Why do glass windows still exist? (Why haven't they been replaced by plastics?)



Glass is fragile and impractical to transport, install and repair. Even worse, glass kills and hurts people when it breaks. Falling to the streets like guillotines during earthquakes and bomb raids. During wars people put tape on their windows to prevent shattering. When that meteor exploded over Chelyabinsk, people got hurt by standing inside of a window watching the sky when the shock wave hit them.

There are perfectly transparent plastics, for example the PET material used to make coca cola bottles. Why aren't windows made out of that instead of glass (fragile ceramics)? It seems to be much cheaper, safer and more practical to handle. Is there any advantage at all to make windows out of glass? Is this a billion dollar business idea, and if so, why haven't anyone realized it yet?


Posted 2016-02-13T06:20:28.890

Reputation: 463

18If you want to reinvent architecture so that it can cope with bomb raids, war, and meteor impact as typical use cases, what about flammability of plastics?Hagen von Eitzen 2016-02-13T16:21:51.013

22Glass is sturdier than you make it out to be. Consider that a beer bottle is notoriously stronger than a human skull. Fragility is part of the "cost" to pay for hardness - basically, glass will either stay mostly unaffected by stress, or it will fail completely. (Where plastics would scratch, bend, buckle, etc.) Dumping glass it because of those failure modes as opposed to addressing them directly seems like throwing the baby out of the bathwater.millimoose 2016-02-13T19:09:03.400

@HagenvonEitzen - PMMA specifically actually seems reasonably safe in a fire in that it burns clearly. (See: http://www.pmma-online.eu/faq) It might not be suitable to keep fire from getting inside a building; on the other hand, it might provide an escape route. Google tells me PMMA melts at 160 degrees Celsius, which is a temperature humans could feasibly withstand briefly enough to jump through a window - since you can put your hands into an oven hotter than that for a few seconds.

millimoose 2016-02-13T19:45:13.807

14@millimoose > since you can put your hands into an oven hotter than that for a few seconds molten plastic will transfer heat far more effectively than hot air. It has the added 'bonus' of probably sticking to your skin.Bob 2016-02-13T20:24:33.060

5There is also the consideration that in an emergency you can break a domestic glass window by hitting it with a chair to provide an escape route or at least access to fresh air, In this case the risk of cuts from broken glass is probably still acceptable if the alternative is dying from smoke inhalation.Chris Johns 2016-02-13T20:38:19.607

6There's is an issue I don't see anyone mentioning. Windows are already a key point in homes where insulation is lost (and hence energy and money is wasted). Do potentially plastic windows have similar properties in terms of heat transfer as their glass counterparts?PVAL 2016-02-14T03:35:42.527

Trade fragility for longevity, among other obvious reasons.Carl Witthoft 2016-02-14T17:20:07.337

@Bob I was working off the assumptiom that the blaze would be close to the window, and the acrylic woulf either melt into a neat puddle fairly quickly, and/or plain burn away. (I couldn't really find anything that would substantiate or disprove this.) But I was addressing the concern of "flammability", when that seems to not be the issue in a fire - on the contrary, it seems to be worse if the window pane would not catch fire. (I.e. if the blaze isn't close to the window, which is now unbreakable.)millimoose 2016-02-14T18:23:15.307

Car headlights are the best of both worlds. Glass for all the mentioned reasons, and a replaceable plastic cover to absorb stone impacts.geometrikal 2016-02-14T22:37:54.153

1@ChrisJohns: I agree with the smashing of glass as an escape route. As for "fresh air", that's a Bad Idea (tm), because not only you get fresh air, the fire causing the smoke does, also. Only break windows if you can get out that way soon-ish, because burning up is just as bad as smoke inhalation.DevSolar 2016-02-15T12:49:26.973

3@PVAL: Windows are actually indended to be the weak point in house insulation. If they weren't, you would get the condensation (and, eventually, mold) on the walls instead. A common problem with old houses that get new, top-of-the-line insulated windows but don't up the insulation of the walls as well...DevSolar 2016-02-15T12:53:02.200

1@ChrisJohns It is dangerous to smash a window and move through it. And most often useless because it is high over the street below. It is safer to simply OPEN IT.LocalFluff 2016-02-15T13:28:11.013

1@millimoose: I believe it's preferable for the window not to break on its own in a fire, but for fire crews to be able to break it. It seems to me a particularly bad scenario for the windows to melt, in a burning building, and to thus oxygenate the fire. I've often noticed the wire mesh glass in fire doors, in institutions (schools, say), wherein the idea is to prevent such breaking, in order instead to contain the fire.Mathieu K. 2016-02-15T21:35:12.073

Glass is a very interesting substance, from a scientific point of view. It's quite phenomenal for various reasons, such as how it doesn't interact with many things, chemically, which combined with its scratch-resistance makes it extremely durable. The Neal Stephenson sci-fi novel The Diamond Age is so called because in the setting, nanotechnology advances mean that windows can be easily produced from pure carbon, as a sheet of diamond. Because this process is so cheap and simple, most windows are now diamond rather than glass.Max Williams 2016-02-16T09:53:12.947

There are types of glass which don't break - like the roof of my conservatory, it's hail-proof and I challenge anybody to break it.RedSonja 2016-02-17T08:10:18.740

There are many kinds of plastic; each has different characteristics for hardness, opaqueness, toughness, flammability... which one did you have in mind?RedSonja 2016-02-17T08:11:46.150

@RedSonja I had in mind the plain cheap unbreakable transparent plastics used in soda bottles.LocalFluff 2016-02-17T08:18:39.127



There are two main reasons why glass is still preferred over say PMMA.

The first is durability. As long as it isn't broken, the glass in a window can easily last for hundreds of years in good condition. In particular it is a lot more resistant to scratches than comparable plastics and isn't really subject to much in the way of environmental degradation. Windows are very prone to getting scratched when they are washed as they accumulate small particles of grit on their surface which gets rubbed around the surface during cleaning. Even with scratch resistant coatings no transparent plastics get anywhere near the hardness of glass.

Most glasses are also much more resistant to environmental degradation from sunlight and various chemicals in the environment. Even the most resistant plastics start to discolour and become brittle over time.

The second factor is stiffness. Glass has a much higher Young's Modulus than PMMA. In bottles etc which are stiffened by their shape this doesn't matter much but, as windows tend to be large, flat, thin panels stiffness is a big issue, affecting their ability to be sealed into their frames and their optical properties. So a plastic window would need to be substantially thicker than a glass one to have the same stiffness with consequences for optical quality and cost.

There may also be issues with gas permeability in the context of double glazed windows.

In addition many of the safety concern raised in the question are addressed by laminated and tempered glass. Tempered glass is heat treated to control internal stresses, making it significantly stronger than float glass with the additional benefit that if it does break the entire plate fractures into small granules rather than sharp shards. Laminated glass consists of alternating layer of glass and a polymer film, producing a composite sheet with very high strength and toughness, potentially to the point where it can be usefully bullet resistant.

Another aspect of this is that side and rear windows in vehicles are often required to be made from tempered glass for safety reasons as they can be safely broken to allow access and extraction of passengers in an accident if doors are jammed or inaccessible.

Chris Johns

Posted 2016-02-13T06:20:28.890

Reputation: 12 837

19+1 good answer! The only things I would add to this are environmental properties, specifically UV degradation of polymers (they turn yellow and crack), and creep and sagging from elevated temperatures. Both can be mitigated, but not prevented entirely in bare sunlight. Silica based glass suffers from neither problem on human timescales.starrise 2016-02-13T23:22:23.013

1Forgive the naive question, but how does laminated glass avoid introducing the problem you mentioned of having a more scratchable, less weather-proof surface? Also are tempered or laminated glass commonly used in modern building construction?user568458 2016-02-15T10:11:26.517

1The other layers are glass so a hard surface is maintained, it is essentially a layer of plastic sandwiched between two thin glass layers. The inner layer may also have others films to provide tints or control reflectivity etc. Toughened glass is widely used for glass doors in particular. Usage will depend on specific design decisions and local building regulations but windows in 'skyscrapers' are very likely to be laminated glass.Chris Johns 2016-02-15T10:21:04.190


@starrise "Silica based glass suffers from neither problem on human timescales" - maybe, maybe not: consider the purple windows in Boston's Beacon Hill neighborhood.

davidbak 2016-02-16T19:24:12.693

Neat find, I seem to recall that certain eras of glass bottles can turn blue/green in sunlight as well. I suppose my statement should be amended to state "properly manufactured modern window glass" instead. http://libanswers.cmog.org/mobile.php?action=5&qid=283712

starrise 2016-02-16T19:48:48.670


Here is a plastic (most likely PMMA) window, in a boat, after only 37 years.

enter image description here

In addition to the obvious scratches, the outer surface has developed a cloudiness : possibly from degradation due to UV light, and (towards the LH end) you can see a cubelike pattern of stress cracks, rather like a toughened glass windscreen after a pebble hit it.

You really can't tell what you're looking at through it.

In this application, a PMMA window is still the best solution, rather than the difficulty of making a glass window fit the curve of the hull. But it shows the limitations compared to glass, which remains usable for centuries.

As far as replacing glass windows, the cost of a specialist will be high for any type of repair in any technology.

But absent custom curves, the raw material (glass) is cheap : about £1/sq foot, much cheaper than perspex or polycarbonate, and much easier and faster to cut. (Watching a professional exploit the special fracture properties of glass is impressive. Contrast with the difficulty of cutting perspex or polycarbonate!)

Brian Drummond

Posted 2016-02-13T06:20:28.890

Reputation: 1 369

That window has obviously has seen some tough times, even the metal and paint around it needs maintenance. Imagine if one could buy a replacement window made out of plastic for a few cents (like any soda bottle) and install it as easily as one washes a glass window. Maybe on a ship that is not easily implemented, but in ordinary buildings of most kinds. I still think that low cost and ease and safety of handling is a good solution to the problems with softness. Safety and unbreakability might have been the reason for choosing plastic windows when that ship was constructed.LocalFluff 2016-02-14T14:36:11.183

5Any window as thin as a soda bottle won't survive a winter, and if it's as easy to replace as wash, it's not well fixed or sealed - OR that's a very impressive windowframe! (The metal was fine : the paint was cheap household paint in a damp environment, the original lining - more plastic! - having died long ago)Brian Drummond 2016-02-14T15:12:26.980

The inner two frames could be well fixed and permanently installed. The outer one, which takes the damage, could be easily replaced. People have no problem with setting up a tent when camping or covering their food in plastic foil. Now, I'm not an engineer, but this does not seem so hard to me. I still don't understand glass windows as anything but a pre-plastic historic remnant. Sure, wine should be served in nice glasses, but office windows are not luxury goods. They are always dirty anyway, who cares about a few scratches?LocalFluff 2016-02-14T15:56:54.510

3This is a great answer. Personally, I have a hard enough time trying to keep windows and doors sealed at home when (such as now) the temps are single-digit outside. I think you sacrifice the efficiency of such a seal if you make the pane easily replaceable, no matter how good your design. There's a reason we still seal our tubs, sinks, sills, etc. with caulk or silicone, for example.trpt4him 2016-02-14T16:03:58.073

1@LocalFluff : then you can demonstrate the idea with low cost plastic sheeting and whatever outer frame you choose, applied over existing windows and frames. If you can get it to survive 5 or 10 years, no misting over, no water leakage, cheap and easy replacement (even upstairs) you may be on to something.Brian Drummond 2016-02-14T16:27:30.527

1I'm curious how this window would react to a heat-gun. I suspect it'd fix it up, but there's a good chance it'd do the exact opposite.Dewi Morgan 2016-02-15T20:25:41.023

1@DewiMorgan Heat gun might clear the cloudiness (if you're lucky and don't melt the surface) and maybe smooth out the scratches to some extent, but I don't see it doing the stress cracks (right through the interior of the material) any good.Brian Drummond 2016-06-10T17:43:40.037


What are the requirements for window glass?

  • Resistance to moisture
  • Resistance to UV radiation
  • Resistance to cleaning agents
  • Very high transmission in the visible spectrum
  • (There are many more, from an engineering point of view)

Most transparent plastics are not resistant enough. Plexiglas is one example which fulfills the requirements enough to be used that way. See this brochure from evonic where they guarantee you 30 years without noticeable yellowing.

The biggest problem with PMMA for household use is its "softness" (more prone to scratches) and mediocre resistance to cleaning agents as compared to glass. Now, there are many different flavours of PMMA, and you can coat a Plexiglas sheet to make it more durable.

PMMA has many advantages over soda-lime glass:

  • Higher transmission in the visible spectrum
  • Lower density
  • Easier processing
  • ...

It is at the moment probably just not cost effective enough to make PMMA as resilient as glass for use as window glass (for household use!, PMMA is used in many different application as a better alternative to the classical inorganic glass). This may change in the near future, as material science and the transparent plastics industry progresses.

John H. K.

Posted 2016-02-13T06:20:28.890

Reputation: 1 061

6I think scratch resistance is the killer feature. I use plastic lenses on my perscription glasses for shatter resistance, but eventually the lenses get so scratched beyons use. Coatings will also eventually break as heat variation occours. In practice i have ever only broken one window in my life even that was due to building element shift.joojaa 2016-02-13T16:57:37.437

I can imagine replacing the outer plastic window at no higher cost or trouble than cleaning a glass window. Like a tape or curtain. I put some kind of wax on my car windows in order to prevent dirt from sticking to them. Cost efficiency should come with economies of scale, considering how many billion of windows there are in the world.LocalFluff 2016-02-14T11:52:03.213


I actually did make windows out of plexiglas for an outbuilding. I could work them to the shape I wanted and they were lightweight. However, I discovered that they indeed scratch easily: just trying to get the protective paper off I both scraped and melted the plastic.

We want hard windows for houses and primary buildings. So what you describe would be addressed by laminates, as used in automobiles. I'm sure you can buy that. But most people find having shutters to be cheaper and satisfactory if severe weather may cause breakage, along with breakage warranty and insurance, or the cheapness of replacing plain glass panes.

There are regulations for requiring tempered glass for some windows that a child may fall into. Tempered glass in general addresses the dangers you raise. I don't know about regulations for glass clad skyscrapers where earthquakes are prone, but I'll bet they don't fall even if shaken because they are hung and can move independantly.


Posted 2016-02-13T06:20:28.890

Reputation: 233


After some research,

we are using glass windows as it was used traditionally. Actually plastic windows exists. Though it may take time for most of us to adapt to it.

While the reasons to consider glass windows are:

  • Up to 80% of all recycled glass can be reclaimed.

  • Recycled glass uses 40% less energy than manufacturing new glass.

  • Recycling doesn’t compromise glass’ quality or structure and no toxins are produced in its recycling.


Plastic windows availablity


Posted 2016-02-13T06:20:28.890

Reputation: 448

I actually think that recycling is yet another argument against glass windows. A couple of times a window has broken for me, and it has been expensive to call a specialist to reinstall a new one. But I have consumed thousands of plastic transparent soda bottles and none of them ever broke. Nor have I heard of anyone ever getting hurt by a plastic bottle. There's an elaborate consumer recycling system for PET bottles, melting and reforming them, although they really only need to be cleaned and refilled since they are practically unbreakable.LocalFluff 2016-02-13T08:46:01.857

Compare the situation. Drop by drop makes the ocean. If everyone of us avoid glass windows then the production rate of it will drop and the effects of the plastic on environment will decrease (by a small number, but it will surely decrease). That's also one of the reason we try to use eco-friendly products. Example R-134a rather than R-22Fennekin 2016-02-13T09:11:47.320

The "Plastic Windows" in your link have (mostly) a plastic frame, no?John H. K. 2016-02-13T09:35:51.443

most of them have plastic frameFennekin 2016-02-13T10:04:56.793

some of them like "http://4.imimg.com/data4/FA/TC/MY-4842325/window-250x250.jpg" seem to have transparent plastic instead of glass

Fennekin 2016-02-13T10:05:36.060

3@LocalFluff ive also reclaimed many glass bottles and most of them didnt break. I also broke a plexiglass window which also needed a specialist that was hard to find and even more expensive than glass windows. Also glass has near infinite recyclability, that is it does not degrade as it is recycled.joojaa 2016-02-13T16:49:18.663

1@LocalFluff, reglazing a glass window (at least a traditional wooden-framed one) is something anyone can learn to do in an hour or so. Why do you think repairing a glass window requires more of a specialist to do than repairing a plastic one?The Photon 2016-02-15T23:46:20.310

2@LocalFluff: PET doesn't make good windows, and PMMA isn't easy to recycle.MSalters 2016-02-15T23:59:35.440

1I've diy-installed both glass and plastic windows (plastic for safety, glass for durability and clarity) and I've found that glass is easier to work. Polycarbonate needs a special saw blade or a lot of elbow grease with a scoring tool and it's really easy to mar or scratch up.Adam Brown 2016-02-16T21:33:54.647

@LocalFluff I had more than one PET bottle "break". They got scratched while rolling through my car's trunk and started leaking their content into the trunkmat...Alexander 2016-02-17T10:42:18.530


Another issue is that the coefficient of thermal expansion of many plastics is higher than that of window glass. Frames for plastic windows would have to be able to accommodate more expansion / contraction than those for glass windows.




Posted 2016-02-13T06:20:28.890

Reputation: 51


Hardness aside, weathering effects (from e.g. rainfall, snow, hail, temperature and moisture variations etcetera) and UV degradation are of course important to consider for windows. In general, glass is much more resistant to such effects compared to plastics.

There is also something called creep, which is essentially time-dependent deformation during long times (months or years). You might have seen it if you've ever left something heavy on a cardboard box for a long time. Plastics creep pretty easily. I don't know if this actually would be a major problem for a regular window perpendicular to the ground, but it might not be good for a window that will carry static loads.

Plastics will also whiten due to deformation. They might also discolor.

Of course, these properties are modifiable (to some extent) through various additives.


Posted 2016-02-13T06:20:28.890

Reputation: 31


Plastic is not used for windows mainly because it is not durable, in fact:

  • it is degraded by UV radiation, which breaks atomic bonds, yielding a matt and thinned (and fragile) surface, not so desirable for a window;

  • because of the previous point, plastic generates micro- and nanoparticles which diffuse inside the house and inhaling them is not healthy;

  • it is less hard than glass, then it can be easily scratched with wider variety of materials;

  • it is flammable(!) and can produce dioxin as it is burnt (especially if it contains chlorine atoms in its structure), an extremely toxic and carcinogenic compound.

Moreover, glass is able to shield approximately 10% of solar radiation and almost 100% of infrared radiation (emitted by human body as heat), so it is a powerful insulating material, which helps keep your house relatively fresh in summer and warm in winter.


Posted 2016-02-13T06:20:28.890

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