There is no doubt that glass is an essential component of building facades, and it’s very good at doing certain things, such as letting daylight and solar radiation in and keeping wind and rain out.

However, conventional glass performs terribly when it comes to controlling heat gain and loss and keeping bodies comfortable against temperature extremes.

But we solved all those issues when we invented ‘smart glass’ right? Surely, our glass box skyscrapers wouldn’t be tenable unless those issues were a thing of the past. Well actually no, that’s not case. The only reason unshaded glass box buildings work is that we pump fossil fuel energy into them in the form of air conditioning to keep them comfortable. Even then, there are limitations.

Thermal design engineers will tell you that with double (or triple) glazing and smart Low-E and heat absorptive and/or reflective glass that they have it all covered, and that glass alone can be used successfully. Except it can’t.

Glass alone cannot cover all contingencies, and the hotter the summer sun, the less successful it is. In winter in cold and even temperate climates high transmissivity (untinted) highly insulated glass units with Low-E coatings are typical and work very well. Problem is, in summer unshaded glass lets a lot of heat in, and the very factors that work for the building and occupants now works against them letting heat in and keeping it in, very efficiently overheating buildings and people all the better for the air conditioning engineers (whose fees are often based on the size of the air conditioning plants) and suppliers (who designed some of the major software tools engineers commonly use to design buildings).

So to overcome this, heat absorbing and reflective glasses were developed and to greater or lesser extent are used in all glass facades in climates where there is any bite at all in the summer sun.

Herein lies some of the problem; heat isn’t just ‘heat’. Solar energy does enter buildings by conduction (efficiently offset by insulated glass units or IGUs) from the atmosphere but the majority enters via direct and diffuse solar radiation.

If we make the glass reflective enough to stop enough solar radiation to avoid overheating of unshaded all-glass facades, we create strong rogue reflections that cause accidents and literally melt cars. So we wind the reflectiveness back and bump up the tinting levels to absorb the radiation in the façade and this is where the next problems begin.

Some years ago I was approached by a very senior public servant responsible for the oversight of a major inner urban capital city redevelopment project, including a number of major residential towers. He asked me why, when all of the engineers were telling him these market leading residential towers were  fine, he had a large group of unhappy purchasers and their lawyers complaining that the buildings were too to hot for comfort, even with air conditioning and even in winter.

I told him the only way that would be happening is if the buildings were all glass, unshaded and probably deeply tinted facades.

He paused for a second and replied ‘you just described the buildings exactly’ and then asked ‘how come in a few seconds you can diagnose the problem when all these expensive and supposedly talented engineers have signed off on these buildings?’

I told him you have to understand physiological comfort and how the typical thermal design software works- or in fact doesn’t work.

The human body is up to four times more sensitive to radiant heat than any other form of heat. In fact, we are more than two times more sensitive to radiant heat than all of the other heat loss or gain pathways (convection, conduction, respiration, and evaporation) combined. The body is highly sensitive to even small changes in radiant temperatures; that’s why we love radiators, heated floors and mass surfaces in buildings to retain heat and keep us both warm and cool.

But as you tint glass, it progressively absorbs more and more heat itself and it becomes in effect a large plate radiator. The darker the tint, the more heat it both absorbs and then eventually re-radiates even when it’s double-glazed, such as in IGUs. Its important to note that this doesn’t cause the room air to heat up much, so its not picked up by the room temperature sensors of the HVAC, but it does heat people up easily and effectively, even through clothes.

Even a few degrees of radiant temperature can make a room very uncomfortable. A radiant temperature increase (or decrease) of  five degrees can make the room feel eight degrees hotter (or colder) without changing the temperature in the room.

What’s more, unless thermal design engineers use specialised simulation tools to specifically test this during the design stage, it will go unnoticed. Most software tools don’t assess this at all because they are focussed on building air temperature and HVAC design, not occupant comfort. Why do I make this distinction? Well, if you are in a commercial office building with big floor plates, the only people likely to experience this are within four to six metres of the curtain wall. As bad as this is in a commercial setting, in apartments those distances comprise the entirety of all typical room sizes, so there is no escape. Hence lawyers get used to sort out problems after the fact, when intelligent design could have eliminated the problem during design stage.

I’m a curious person and self-admitted geek, and I have had a lot of pleasure over the years playing with a little toy called an IR (infrared) thermometer. Anyone can buy them for around $80 and they open your eyes to the normally invisible (but easily sensed by the skin) world of radiant temperature. IR is an enlightening tool and one being used increasingly to determine building faults after construction especially in insulation defects.

A few years ago I headed up the team of consultants that implemented the world’s first coordinated Sustainability Guidelines for the Emirate of Abu Dhabi. The guidelines integrated sustainability into the core of all building-related development policies, regulations, building codes, New Capital City Local Plans, and helped in the development of the Estidama Green Building Rating tool that among other things mandated external shading on buildings across the Emirate. This was achieved by presenting evidence collected from existing buildings in Abu Dhabi and Dubai.

You might not think there are very many parallels between Australian climates and the extreme temperatures of the Middle East, but all the temperature samples I took were in winter. Winter in UAE is very similar to a mild to hot day in any capital city in Australia; I even have photos of January flooding and hail on the window sill of a Dubai hotel on one trip.

Let me share my experiences of a building I visited in mid-winter (January) in the desert, midway between Dubai and Abu Dhabi. It was comprised of two 12-storey office towers of azure blue (heat absorbing) reflective (silver blue mirror look to the outside) double glazed IGUs linked by a three-storey entry podium of clear IGUs. On that clear sunny and mild day, at 9 am the external air temperature was 21 degrees and inside, the HVAC was set to what felt like a very chilly 21 degrees. I wondered why they had set the air conditioning so low, so I took a few IR measurements. The internal skin temperature of the blue IGUs was 33 degrees even though it was only 21 degrees outside. Anyone within four to six metres of the walls in sun would have been hot on the side of their body facing the glass and cold elsewhere. Their average temperature would be in the comfortable range, but their body would be under stress to feel that way. It would be like having heaters running on one half of the body and the internal air conditioning running at the same time on the other half.

At midday, things got even more interesting. The outside temperature had risen to 35 degrees, the inside air temperature was still 21 degrees, but the outside skin of the building in the sun measured 65 degrees and the inside skin temperatures of the blue IGU in the offices were 35.4 degrees. Interestingly, the clear insulated glass podium was an astounding 37.2 degrees – because it was not reflective like the blue IGUs, it absorbed more heat. Amazingly, the temperature of the dark brown concrete pavers on the ground in the carpark in full sun was only 50 degrees! I would also note that the solar intensity in UAE at that time of year is similar to Australia in summer.

Anybody occupying this structure (and any other like structure in any city that experiences equivalent temperatures) and sitting within a reasonable distance of the glass curtain wall would be very uncomfortable and wouldn’t know why. Their bodies would be in thermal stress, and they would feel stressed without knowing why, would possibly be cranky, and would have difficulty concentrating. Over long periods, this constant stress is likely to affect an individual’s immune system. There is no doubt in my mind that unshaded glass buildings in our climate, apart from their inherent energy inefficiency, represent health and productivity draining building design.

Yet the solution is simple and passive. External shading, either fixed or operable, solves all of these issues and allows less deeply tinted glass. While this can create nuisance glare in offices that requires management, in multi-unit residences, shaded windows and covered outdoor entertainment areas open up a multitude of design possibilities that can create much higher levels of energy efficiency, more comfortable thermal conditions and keep the lawyers at bay.

All glass buildings, especially if they incorporate recent innovations in 3D printing of structural glass elements and even if they contain building integrated photovoltaic (BIPV) panel glazing, will have no part in a sustainable city of tomorrow. Predominantly glass buildings will only become part of the solution delivered by sustainable cities if they use smart glass elements in smart ways that combine high levels of insulation and low-E surfaces with external shading or some other smart way of stopping radiant heat gain through and of the glass elements themselves in summer while allowing passive heat gains when desirable.

Unshaded all glass buildings as we know them now, have no place in sustainable cities of the future.

Cover image: Al Bahar Towers, Dubai
  • Great breakdown of a complex issue.

  • I don't share David's view of the future of sustainable "all glass" buildings. Whilst solid walls and external shading devices perform important functions the most important aspect of façade development over the past 50 years has been performance glass. Numerous factors continue to drive this evolution:

    Daylight Access – Full height glazing provides much needed natural day light particularly for building with deep floor plates.

    Internal shading devices – reflect solar radiation back through a façade and can respond to different levels of solar radiation.

    Active facades – treat façade zones as micro-climates, double facades, pelmet extraction and other methods can be used to improve energy efficiency.

    Advanced glass technology – today triple low E coating can cut solar radiation by 75% while achieving 50% visible light transparency, with a reflectivity of less than 20% "Amazing".

    Future trends – clear photo-voltaic coatings, electro-chromatic, gas-chromatic and thermal-chromatic glazing will become commercially viable.

    "All glass" buildings will continue to grow in popularity and technologies will continue to evolve to make them increasingly energy efficient.

  • Great piece, David. I know all about this, and from what I can see, no one is talking about it except for you and John Straube. It's a huge mystery to me why architects and engineers aren't more aware of radiant properties of glass, it's not very difficult to understand.

  • Great article, David Baggs. Thanks for writing clearly on such an important topic. Gives us some nice insight into why we are producing such rubbish. Of course overall power consumption would be dramatically improved using a building physics modelling program like the Passive House PHPP. But also your comments about occupant comfort are fascinating. Yesterday I was in a 5 star hotel in Sydney. Early morning temp outside was around 17 degrees. On the single glazed glass it felt about 45 degrees. Crazy!
    Also I am very interested in the fact that human comfort is impacted by differences in temperature. In our passive house I have noticed some things like a) a concrete floor (which is nothing to do with passive house design) has a dramatic cooling effect in summer, due to thermal conductivity of the material. And b) the stable temperatures and lack of drafts immediately causes the body (and mind) to relax.

    We experience all of this stuff and hardly anyone writes about it. Nice piece!

    • I thought concrete floor had quite a bit to do with passive design.
      Absorbing solar during winter, radiating back into house at night?

    • Thanks Fiona…good example…this is EXACTLY the effect I was referring to,

      " In our passive house I have noticed some things like a) a concrete floor (which is nothing to do with passive house design) has a dramatic cooling effect in summer, due to thermal conductivity of the material"

      only in reverse, passive radiant cooling….that is the effect to achieve…has little effect on the air temperature…but major effect on people! The reverse is true of unshaded glass- heat it up…massive sized radiator…very uncomfortable.

  • Very interesting article.
    Last year I was asked for recommendation to fix largest facade panels made from Corian Nocturn – Afrykarium ZOO Wroclaw using secret fixings. Panels were 5600mm high and 3600mm wide. Thermal linear expansion was very big and reached 28mm between winter night -20C and summer day +35C. We started to measure panel temperatures with IR thermometer, air temperature and the temperature of air in the cavity. Once I have noticed that higher air temperature does not have direct correlation with panel temp. There were days (for example cloudy), when air temp was higher and panel temp was lower in relation to the other days, when air temp was lower and panel temp was higher (and the sky was clean).
    I have realised that the air temperature is only one factor here. Most important is UV factor.
    Summarizing when the sky is covered, air temp is high there is less UV radiation than next day, which may have lowe air but higher UV radiation.
    On the other hand I can confirm, as a person working in ventilated facade branch (stone, ceramic, concrete…and glass), that the market goes direction to all glass facade (if I understand your definition).

  • David, interesting and great article!
    You are spot on!

  • An excellent article to help more architects and users understand the way 'simple' unshaded glass facades work, and especially to explain why just having 'smarter' glass will not solve most of the problems associated with overuse of windows.

    But not all the glass in a facade has to coincide with 'windows', which may be defined as sources of light (mostly outlook). Glass is also a very useful cladding panel for the 'opaque' parts of walls. Glass in curtain walls may be backed by remarkable levels of bulk insulation, and/or more complex insulating assemblies including low emissivity cavities. A competent facade designer could easily design a glass clad wall assembly which outperforms more traditional buildings for both summer and winter.

    Glass also happens to weather well, provide a near perfect substrate for emerging thin film and nano photovoltaic technologies, and can be relatively easily disassembled, repurposed and recycled.

    In short, the problem is the title of the piece. All glass facades will actually survive, because glass has other advantages in facades besides being a window.

    Glass is also made from some of the most abundant resources on the planet

  • An astute article, relevant to the blight of all-glass buildings…

  • Glass is responsible for a huge amount of improper energy use in buildings. Any input to acknowledge this is welcome.

  • Would triple glazed argon filled facades work?

  • The best article I have read in many years.
    It shows that the late Harry Seidler,architect of many major office buildings in Sydney knew what he was doing when he shaded external glass.
    Why has the profession learnt nothing.I suspect that the so-called curtain wall is easy and cheap to design and thus finds currency in architectural offices where work is done for very low fees and design solutions have to be found very quickly and cheaply.

    Where could I get a printed copy of the article by David Baggs?

    Guy Maron AM LFRAIA

  • great article
    articulates in a very clear pragmatic way what we all sort of knew but have not seen described so succinctly.