Building up the perfect wall

There are dozens of facade consultants, facade engineers or building envelope specialists Webpages out there. Many of them are being listed in this blog, in the column Engineers & Facade consultants. There is a constant in these pages: you won't find almost any information about what we facade specialists really do for a living. We don't write there. We have no opinions about our field of experience. We seem to be on the hyperspace trying to convince potential clients that we are the right folks for them, just because our Webpage - not designed by us - looks great or professional. The fact that it almost always looks boring doesn't seem to bother us.

There is one exception at least, one that clearly jumps above all others. This post is dedicated to a bunch of building science specialists - mainly building envelope related - who are brave enough to write and say what they think and do. Their Webpage is called Buildingscience.com. Against all odds, it's not another governmental agency or something paid by a guild of construction materials suppliers. Building Science Corporation is a firm of building consultants and architects, located in Massachusetts with a branch in Ontario. They specialize in building technology consulting, more specifically in preventing and resolving problems related to building design, construction and - yes - operation. They seem to be experts in energy efficiency, buildings retrofit, moisture dynamics, indoor air quality and building failure investigations.

The difference between this team and other building envelope specialists is the people they have and the way they market themselves. Two of the principals, Joseph Lstiburek and John Straube, are also the most active writers of articles in the information part of the Webpage. These guys sum up a huge field experience with strong academic and research roots, combine knowledge with an entertaining writing style, and deal with issues one rarely finds treated with such clarity. Lstiburek founded the company, Straube joined later. Lstiburek seems to be the one with practical roots, reinforced by being part of the 'Building America' program at the US Department of Energy. Straube seems to be the professor in the team, teaching building science in the Civil Engineering Department and School of Architecture at the University of Waterloo, Canada.


There are several document files available at their Webpage. The most interesting papers can be found under the labels Building Science Digests (BSD), Building Science Insights (BSI), Guides and Manuals (GM) and Research Reports (RR). There is also a complete Glossary of Building Science terms. Digests and Insights are much less dense and really fun to read. Let's have a look for instance at BSI 005: A bridge too far by Joseph Lstiburek. The topic is obviously thermal bridges. You can find sentences as these:

For a bunch of supposedly clever folks we sure do dumb things. One of the big ideas of the past couple of decades or so is to keep the heat out during cooling and keep the heat in during heating. The better we are at this the less energy we need to use to condition the interior. Apparently this concept has not caught on. (...) If an alien from another planet looked at our construction practices he would conclude that we have too much heat in buildings and we want to reject that heat to the outside.

The paper is illustrated with images as clear as the one below (the caption has been copied from the original):

"Clint Eastwood" Thermodynamics—“The Good” uses offsets and exterior insulation. “The Bad” only uses exterior insulation. “The Ugly” uses neither. 

Why can't we be as clear as these folks when discussing about things we all know - and can be measured?

Let's go back to the paper that bears the name of this post, BSI 001: The perfect wall, another example of must read building envelope science. The author is again our entertaining but precise Joseph Lstiburek:

The perfect wall is an environmental separator—it has to keep the outside out and the inside in. (...) Today walls need four principal control layers—especially if we don’t build out of rocks. They are presented in order of importance: a) a rain control layer, b) an air control layer, c) a vapor control layer, and d) a thermal control layer.

In concept the perfect wall (see image to the left) should have the rainwater control layer, the air control layer, the vapor control layer and the thermal control layer on the exterior of the structure. The cladding function is principally to act a an ultra-violet screen and a first rain screen. And yes, architects also consider the aesthetics of the cladding to be important.

At this point Straube goes for a second to Canada, and refers (without mentioning their names) to the seminal works of the Norwegian O. Birkeland and the Canadian G.K. Garden about the concept of rain screen cladding and the control of rain penetration. I will dedicate a post to these guys and their papers, written in the first half of the 60s, since their influence is greater today than at the time of writing. Another old Canadian professor is cited here, N.B. Hutcheon, whose Principles Applied to an Insulated Masonry Wall (1964) are also completely up to date. The images shown here below, taken from Hutcheon, still resonate in their clarity.

It is interesting to follow Hutcheon's reasoning when he compares these two sections 46 years ago:

Wall to the left is representative of a number of current designs that have been used quite extensively in recent buildings. It is of a basic form consisting of 8-in. back-up and 4-in. facing, in this case stone, which has been widely used in Canada over the past 50 years or more. Insulation is now commonly added to the inside, and may take several forms including mineral wool between strapping or foamed plastic serving also as plaster base. Full mortar backing, which usually requires a very wet mortar, is commonly used behind the stone.

(...) Reference to the winter temperature gradients for Wall to the left will show that all material outside the insulation will fall below freezing. (...) Rain penetration through cracks, occurring as a result of temperature movement in the exterior cladding, can also allow the entry of water and the wetting of the wall.

A dramatic difference in temperature conditions and their attendant dimensional changes can be effected by moving the location of the insulation, see Wall to the right. The main wythe and all the parts of the structure in contact with it are subjected to a much smaller range of temperatures. The possibility of disruptive dimensional changes arising from temperature effects is greatly reduced for all but the exterior cladding and, as will be discussed, these can readily be accommodated. The window frame, now bedded in or fastened to the warm interior wythe, is relieved of the substantial edge-cooling effect of the former arrangement. Advantage can be taken of the inside metal sill to collect and conduct heat to the frame, and a thermal break may be incorporated on the outside to minimize the loss of heat in winter.

(...) The exterior cladding can be arranged as shown for Wall to the right in the form of an open rain screen. It may be set out to form an air space and supported by ledger angles and ties as before. The air space, being heavily vented by suitably designed open joints at both horizontal and vertical intervals, will at all times follow closely the outside air pressure so that the rain screen is substantially relieved of wind pressure differences. This not only removes the major force causing rain to penetrate the cladding, but also eliminates the wind loads on it.

Isn't it amazing? We are still - 46 years later - teaching this exact lesson to new generations of equally astonished architects. Even worse, we still see in 2010 a number of projects with wall sections similar to the left detail instead of to the right one. Lstibureck goes one step further in his paper to discuss the preferred position not just of the thermal barrier, but of the four control barriers as he calls them: rain, air, vapor and thermal. The details are clear in his article. I will summarize here one of the conclussions because it is of great help for us to do a good detail of any building envelope - be it a wall, a roof or a slab in contact with the earth.

Lstiburek describes first the roof adequate build-up (image above to the left) and then the slab in contact with the earth (image to the right), to find out a striking similarity between those two and with a perfect vertical wall:  

The perfect roof is sometime referred to as an “inverted roof” since the rainwater control layer is under the insulation and ballast (i.e. roof cladding). Personally I don’t view it as inverted. Those other folks got it wrong by locating the membrane exposed on the top of the insulation—it is they that are inverted. The perfect slab has a stone layer that separates it from the earth that acts as a capillary break and a ground water control layer. This stone layer should be drained and vented to the atmosphere— just as you would drain and vent a wall cladding.

Notice that in the perfect roof assembly the critical control layer - the membrane for rainwater control, air control and vapor control is located under the thermal insulation layer and the stone ballast (i.e. “roof cladding”) so that it is protected from the principle damage functions of water, heat and ultra violet radiation.

What happens where roofs meet walls?. The classic roof-wall intersection is presented in the figure to the left. Notice that the control layer for rain on the roof is connected to the control layer for rain on the wall, the control layer for air on the roof is connected to the control layer for air on the wall . . . and so it goes. Beautiful. And when it is not so…ugly.

In a beautiful bit of elegance and symmetry if you lie the perfect wall down you get the perfect roof and then when you flip it the other way you get the perfect slab. The physics of walls, roofs and slabs are pretty much the same—no surprise. 

This insight was shown into a whole generation of practitioners by the good building envelope specialists since back in the sixties. Where? Our friend Lstiburek is proud to have got it at the University around the eighties - he is a mechanical engineer. Others can not be that lucky: I found this piece of information by myself after finishing my architectural studies. It doesn't matter when - what matters is that, once you get it, you should never again forget it. Articles as clear as this one remind us this lesson. And there are many others at the Webpage... so please, go there and have a look, for your own benefit.

Large glass installation: miracles of vacuum lifting

The cladding world has gone mad. Facade units get bigger and bigger every year. Glass and panel dimensions are exceeding any reasonable measure. Who are to be blamed for it? Architects are partly responsible, for sure. The last gossip says Foster + Partners are designing a big company HQ in the Pacific coast with 15m long insulated glass units. Seismic movements in the region don't seem to refrain the architects from trying the impossible once again...

Henze-Glas DGU in the factory, before shipping to Glasstec 2010.  35 chaps are sitting on top of the 18m long glass unit

The monster DGU unit as shown at Glasstec 2010

But industry has also entered the race with pleasure. The 'jumbo size' glass, that is, the maximum dimensions of a glass sheet, was 6,000 x 3,210mm up to 2007. Since then a new jumbo size appeared: 9,000 x 3,210mm. Double glass units of 7,500 x 3,200mm or even 9,000 x 3200mm are now commercially avilable. Visitors at the last Glasstec Düsseldorf in October 2010 could see a huge insulated glass panel of 18,000 x 3,300mm! It had three layers of 10mm and weighed 4,5 tons. The producer was the large-dimensions glass specialist Henze-Glas from Hörden, Germany.

The Henze-Glas guys usually take care of the fabrication and supply of their glass units to jobsite, all in one, since large dimensions are not easy to handle. But the tricky question is: how can a facade contractor install glass or metal panel units around 10m long in a facade? This is the issue we are going to discuss in this post.

Mankind has been aware of the power of air pressure since Otto von Guericke’s demonstration of the Magdeburg hemisferes in 1656, when 16 horses were unable to separate two hemispheres which had been pumped free of oxygen. Boyle and Hooke, two good old names of physics, worked together to design and build an improved air pump. Their work was the origin of the Boyle's law: the volume of a gas body is inversely proportional to its pressure.

Early Wood's Powr Grip cups, beginning of 1960's

It wasn’t until the 1960s that air pressure power started to be used with vacuum lifting equipment for transporting and installing glass panels at construction sites. One of the founders of the guild was Howard Wood, who in 1961 designed and built the first Wood's Powr-Grip Valve Grinder. The tool consisted of a small, spring-action vacuum pump mounted in a wooden handle, opposite a rubber suction cup which attached to the flat surface of an engine valve. The demand for the unique little lifting tool grew, and a glazier friend from Wood's suggested that he develop a vacuum cup for handling glass. With support from friends, Howard began manufacturing vacuum cups for glass handling in 1963, and obtained a patent for his design in March 1966.

Hydraulica 2000 vacuum lifter

Vacuum cups were soon attached to cranes or lifting devices, and soon a new machine came into play: vacuum lifters. The generally smooth and gas-tight surface of glass means vacuum lifting devices are just right for the job. That is also the case with metal panels. These days even stone and prefab concrete panels are lifted and moved using air suckers.

Vacuum lift atached to a crane from Anver

Wirth GmbH is a German company that builds applications for vacuum lifting. The first version of their Oktopus lifter appeared in 1992. With devices like that installing large-format roofing, ceiling and facade panels made of sandwich, profiled sheets and glass has become much easier. Today's vacuum lifting equipment is based on individual suction cups attached to thin structural elements hung from a crane. These systems allow for the installation of vertical wall panels up to 12m long, or even horizontal roof panels up to 22m long.

The lifting devices can be hung from a crane, attached to a forklift, to a truck-mounted crane or to an elevated working platform - also known as cherry picker. Several hydraulic functions integrated into the vacuum lifters allow panels to swing up and down, be raised and lowered, twist left and right, or move forward and backward horizontally.

One of the best options is to use a vacuum lifter attached to a minicrane, also called a spider crane. Two European companies are well-known builders of minicranes: Unic Cranes from Scotland and Riebsamen from Germany, the latter being better known for their brand Glasboy. These minicranes can be used for mounting curtain wall units from the floor above, or for mounting glass in a skylight from the atrium below. The dimensions of a minicrane when its legs and arm are folded are really minimal, allowing the smallest minicranes to be lifted inside an elevator.

Minicrane Glasboy Frey 860 from Riebsamen

Some special devices can solve typical installation problems. One of them is the presence of overhangs in high level installation works. If a cantilevered slab prevents cranes from lowering their load flush with the envelope, an overhang beam provides an ingenious way of overcoming the problem. The  Libro 500 overhang beam for example provides a reach from suspension point to pad extension that allows glazing under overhangs up to a depth of 1,750mm.

All the options of cranes, minicranes and vacuum lifting devices can be checked (and hired, if you need them) at the UK webpage of GGR Group, a must see for those with a lifting problem to solve. If your site is in the US, then go visit the founding fathers, the guys of Wood's Powr Grip. If your doubts are more general or you want to have an overview of the crane and lifting world, have a look at Vertikal magazine.

Now, let us enter a slightly tougher issue. Which lifting device do I need for my load? How many suction cups are required considering the glass dimensions? Is suction lifting really safe? Depending on the application and the device, the load bearing capacity of a vacuum lifter varies between 250 kg and 1,000 kg. Vacuum lifting devices suitable for construction sites must be battery powered and therefore completely self-sufficient. A safety system inside the device constantly monitors the condition of the vacuum circuit and the batteries. Optical and acoustic warning signals give early indication of deviations from the normal conditions. A reserve vacuum system maintains load bearing capacities even in the event of a loss of power, so that there is enough time to safely deposit the load once an alarm goes off.

Robotic (left) and Libro 500 overhang crane lifts

European norm EN 13155 defines how to verify the load bearing capacity of vacuum lifting equipment. Load bearing parts are to be checked at three times the nominal load bearing capacity of the device. Vacuum lifters must be able to hold a load, in all positions at the end of the vacuum range, of at least two times the nominal load bearing capacity of the device. The combination of vacuum lifting device and suctioned load must, obviously, not exceed the load bearing capacity of the lifting equipment (crane / forklift / working platform). If you are interested in the same topic from the US, your document should be ASME Standard B30.20, addressing vacuum lifting and general materials handling products.

The load bearing capacity of the suction cups depends mainly on the following four factors: a) Size of the suction cups; b) Pressure difference between the level of vacuum in the suction cup and the ambient pressure; c) Load direction (vertical, parallel or sloped to the suction cup surface); and d) Surface properties and porosity of the suctioned material.

As a rule of thumb, a vacuum lifter used at a height of 1000m admits 10% less weight than the same device at sea level. The load direction is even more critical. If the load to be lifted is picked and released in a horizontal position, the maximum load capacity will be two times higher than if the load has be hold in vertical. Finally, the suction cup diametre defines the load bearing capacity of the system. The chart below, taken from Wirth Webpage, shows the relationship between suction cup diametre and load bearing at sea level for both horizontal (blue line) and vertical (red line) load directions.

We started this post talking about large glass units. The vacuum lifters you can find in the market have a maximum load bearing capacity of 1 metric ton at best. What if your glass is really large - and heavy? No problem, there is always a German wizard with a solution for that - regardless the names they give to their inventions. Bystronic Glass has built the world's largest glass vacuum lifter up to now: the Glasmaxilift 5000 is able to handle glass lites up to 15 meters in length and 5 metric tons using just air pressure. You got it. And Foster + Partners will have their huge glass installed as well.

Will transparent polymers kill glass?

A silent revolution is taking place these days. Due to a number of reasons, the glass position as the one and only transparent filling for curtain walls is being threatened. Who is the new kid on the block? Well, it has been around for a while, but it has grown bigger now: transparent recyclable polymers, commonly called thermoplastics.

Tokyo glass and acrylic cantilevered structure, Dewhurst Macfarlane

The attack of the polymers has already started, in the way barbarians entered the Roman empire: as an alliance. If you need a good bullet resistant glass you will end up in a laminate called glass-clad polycarbonate. Beware: the higher the bullet resistance requirements, the less glass there will be in the laminate. If you are after a blast-resistant curtain wall, the options are heavy PVB laminated glass (1.52mm or more of polyvinyl butiral layers) or glass combined with an ionoplastic interlayer as SetryGlas, in widths of 2.28mm or more. If you want to have overhead glazing or horizontal glass with live loads, there you will find the plastic companions again. Structural glass is in fashion, and you may want to achieve all-glass, non metal-supported transparent structures. When you do that, most of the time it's due to the help of polycarbonate sheets glued to the glass with transparent polyurethane interlayers.

Tokyo International Forum, glass canopy details. PMMA was not required structurally but it was added as a safety measure against typhoons and earthquakes.

This was the situation up to now. Glass is still in complete command if we want to clad a facade with a transparent, low U-value, durable and non combustible material. A curtain wall is still synonym for a glass curtain wall. But things are starting to change, and the big polymers suppliers have focused their attention onto the new frontier: to introduce Transparent Composite Facades (TCF) in lieu of Glass Curtain Walls (GCW).

I have taken the TCF name from a PhD dissertation in the University of Michigan, submitted by Kyoung-Hee Kim in 2009, and advised by Professor Harry Giles. The title is Structural evaluation and life cycle assessment of a Transparent Composite Facade system. I have also got ideas for this post from a presentation at the last Glasstec conference in Sept 2010: New materials for transparent constructions, by Eckhardt and Stahl.

Prism Cultural Centre in West Hollywood, California by PATTERN Architects. Translucent facade in resin-based composite polycarbonate. 3Form, an advanced material fabrication company specializing on resin-based composites, has collaborated on the facade solution.

The contenders to take the place of glass in curtain walls are four thermoplastics: polycarbonate (PC), polymethylmethacrylate (PMMA or acrylic), polyethylene terephthalate (PET or nylon) and polypropylene (PP). PET and PP are still lagging behind in the race, mostly because of their low stiffness and low ultimate strength. PET and PP have a Young's modulus 1/7 and 1/2 that of polycarbonate and acrylic, and their ultimate strenght is 1/3 that of polycarbonate and acrylic. The main advantages of polycarbonate and acrylic, on the other hand, is that they are 20 times less brittle than glass and their ultimate streght can be two times higher than glass (see data on the table below). So, we will focus on polycarbonate and PMMA / acrylic from now on.

From 'Materials and design' by Ashby and Johnson, plus www.matweb.com

Some may say these two materials have been around for too long to worry about them now. That is true, but doesn't tell the whole story. Let's call them by their best-known brands: polycarbonate is better known as Lexan, Makrolon or Danpalon. PMMA / acrylic is sold under the brands Plexiglas, Lucite or Perspex. Polycarbonate, either in solid or in multilayered sheets, has found a safe place in architecture as a translucent wall cladding, not transparent. A great example is the Laban Dance Centre in Southern London, a proyect by the Swiss architects Herzog & de Meuron with PC sheets supplied by Rodeca in Germany.

Laban Contemporary Dance Centre, London. Herzog & de Meuron architects

The cladding on the Laban Centre has four layers with a U-value of 1.45 W/m2°K, better than a low-e double glass unit. But present multiwall polycarbonate sheets, only 60mm thick, have already reduced the U-value to 0.85W/m2ºK, in the range of a triple glass with argon and low-e coatings. Polycarbonate can also reduce reliance on secondary solar control; the panels used at the Laban Centre feature dimpled inner skins that diffuse the light. Concerns over the durability and UV resistance of polycarbonate have now been reduced thanks to new film protection technology. Manufacturers now guarantee that the polycarbonate will lose no more than 1% of its transparency over the first 10 years. All this is fine, but this is still a translucent wall, a cladding material for gyms, dance centres, swimming pools or industrial buildings.

V-profile by Rodeca, a mullion in extruded polycarbonate

The next step for solid polycarbonate and acrylic is to become the transparent filling of a curtain wall, both in the fix elements and in the windows. A transparent composite facade (TCF) is well described by Kyoung-Hee Kim's dissertation as:

A composite construction consisting of a polymer double skin with an inner composite core, configured to provide a stiffer, safer, energy efficient and lightweight alterative to a glass façade system. 
This new 'glazing' system has spurred studies that evaluate the material performance of polymer and composites as a cladding material. The polymer skin has a sustainable characteristic due to its recyclability, which can help to reduce the environmental impact associated with raw material depletion and disposal.

Let's use an existing example to visualize the new TCF coming. Kalwall is a well known US translucent facade and skylight system, whose filling material by the way is a thermo-set polymer reinforced with fiberglass, with modified properties regarding UV-resistance and reaction to fire. The basic panel comes in standard dimensions and is installed as a very simple curtain wall unit sistem.

City & Islington College, London. Van Heyningen and Haward architects. Kalwall and glass facade.

The best version of Kalwall, pushed by Stoakes (the UK distributor) to comply with EU directives, has really low U-values by using thermally broken profiles combined with aerogel insulation. Even without aerogels you can have a 100mm panel, filled with polycarbonate fibres, with a U-value of 0.83W/m2ºK and a solar factor of 0.15. But, alas, it's still translucent. Now, imagine you replace the fiberglass reinforced thermoset at both sides of the panel with a high resistant solid policarbonate sheet, and add some intermediate transparent sheets to improve its U-value and acoustic performance. The outcome would be something similar to Kalwall - with the same Japanese paper-like rectangular pattern - but wholly transparent, no glass required whatsoever. Stop imagining, this concept is already being developed somewhere in Europe.

If we move to PMMA / acrylic, a similar story is being written these days. The material can be easily molded to achieve fuzzy shapes at a cost which is only a fraction of glass. In solid state, acrylic sheets can be cut and mechanized using laser cut CNC machines to provide extrusion-like profiles. A great example is the facade of the Reiss HQ building in London by Squire and Partners architects, with a machined PMMA external skin, and lit with an LED system at the bottom of each floor level.

Reiss London. Double skin facade with acrylic and glass. 

Reiss London. Acrylic milling process and finished facade panel.

Evonik Röhm, the company that owns the Plexiglas brand, was founded by Mr Röhm, the inventor of PMMA in 1933. After almost 40 years, the Olympic stadium in Munich by Frei Otto is still a forward-looking piece of architecture. The complex grid of steel cables was clad with a solid, tinted Plexiglas sheet to provide shelter against the summer sun rays. Need versatility? The monolithic thickness of PMMA is 200mm in a dimension of 3 x 8m - larger than glass, and you can even weld acrylic with hidden joints for bigger units. In the extrusion process you can obtain 25mm thickness, a width of 2m and no limit with length.

Olympic Stadium, Munich 1972. Frei Otto and Günther Behnish.

Curving a polymer is easy, but equally so is molding. Transparent facades as the one at the Liquid Wall in Berlin, the flagship store of Raab Karcher, would be a nightmare in glass, but are feasible in acrylic. Home Couture Berlin is a showroom for tiles and spa accessories. The store provides an ideal presentation platform for Raab Karcher and its joint-venture partners from the premium tile and bathroom fittings sector. The store functions as an elongated shop window for passers-by.

Raab Karcher flagship store, Berlin

The ‘liquid wall’ installation of milled Plexiglas appears as a vertical wall of water and serves as an eye-catcher in the Ku'Damm facade. The distorting lens makes the illuminated back wall oscillate as you wander past it. The shop window has been made after a 50mm Plexiglas sheet milled, formed and polished to get convex and concave surfaces. This form creates an effect of a moving room while walking by the window, as if the inside were a swimming pool.

If you need more inspiration, have a look at these four polymer materials suppliers: 3form, Panelite, Lightblocks and Krystaclear. These are all US-based companies at the verge of another revolution: they are not just suppliers, but also fabricators, engineers, materials researchers and high-end designers. They are in the front of the 'design to manufacture' concept that is changing the way materials are used. And they are all based in polymers.

When you can't win your enemies, join them. Another interesting product is Gewe-composite by the German glass supplier Schollglas. This laminated safety glass made with two sheets of glass and an intermediate 2mm polymeric membrane, can replace thicker glass-PVB laminates, is very easy to cold bend and does not stop UV radiation, thus making it very appropriate for winter gardens. The Amazonienhaus botanical greenhouse in Stuttgart is a good example of high UV-transmission, low-e coating composite transparent cladding. Another striking use of this composite in a bent application - non heat mold required - is the Mobile Formula 1 Event Centre for McLaren in the UK. The cold bent composite here integrates metal sheets and comes with a highly selective coating.

Amazonienhaus Stuttgart. Glass laminated panels with high UV transmission from Schollglas

Gewe-composite from Schollglas

There are still issues to solve and improve with thermoplastics before they can replace glass in curtain walls. Even if their mechanical properties are OK in general (impact resistance being great in particular) long term creep deformation is a clear disadvantage. The elastic modulus of extruded polycarbonate, for example, can be reduced to 40% after 1000 hours of constant loading. Regarding durability, polymers and acrylics offer a lower durability and weatherability under outside exposure compared to glass. Coated protections against UV have improved this, but there is still a way to go. The yelowness index (YI) measures discoloration levels under UV exposure, and values above YI-8 are not recommended for external use.

Another issue that must be integrated in the design is the thermal movement of plastics. The coefficient of thermal expansion of both polycarbonate and PMMA is 6 to 7 times greater than that of glass. Beware of the expansion pockets and frame movements! Abrasion resistance of plastics has improved with external coatings, but its value is still 2 to 4 times lower than that of glass. The biggest issue with plastics as external facade elements is probably their flammability or fire reaction. On one side, PC, PMMA and glass all conform to the flammability requirements of the ASTM codes. However, full compliance with the International Building Code (IBC) has to be checked case by case. The IBC limits the installation of plastic glazing to a maximum area of 50% of a building facade.

It is interesting to note that the U-value of a single layer of 6mm of transparent glass, polycarbonate or acrylic is practically the same: between 5,2 and 5,8W/m2ºK (glass being the highest). We get a similar result with g-value and light transmittance: uncoated glass and polycarbonate transmit the same amount of solar and visible radiation, while acrylic is slightly more transparent in both cases. But when we introduce selective coatings, glass performs much better than plastics in energy and visible light terms. Up to now, though.

Nobody knows the end of this story. Will new recyclable polymers completely replace glass as a transparent filling for curtain walls? It seems uncertain, but at least I would vote for a future co-habitation of both materials. If I had money to invest in the stock market, I would buy plastic shares rather than Saint Gobain ones. Well, don't follow my advice too quickly: Saint Gobain is investing in plastics right now, so think twice...