormous endeavor. It’s costly, time consuming, and disruptive, if it’s even feasible at all. For many building owners, that just wasn’t realistic. The alternative was much simpler and cheaper. So that’s what most did. In reality, enforcement of the new law was lax, so many buildings didn’t comply with the new rule at all, but where buildings were upgraded, it was almost exclusively to facilitate egress on the outside.
The rules for alternative egress routes would be extended to other types of buildings over time. But they weren’t very specific about what form those escape structures should take or if a “structure” was even necessary at all. So inventors got pretty creative. All types of rope and chain ladders, chutes, slides, slings, baskets, and tubes were proposed. For a time, New York City building officials accepted just about any type of escape device as compliant with the law. One of my favorites is the parachute hat you could keep stored in a closet by the window, along with rubber overshoes to soften the inevitable hard landing. I doubt anyone took this seriously, even at the time, but you have to wonder if the inventors actually tested some of these wacky ideas.
The other large cities mostly followed New York, requiring some kind of alternative escape route on tall buildings in the event of a fire. And eventually the classic iron or steel ladder-and-balcony system coalesced as the default solution to fire escapes in the early 1900s. It definitely seems like a better system than some of the others being proposed at the time, but not everyone was convinced.
Even before the skeleton-style fire escapes became somewhat standardized, Americans were trying to apply some scientific thinking to the problem. You have to realize that building regulations are, by necessity, kind of onerous. If people were doing these things already, there would be no need for the rules. Practically every aspect of the urban environment looks the way it does because of the codes enshrined by some committee or group of professionals in what’s usually a fairly opaque process. They’re blanket requirements that often affect every single building, they’re expensive to comply with, and they have wide-ranging effects that literally shape the city. So it’s important that we get this stuff right.
One of the first technical considerations of the modern fire escape was carried out by the Franklin Institute in 1881 after the Randolph Street mill fire in Philadelphia trapped 35 people inside. Their conclusion was that external egress paths, in other words, the bolt-on fire escapes had, quote, “comparatively little value.” They recognized the fact that it was the only practical choice in some situations, but that internal stairways, properly constructed, afforded the best means of escape.
Decades later, their conclusion and recommendations were made clearer in one of the most famous and also worst industrial disasters in American history. In 1911, the Triangle Shirtwaist garment factory caught fire in the top floors of the Asch Building in New York City. The building was out of compliance with the exit laws of the time. The doors in the narrow, winding staircases opened inward, making them hard to open in a crush of panicked people trying to escape, and one of them was reportedly locked. The building had an exterior fire escape, but it buckled under the weight of fleeing garment workers, dropping them to the ground below. Ultimately more than 140 people died in the tragedy, most of them women and girls. Among the many reforms that resulted was showing how a fire escape could do more harm than good.
This debate continued for decades. One side said, essentially, we should not be using these external fire escapes. A report in the National Fire Protection Association’s quarterly journal called them, quote, “a pitiful delusion,” saying that they “contributed to the principal element of tragedy to all fires where panic resulted.” The worry was that the widespread use of these iron escapes was just making it harder to enact regulations that would be more functional. That organization eventually released a model building code that recommended prohibiting outside stairs as fire escapes in 1923, but it wasn’t widely adopted.
The other side said that, since external escapes are already so widely used, we should be focused on making them better. The 1949 National Building Code, which was largely developed by insurance companies, took a more pragmatic approach, allowing exterior escapes but regulating them with minimum sizes. Basically they made them into full outside stairways as robust and large as the ones inside. But by then, the list of disadvantages of the traditional fire escape had just grown too long:
Architects didn’t like them because they cluttered up a building’s facade, sometimes even blocking views. They could make burglaries easier, giving anyone nefarious access to the upper floors of a building. They required a lot of maintenance to prevent rust and maintain stability. No one really practiced using them, so they were rarely the first inclination in an actual emergency. Even if they were used, the bottom stairway, which was often hinged up to avoid intrusions, was sometimes inoperable from lack of regular use. They weren’t suitable for many types of buildings, like hospitals, where getting people through a window or down a steep staircase is a challenge. And, as nice as they sometimes are for tenants to have a little balcony, it is difficult to preserve their function as an actual egress route. It’s unused space, after all, so people naturally take advantage of them, putting furniture or using them for storage, blocking that path if it’s ever needed.
The only real advantage is that they didn’t take away from the expensive interior floorspace, but that just wasn’t enough of a benefit anymore. When New York City updated their building codes in 1968, exterior exits were officially prohibited for new construction. Many other large cities across the country had either already banned them or would follow soon after. What took their place? The exact thing the Franklin Institute proposed in 1881, nearly a century earlier. I have to read this quote because of how prescient it was:
“As a general rule, applicable to all buildings, the main reliance should be placed upon stairways in daily use, for the obvious reason that the occupants of a building instinctively seek such an escape in case of fire, that they are familiar with it, and that, being in daily use, it is reasonably certain to be unobstructed and to have the approaches to it clear.”
Even more than that, they suggested that stairways be enclosed in brick, built of incombustible materials, and have self-closing doors to keep out smoke. And that’s practically a perfect description of the modern tower stairway used in tall buildings today. And you might not even notice it. All the safety features are a lot less conspicuous than a spindly iron assembly bolted to the outside of the building, but we’ve just found that it’s so much safer. And it really took these gradual improvements over decades to reach this consensus: safer consumer products, fire rated building materials, sprinklers, alarms, and even reliable air-conditioning systems that make windowless, interior, smoke-proof stair towers possible.
And that’s kind of the story of lots of building codes. They have this weird momentum. For one, they normally only apply to new construction, so they’re kind of like planting a tree. We don’t see the benefits until years later. But once they are in place and the industry evolves to accommodate those regulations, there’s a lot of resistance to getting rid of them later. So it can be really hard to imagine the repercussions a specific code will have years into the future. Here’s a perfect example:
The Center on Building in North America recently completed a report on elevators, finding that they cost at least three times as much as those in other developed countries around the world. One of the reasons for this is just that they’re much bigger in America. And the reason for that is one fire inspector in Glendale, Arizona submitted a proposal to the International Code Council to require elevators fit a horizontal stretcher, erroneously stating on the form that there would be no cost impact. At face value, this is sensible. If you have a medical emergency high up in a tall building, it would be ideal if the first responders could use the elevator to get you down.
The amendment slipped through without close scrutiny, and now, if you want to put an elevator in your building in the US, it has to be a big one. But the result of that change wasn’t only that our elevators got bigger, but also that more and more building owners now just decide it isn’t worth the cost at all, opting for walkup designs instead. So the change actually led to fewer accessible buildings, and ultimately hurt the very people with mobility issues the rules were originally intended to protect.
In a similar way, New York City’s first fire egress law in 1860 applied to existing buildings as well as new construction, essentially ensuring that the prevailing solutions were going to be bolt-on retrofits to the outside of buildings, and that momentum carried for practically the next century and a half. Writing building regulations is a messy process, and there really is rarely a single “right” answer. There are just tradeoffs that affect every building differently and ripple through space, time, and the economy. There’s something nostalgic and charming about the old iron fire escapes across American skylines, but I think it’s probably best that they’ve finally gone the way of the parachute hat.
https://practical.engineering/blog/2026/10/20/whatever-happened-to-fire-escapes
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