Acoustic properties of glass: not so simple

At Arup, working with different specialists creates many opportunities to learn from each other. Sometimes one forgets that engineers who know everything about dark matters as climatic loads in glass or intricacies of structural silicone may not have a clue about the acoustic performance of a window.

That's why these questions keep coming to my desk (remember I'm an incurable generalist in the façades world): what effect does glass thickness have in the acoustics of a double glass unit? Or what matters more in the acoustical performance of insulated glass: the thickness in a monolithic pane, the effect of lamination or the dimension of the cavity? Here you will find some graphical answers to these questions. As usual a number of hidden surprises will come out from the data mining.

Let us start by reviewing two concepts that are paramount to measuring glass performance against noise: loudness (in particular sound pressure level, the decibels thing) and frequency (the Hertz, not related to car rental)

1/ Loudness: sound intensity, sound pressure and sound pressure level

From physics to applied acoustics in buildings. No pain, promised. Loudness is an intuitive concept: a loud noise usually has a larger pressure variation and a weak one has a smaller pressure variation. Depending on what we are looking for - the cause, the effect or the perception of noise - we use different variables and units:

• Sound intensity refers to the cause of noise (not of our concern, only of interest for acusticians). It measures energy flow at the source, so its unit is W/m2.
• Sound pressure refers to the effect of noise as a wave impacting any given surface, that is, noise as energy being transfered through air. Not of our concern either, more for physicists. Its unit is the Pascal or N/m2 (1Pa = 1N/m2). 
• Sound pressure level or SPL (here comes the fun) refers to the perception of noise in humans as it can be "read" by our ears. So SPL is what matters to us, poor construction buddies. For ease of numbering SPL is measured in decibels (dB). A dB is a dimensionless unit used to express logarithmically the ratio of a value (the measured sound pressure) to a reference value (the lower threshold of hearing). Decibels are used since sound pressure level expressed in Pa would be too wide. 0 dB (the lower threshold of audition for humans) equals 0,00002 Pa; whilst 140 dB (the upper human threshold or threshold of pain) equals 200 Pa. This is a range of 140 against 10 million. But logarithms are not "natural" to understand, so some examples will be of help.

Sound intensity, sound pressure and sound pressure level are obviously related, but they measure different things and they should not be confused. The table below, taken from the very useful Sengpiel audio webpage provides some tips for getting it right, at least conceptually:

SPL variation (left column) related to sound pressure (field quantity) and sound intensity (energy quantity)

Lessons from the table above:

• A raise in sound pressure level (SPL) of 3 dB equals an increase in sound pressure (field quantity) of 1.414 times, and (everything else being equal) it comes as a result of doubling the sound intensity (the source of sound).
• A reduction in sound pressure level measured inside a room of 10 dB equals a reduction in sound pressure of 3.16 times, and it comes as a result of dividing the sound intensity (noise generated on the outside) by ten.

A typical opaque façade (not glass) can have a sound reduction index (a reduction of SPL) of around 40 dB. This means that if the SPL measured at the street is 70 dB, inside the façade one would perceive only 30 dB. Up to here, just arithmetic. 

Now, if the sound reduction index of the façade could be raised from 40 to 43 dB, the perceived noise coming from the street would equal that of reducing the source of noise by half. Even more, if the façade could be acoustically improved so that its sound reduction index raised from 40 to 50 dB (difficult but it can be done), the perceived noise coming from the street would equal that of reducing the source of noise (sound intensity) by ten: ten times less cars in the street, ten times less people celebrating the victory of their football team outside.

Expected sound pressure levels for different noises and their equivalent sound pressure and sound intensity. Source: Sengpiel Audio.

We got the point: sound pressure level measured in dB (sometimes indicated as dB-SPL) is critical for architectural physics - a small variation can make a lot of difference. But loudness (sound expressed as pressure variation) is not the only story. Noise - what we want to avoid inside our buildings - is the mixture of sounds of different "quality", some are bass, some are treble. Is our façade or our glass pane capable of stopping each of these "noise qualities" in the same percentage? Could an envelope act as a barrier for bass and a filter for treble? What do bass and treble have to do with noise?

2/ Frequency of sound

Sound is the quickly varying pressure wave travelling through a medium. When sound travels through air, the atmospheric pressure varies periodically (it kind of vibrates). The number of pressure variations per second is called the frequency of sound, and it is measured in Hertz (Hz) which is defined as the number of cycles per second.

Graphic representations of a sound wave. (A) Air at equilibrium, in the absence of a sound wave; (B) compressions and rarefactions that constitute a sound wave; (C) transverse representation of the wave, showing amplitude (A) and wavelength (λ). Source: Encyclopaedia Britannica.

The higher the frequency, the more high-pitched a sound is perceived. Sounds produced by drums have much lower frequencies than those produced by a whistle.

The unit of frequency is the Hertz (Hz). For a sound vibration to be audible to human beings the object must vibrate between 20 and 20,000 times per second. In other words the audible sound has a frequency of between 20 and 20,000 Hz.

High-pitched sounds (treble) have a frequency much greater than bass sounds. The treble frequency ranges between 2,000 and 4,000 Hz while the bass range from 125 to 250 Hz. 

Above: measure of loudness (wave height). The higher the louder.
Below: measure of frequency (wave length). Bass sound has long waves, treble has short waves.

Bad news: frequency and loudness are interrelated in the human ear. The range of 20 Hz to 20,000 Hz is called the audible frequency range - we know this already. But the sounds we hear are a mixture of various frequencies, and we don't perceive all of them with the same clarity. Let's see what the implication of this is.

The entire audible frequency range can be divided into 8 or 24 frequency bands known as octave bands or 1/3 octave bands respectively for analysis. An octave band is the band of frequencies in which the upper limit of the band is twice the frequency of the lower limit. Any particular sound or noise can be represented as a number of 8 (or 24) sound pressure levels in the frequency bands, as illustrated by the diagram below.

A real sound shown as a combination of different sound presure levels, one per each of the 24 frequency bands. Column width: 1/3 octave band (24 in total). Column height: SLP at each frecuency band, measured in dB.

The response of the human ear to sound is dependent on the frequency of the sound. The human ear has its peak response around 2,500 to 3,000 Hz and has a relatively low response at low frequencies. Hence, the single sound pressure level obtained by simply adding the contribution from all 1/3 octave bands together will not correlate well with the non-linear frequency response of the human ear.

This has led to the concept of weighting scales. The following diagram shows the "A-weighting" scale:

Reduction of SPL (in dB) at frequencies below and above 2000 to 3000 Hz to reflect the frequency response of the human ear.

In the "A-weighting" scale, the sound pressure levels for the lower frequency bands and high frequency bands are reduced by certain amounts before they are being combined together to give one single sound pressure level value. This value is designated as dB(A). The dB(A) is often used as it reflects more accurately the frequency response of the human ear. 

Other, less used weighting scales, are dB(B) and dB(C). The decibel C filter is practically linear over several octaves and is suitable for subjective measurements at very high sound pressure levels. The decibel B filter is between C and A. The three filters are compared below:

Noise filtering at different octaves of frequency applying decibel filter scales A, B or C.

That was enough for theory. Let us now see how all this affects the performance of glass as a real acoustic barrier.

The four hand-sketched graphs shown here below are all taken from the first edition of a great book called "Detailing for acoustics", written by Peter Lord and Duncan Templeton. There are three editions by now and I highly recommend buying one if you are an architect interested in acoustic issues applied to buildings.

3/ Glass thickness effect

The sound attenuation of any material depends on its mass, stiffness and damping characteristics. With a single glass pane the only effective way to increase its performance is to increase the thickness, because stiffness and damping cannot be changed. The sound transmission loss for a single glass pane, measured over a range of frequencies, varies depending on glass thickness.

Thicker glass tends to provide greater sound reduction even though it may actually transmit more sound at specific frequencies. Every glass pane thickness has a weak frequency value; that is, a frequency for which that glass is less 'noise absorbent' than for the others. That value is known as critical frequency. See the graphic below:

Sound reduction (in dB) measured at different frequency bands for glass panes of different thickness. Source: Detailing for Acoustics, Lord and Templeton.

A 4 mm-thick glass is rather transparent (poor attenuation measured in dB) for high frequencies at the range of 3500 Hz; 6 mm-thick glass is poor for frequencies around 2000 Hz; and 10 mm-thick glass performs bad at 1300 Hz. The higher the mass the less of a problem critical frequency appears to be: 25 mm-thick glass has no weak point as it can be noted from the graph above.

An insulating glass unit built with two panes of the same thickness experiences the issue of critical frequency: it is said that the two panes vibrate (resonate) together at that frequency, thus reducing the glass overall acoustic performance.

For this reason we recommend using different thickness in a double glass unit. A 6-12-4 mm glass will absorb more sound at high frequencies of 2000 Hz (claxon noise) than a 6-12-6 mm glass, in spite of having less mass. On the other hand, at lower frequencies between 125 and 250 Hz (traffic noise) this is not the case: a 6-12-6 mm glass reduces sound more effectively than a 6-12-4 mm glass. At low frequencies sound attenuation is directly proportional to mass.

4/ Laminated vs. monolithic glass

A laminated glass will attenuate sound transmission more than a monolithic glass of the same mass. See the graph below:

Sound absortion of monolithic (solid) glass compared to laminated glass with the same mass. Source: Detailing for Acoustics, Lord and Templeton.

A laminated glass of 2+2 mm reduces sound at high frequencies considerably more than a monolithic glass 4 mm-thick (that's 8 to 10 dB of additional attenuation). Why? because the critical frequency effect disappears due to the sound damping provided by polyvinyl butyral (the soft interlayer used to permanently bond the glass panes together dissipates energy by vibration). The same applies to the 3+3 mm laminated against the monolithic 6 mm. In contrast, at low frequencies (traffic noise) the effect of butyral is less pronounced, although it is still positive (about 2 dB increase).

5/ Air cavity effect

Surprise: a standard double glazed unit does not reduce sound transmission much more than a monolithic glass. What matters is the thickness of the air space between glass panes, but only for really wide cavities.

Effect of air space width on the acoustic performance of double glazing. Source: Detailing for Acoustics, Lord and Templeton.

The acoustic attenuation of a 6-12-6 mm glass is generally superior to that of a monolithic 6mm-thick glass, but only by 2 or 3 dB, and still there may be low frequency bands where the DGU performs worse. Of course if we compare a 6 mm-monolithic with a double glazed 12-6-10 mm, the sound reduction is much better at the double glazed unit.

What really matters is the width of the air space, not the small one found at double glazing but the one of a double skin. The ideal cavity width to boost sound attenuation is 200 mm. For widths less than (or greater than) 200 mm the effect is less noticeable (although a wide air space will always perform better than a narrow one). A double glazing with 10 mm air space performs almost like a 20 mm airspace.

6/ Combined air cavity & glass thickness effect

The conclusion comes in the last graph: a combination of large thickness, different one between the two panes and wide air space distance (even better if we use laminated glass) provides the maximum noise attenuation. We can reach up to 45dB.

Combined effect of glass thickness and air space on the acoustic performance of double glazing. Source: Detailing for Acoustics, Lord and Templeton.

To achieve this with a conventional double glazing width (about 28-35mm only) we have to employ an acoustic interlayer or a sort of resin between two panes in a laminated glass combined within a DGU. These acoustic interlayers or resins dissipate sound waves much more than two or three PVB interlayers as in a typical laminated glass. Some brands of enhanced acoustical laminated products are:

• Pilkington Optiphon.
• Saint Gobain Stadip Silence.
• AGC Thermobel Phonibel.
• Viracon Saflex SilentGlass.

SGG Stadip Silence effect as part of a double glazed unit. Other brands perform similarly. By the way, the scale below is not frequency but loudness (it measures dB). Taken from Saint Gobain Stadip Silence brochure.

What about the effect of using argon or krypton instead of air? In theory, a higher density gas in the space between panes should have a positive effect on acoustical performance. Comparison testing of standard symmetrical insulating units indicates though that common gases as argon have virtually no increased effect on sound attenuation ratings. While some improvement was noted at some frequencies, resonance effects actually became more pronounced.

7. Some useful values

Rw index: The Rw index or sound reduction index (expressed in decibels) measures, in just one number, the acoustic performance of a specific glass unit. The higher the Rw index, the better the level of acoustic insulation offered by that glass composition. The Rw index of ordinary double glazing is around 29 dB whereas a good acoustic interlayer offers an Rw index of around 50 dB.

Rw is a single figure rating for the airborne sound insulation of building elements (not just glass). It includes a weighting for the human ear and measures actual sound transmittance. Rw is measured in a laboratory, not on site (the site-measured equivalent value has the Egyptian denomination of DnT,W). The Rw value is merely an average simplifying mutual comparison of various building components. That can be confusing some times. Two glass units can have the same Rw index while one of them performs well at low frequencies and bad at high ones, and the other one performs just the opposite.

C and Ctr factors: To slightly avoid this issue two spectrum adjustment factors: C and Ctr, have been added to modulate the Rw average. For sound waves featuring high frequencies, the factor C is added to the Rw value. For lower frequencies, factor Ctr needs to be added. The acoustic behaviour of a building component is hence defined by three numbers: Rw (C, Ctr). A building component with the values Rw (C, Ctr) = 40 (-1, -4) provides an average insulation performance of 40 dB. For higher pitched sounds the sound insulation is lessened by 1 dB (39 dB) and for lower pitched sound sources it is lessened by 4 dB (36 dB).

The table below, extracted from Saint Gobain, helps showing how these three numbers apply to different laminated units with acoustic interlayers:

Sound reduction index values for several laminated glass units with acoustic interlayers. The thickness shown at the right column is the total one. 13 mm means 6 mm + 6 mm + 0.8 mm interlayer. Taken from Saint Gobain Stadip Silence brochure.

C takes into account medium and high frequency noise sources such as TV, music, loud conversations or aircraft noise a short distance away. Ctr takes into account medium and low frequency noise sources such as urban traffic noise or aircraft noise a long distance away.

Pink Noise:  Expressed in dB(A), this is an assessment of the sound insulating properties of a building material over specified standard frequencies, which represent general activity noise when equal levels of power are applied at each frequency. So, in pink noise each octave carries an equal amount of noise power. Funnily: the name arises from the pink appearance of visible light with this power spectrum.

Ra:  Ra is the abbreviation for the sound reduction index when the spectrum adaptation term C is applied to the single number weighted sound reduction index (Rw), using pink noise as a sound source.

Ra,tr:  Ra,tr is the abbreviation for the sound reduction index when the spectrum adaptation term Ctr is applied to the single number weighted sound reduction index (Rw) using pink noise as a sound source.

So far so good. Acoustic performance of glass should now be less of a dark matter for us. But this is not all: remember that detailing to achieve a proper air tightness between glass and frame will always be required! Loose gaskets can severely harm the best glass selection for acoustics...

Le Corbusier: a French lesson on 'Murs neutralisants'

Is there still anything to discover about Corbu? Hidden inside his extensive writings - and sometimes evident in his projects - Le Corbusier seems to have individuated five building-physic concepts, were these his own developments or not:

• natural ventilation (aération naturelle)
• natural lighting (éclairage solaire)
• solar control (brise soleil)
• thermally active facade in opaque or glazed walls (mur neutralisant)
• internal air conditioning (respiration exacte)

These made for him part of "the modern techniques" (les techniques modernes). Funny they are precisely five points - but this is a coincidence...

Sections showing sun lighting at the Zurich Sanatorium (left) and the skyscraper of Quartier La Marine (right). LC Ouvre Complete 1946-1952.

Ventilation scheme in standard houses for workers. Le Corbusier.

Corbu seems to have always paid attention to the first two items: natural ventilation and natural lighting, be it at his writings and in his projects (the images above are a good example). He was influenced by and a promoter of the hygienist culture of his time in architecture. But in the other three points he went beyond the usual and became the first modern architect really interested in mixing passive and active methods of energy control. Regardless the accuracy of his concepts from a physical point of view, this is a remarcable feat.

Seen from today's distance, Le Corbu was a prophet not only in architectural language and urbanism but also in identifying the importance of a right combination between building design (passive measures, mass, ventilation etc) and active systems (air control and mechanical distribution) for achieving comfort in buildings. But his strength as prophet may be precisely in his weaknesses as master builder. Had Le Corbusier had a rigurous technical knowledge in the five points above, his predictions would not have been as telling for the coming generations as they were.

The two active concepts - the latter of the five, which Corbu developed between 1926 and 1933 - were the 'Mur neutralisant' and the 'Respiration exacte'. In my opinion, Corbu had it clear that both were intended to work together because they were complementary. Corbu tried to implement them at the Cité de Refuge in Paris and the Centrosoyuz in Moscow, unsuccessfully in both cases. Put simply, his intention was to obtain an internal comfortable environment all year round and in all climates.

Corbu coined two names but in fact three aspects were required simultaneously:

1. A very high air-tightness through the envelopes - hence the idea of sealed glass (or sealed opaque walls) being part of the 'Mur neutralisant' concept. The intention was -rightly - to avoid air and heat flowing from inside to outside and viceversa.
2. A mechanical system of controlled ventilation capable of adjusting both air temperature and humidity - that is, a rough description of an air conditioning system. This was the idea behind the 'Respiration exacte', which in fact comes from his colleague the engineer Gustave Lyon.
3. Finally, in order to allow glazed facades to act as external thermal envelopes, an active device that neutralized energy flows (both in winter and in summer) through glazed surfaces: the 'Mur neutralisant'. This acted mainly as a barrier avoiding heat to flow inside-out during winter and outside-in during summer. Corbu delevoped an existing, earlier device (water heat radiators installed between two parallel glazed walls) into a more ambitious idea. By inserting air pipes around a sealed double glazed cavity he suggested that treated air could be blowed, warm in winter and cold in summer, so as to neutralize the outer conditions. This would allow the 'Respiration exact' system to maintain a constant internal temperature of 18ºC.

Respiration exacte and Mur neutralisant as they were envisaged for the Centrosoyuz project in Moscow

'Mur neutralisant'  and 'Respiration exacte' diagram, 1929, as published in 'Précisions'

 Where did these ideas come from? Were they revolutionary or not? The 'Respiration exacte' concept seems to be a somewhat poetic version of the mechanical ventilation system used by Gustave Lyon at the Pleyel Theatre and in other French auditoria. It seems that Lyon called his system 'Aération ponctuelle', not so different from 'Respiration exacte'. In his drawings (see below) for the Centrosoyuz project in Moscow Corbu used the engineer's name in some of the details: "Aération ponctuelle. 80 litres-minute d'air à 18ºC par personne avec régénération dans circuit fermé; système Gust. Lyon" (Point ventilation. 80 litres/minute of air at 18ºC per person with regeneration in closed circuit; system Gust. Lyon).


Image of the Centrosoyuz project for Moscow with references to the Mur Neutralisant and the Aération Ponctuelle (1928)


Lyon himself was not a mechanical engineer but an specialist in acoustics - to be precise, an expert in piano sound and its mechanisms. It seems that he entered the area of mechanical ventilation after having designed the acoustics for many concert halls, and probably having experienced the discomfort of those closed unventilated spaces. But he was not a climate engineer as Willis Carrier or the men at the American Blower Company. His intuitions on air conditioning came from his practical experience at improving the ventilation of the Salle Pleyel or the Trocadero Palace after he had gained reputation as an acustician. Lyon was 70 years old in 1927 at the opening of the Salle Pleyel in Paris: not exactly an eager engineer in contact with the novelties from New York or Chicago. In any case, the idea of an enclosed inner space - sealed from the outside - with some mechanical devices to control air temperature and humidity was not new, and it could very well be passed from Monsieur Lyon to his young new client, l'architecte Charles Eduard Jeanneret (Corbu).

Centrosoyuz plan and external view. Notice the double glazed wall with sliding windows.

In relation to the 'Mur neutralisant', I fully agree with Reyner Banham's position at his legendary book "The architecture of the well-tempered environment". Le Corbusier had experienced a similar concept for the windows in his Villa Schwob in Switzerland by 1916, so that this seems to be his own development. In the Ville Schwob very large windows (one of them two storeys high, see below) were designed in two layers, with heating pipes between them, to prevent down draughts. In t he same details for the Centrosoyuz (1929) he referred to them as "Murs neutralisants de verre ou de pierre; circuit fermé rapide d'air sec chaud (hiver) ou froid (été); systéme L.C. - P.J." (Neutralising walls in glass or stone; quick closed circuit of dry hot air (winter) or cold (summer); system Le Corbusier - Pierre Jeanneret). The 'Mur neutralisant' was his baby; the 'Respiration exacte' was Lyon's.

Villa Schwob, La Chaux de Fonds 1916. Notice the large window pane above the garden entrance: this was a double glass with an intermediate radiator system.

The glass solution was typical to the other Corbu 'Mur neutralisant' schemes; that for the opaque walls in Centrosoyuz reveals another great Corbu's intuition. An enclosed air cavity between two walls of pink tufa stone from the Caucasus (a volcanic, porous stone) would have been a very adequate thermal solution for opaque walls in Moscow, even if there was no hot air circuits inside. The Russian client ultimately dismissed the 'Mur neutralisant' system because of the lack of technical justification. At least they kept the double glazed wall, of which there were some previous examples built in Moscow (see the images of Zuyev Workers Club right below, a project by Ilya Golosov finished in 1926).

The Zuyev Workers Club. Ilya Golosov, 1926. A precedent of double skin glazed walls in Moscow before the Centrosoyuz.

But the opaque wall as it was designed, even without blowed air in the cavity, would have been much better in terms of insulation than the one-layer stone wall finally built, with a thickness of 40cm.

Could these two concepts, the 'Mur neutralisant' and the 'Respiration exacte' really work? Were they logical? The answer depends on the system.

It is not clear to me what Le Corbusier meant by 'Respiration exacte' (I suspect it's not just me; Corbu was not an engineer and he did not describe the concept in depth). The Centrosoyuz drawings show a viable closed circuit distribution, a system which might have worked applying the knowledge in air conditioning already available at the time. Trying to keep the inner temperature at 18º all year round might be onerous in terms of energy consumption, but it was a good intention in terms of comfort. I don't see a real invention here but the application of an existing concept.

Model for the Centrosoyuz complex

In terms of viability the 'Mur neutralisant' is much more difficult to accept; not because it was ahead of its time but simply because it was not reasonable. The concept of a neutralising wall was tested and calculated by two independent companies at the time, one French and the other American. Both Saint Gobain after their twin-box test and the American Blower Company in their calculations concluded that such a system would require an enormous amount of energy to really make a difference on the inner temperature. And I agree without conducting any tests or calculations, because of two weaknesses: too large conductivity (heat in the cavity would scape out in winter) and no control of radiant heat (solar radiation would come in during summer). Let us see this in a bit more detail.

The Centrosoyuz after its opening. The main glazed walls are double glass walls but with no intermediate heating system.

First the conductivity issue: the air-filled cavity would quickly try to equalise its temperature with the outer one - because the temperature gap would usually be higher to the outside than to the inside of the room. A single glass pane has a very high thermal conductivity, meaning that the heat would flow out (in winter) and into the cavity (in summer) instead of warming or cooling (respectively) the air in the room adjacent to the inner side of the glass. The guys from Saint Gobain, who were surely aware of the patents in double glazing already taking place in America, suggested a better alternative: a double glazing on the outside layer instead of a simple glass. And today we would suggest an even better alternative: a triple glass with low emisivity coatings and argon-filled cavities without any active mechanism inside.

Views of Centrosoyuz today

But that would not be enough: we have to take care of the solar radiation, as Le Corbusier would learn the hard way at his Cité de Refuge for the Salvation Army in Paris, based on a project started in 1929 and finished in 1933. The story is well known. This is a text written by Le Corbusier defending his active air-handling principles for La Cité de Refuge in 1931:

“Our Invention, to stop the air at 18 degrees undergoing any external influence… These walls are envisaged in glass, stone, or mixed forms, consisting of a double membrane with a space of a few centimeters between them… a space that surrounds the building underneath, up the walls, over the roof terrace… 

Another thermal plant is installed for heating and cooling, two fans, one blowing, one sucking; another closed circuit… Result, we control things so that the surface of the interior membrane holds 18 degrees”

The South facade of the Cité de Refuge building right after completion in 1933 and as it is now, with the brise-soleils and the sliding windows.

In spite of his great selling capability, Corbu was able to implement only one third of his active principles in the Salvation Army building. The south-facing single glazed facade of 1,000m2 was completely air-tight (no opening windows), but the 'Mur neutralisant' and the 'Respiration exacte' were rejected due to budget constraints. The building remained rather warm during the opening winter, but it proved a complete failure the next summer. Corbu blaimed the absence of air conditioning - true but expensive for a building like this - while the occupants were just asking for opening windows to provide some natural ventilation.

Both (Corbu and his client) were partly right, although the final solution would only come as a consequence of the bombings in Paris. The facade was completely destroyed during the war and Le Corbusier received the commision for rebuilding it. This time, after his trips to Algeria, Argentina and Brazil he had the final answer to the actual problem: an external sun screen to control solar radiation, or 'brise-soleil' was added outside the glass layer. This concept, a passive measure unlike the other two, would become an integral part of Le Corbu's architecture until the end of his career.

The brise-soleil at its most after the War: the Unité d'Habitation in Marseille and sketches by LC on sun control.

How did these trials and errors influence Corbu's vision on architecture? In my opinion the failure of his two active systems, 'Mur neutralisant' and 'Respiration exacte' in Paris and in Moscow led him towards embracing passive control systems after 1935. His architecture becomes more massive, concrete walls take precedence over naked glass and the brise-soleil reigns over every opening. His assistants during the fifties - as Iannis Xenakis in Chandigarh - are better informed about climate control for human comfort. Texts as Victor Olgyay's 'Design with Climate' (1963) emerge partly as a reaction to the excesses of the International Style, but also partly in line with Le Corbusier's view of an architecture more in contact with earth and natural environment.

Bioclimatism, solar charts, wind blow control, ventilation, illumination, healthy spaces... all these terms define Le Corbusier's architectural production after the war until the end of his prolific career. Corbu's strong alignment with these concepts was clearly an invitation for younger generations of architects to act with diffidence in regards to mechanization of internal climate.

Le Corbusier's early intuition about a glazed 'Mur neutralisant' as a way to achieve a 'Respiration exacte' inside his buildings was not right. But he seemed to have learned the lesson, moved on and helped young architects to learn it as well. Others cannot say the same.

Le Corbusier sitting in front of the site for the Centrosoyuz Building in Moscow (March 1931)

Let me finish this post with a small present for Corbu-addicts. These are a few lines of a long letter written by Le Corbusier in 1932 (at the time of construction of the Centrosoyuz, which he calls the Palace)  to the Soviet Commissar of Enlightenment Anatolii Lunacharskii. The translation has been provided by Ross Wolfe, and the whole text of the letter can be found here. Corbu was asking this soviet politician for permission to organize a conference to present his architectural principles. And he was clever enough to sound 'scientific' and progressive in order to get a positive answer...

"In Moscow, I could — outside the Palace — publicly speak of the Radiant City, and explain where progress and the grand view have led us and shown to your country, which is the only one possessing the institutions that permit the realization of modernist programs.  The technical detail of the questions concerning:

architectural reform

the 24-hour solar day and its programme

the new techniques of exact respiration inside buildings (with the recent laboratory experiments at St.-Gobain) (the most pressing problem facing the USSR)

the problems which agriculture poses for the domestic economy

the soundproofing of homes

acoustics

Here are the truths, realities, the long-range items that are informed by the spirit of the five-year Plan — much more than certain restrictive methods, Malthusian and lacking imagination, which have been so warmly embraced in the USSR.

And if anyone wants, I could speak of proportion, of beauty, those things that are the driving forces of my life, because happiness is not possible without a sense of quality."

The solar cycle, LC 1954

It has taken me days to understand the meaning of 'the 24-hour solar day and its programmme' but it is the clue to this story. What Corbu meant by the 24-hour solar day is wonderfully depicted - and described - in the attached later sketch from 1954. Nothing to do with the 'Mur neutralisant', all the opposite.

What this letter tells us is that Le Corbusier was at the time of writing, as early as in 1932, already departing from the mechanistic world of the 'thermal machine' and opting for the order of solar profit, of solar control. A world where bureaucrats or budget constraints would not oppose his inventions any longer. A world where energy would come free and abundant, only requiring control, not production.

But his interest remained the same all around this mental process: the search of happiness through architecture. Because, as his final words resonate like a manifesto:

"happiness is not possible without a sense of quality..."

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.

Cook vs Gehry on designing the best NYC skyscraper

Last August Paul Goldberger, The New Yorker’s architecture critic, spoke with Richard Cook, founder and partner in Cook+Fox Architects and the designer of the new Bank of America Tower.  The Manhattan skyscraper, a.k.a. One Bryant Park, was completed earlier this year and is the largest commercial building to receive a LEED Platinum certification, the highest standard set by the U.S. Green Building Council. Cook and Goldberger indulge in a polite conversation about sustainable design, LEED certification and the meaning of green consciousness for architects nowadays.The critic does not perform as a critic; he seems convinced by the elegant, soft-spoken and well-educated leader of Cook+Fox Architects. My impression - I must admit it - was not so positive. There is something about this glazed tower that seems rather opposite to the concept of a sustainable building, and that's the huge amount of vision glass that covers the facade top to bottom. A similar percentage of vision glass than at the Lever House or the Seagram Building, to name just two icons of New York curtain walls in the 20th century.

The message in the video was well packaged and sent though. A quick review to the Bank of America Web page brings some more info:

Bank of America Tower at One Bryant Park is the heart of our New York operations - and a striking example of our environmental commitment. The 55-story tower, having obtained the U.S. Green Building Council's LEED® (Leadership in Energy and Environmental Design)- CS Platinum certification, is one of the world's most environmentally responsible high-rise office buildings.

Unlike most large buildings, the tower will generate a significant portion of its power on site through a 5.1 megawatt cogeneration system. It also will save about half the energy used by most buildings its size; will filter out about 95 percent of the particules in the air drawn into the building; will use less expensive night-time power to produce ice used to cool the building; and will conserve millions of gallons of water every year through methods such as green roofs and waterless urinals.

Even the Huffington Post, a reliable NYC politics and socialite Web page (not precisely conservative) seems to have joined the praise.

There is another tower in Manhattan, still under construction, which is not known by its sustainable credentials but by its designer, Frank Gehry. The Beekman tower, located just south of City Hall, has recently received a positive review at the art & design pages of The New York Times. The 76-story tower is recognizable by its crinkled stainless steel skin, bringing a new look to an imposing cluster of landmarks from a hundred years ago commanded up to now by the Woolworth Building.

The design of the Beekman tower has evolved through an unusual public-private partnership. In an agreement with New York education officials, the tower’s developer, Forest City Ratner, agreed to incorporate a public elementary school into the project. Forest City was responsible for the construction of the school; the Department of Education then bought the building from the developer. The Beekman tower is thus a curious fusion of public and private zones. Clad in simple red brick, the school will occupy the first five floors of the building. Atop this base will be the elaborate stainless-steel form of the residential tower.

As IBM ads tell, it's time to ask smarter questions. From the available literature, Cook+Fox are the nice, responsible guys whilst old Gehry, in his Southern Californian mood, has come to Manhattan just for the money. Is it as simple as that? Not really.

Let's have a look at the vertical section of One Bryant Park:

Two-thirds of the facade surface - floor to ceiling - are covered with vision glass, only one-third - the edge of slab - is opaque glass with a back-panel insulation. The tower has been clad with Viracon insulated glass with a low-e coating and a silk-screen pattern made of fritted dots on the #2 surface. To allow for higher transparency, the glass is low iron (extra clear). At eye level (sit or standing) the glass has no pattern, providing great views of the New York skyline. The silk-screen pattern extends graduately below eye level to the floor and above eye level to the ceiling to reduce radiant heat gains. These are the best data I could find from the glass supplier, not from the project, so take it as a guess (the glass coating is a project specific combination of VE 15-2M and VRE 15 -59 from Viracon): U-value 1.6 W/m2ºK or 0.30 BTU/hft2ºF, solar heat gaining coefficient between 0.36 and 0.39, visible trasmittance between 55% and 73%.

“Bringing in more daylight deep into the building reduces electricity costs. But it also increases the efficiency of the people that work in the builiding – and that's the greatest cost savings,” says a spokesperson from the developer, adding that financial firms' personnel costs are several factors higher than their energy needs. “If you're 10% more efficient on energy, it's not the same dollar amount as a 2-3% in personnel savings"

This sounds familiar to most of us involved with glass and energy efficient buildings. The architects seem to have convinced the developer that lots of light are good for tenants, and energy losses (or gains) through the glass are secondary. The solar passive behaviour of a glazed tower must be relative, since it doesn't impede the building to achieve a LEED platinum certification. It is true if you use LEED as the only metering system, but it is not true if you really try to minimize the total heat exchange through your facade. Lets have a look now at the curtain wall unit system detail (vertical section through the top-bottom interlocking transom):

The glazing contractor for the project, by the way, is Permasteelisa USA. Do you miss anything in this section? I do: a good old thermal break in the transom profiles. OK, so we have here a non-thermally broken unit system with a combination of 2/3 vision glass (U-value of 1.6 W/m2K centre pane) and 1/3 opaque glass (100mm of mineral wool plus insulated glass, that should be around 0.6 W/m2K centre pane).

We Europeans may be a bit pesimistic when doing U-value calculations, but the combination of those three elements (profiles, vision and spandrel glass), according to our standards, delivers an overall U-value between 1.9 to 2.3 W/m2K. Let's add to it the radiant heat gains: all orientations have the same glass, there are no external shading systems, and 2/3 of the glass is vision, with an average solar heat gain coefficient of around 0.35 (deducting the profiles but adding the radiant heat that enters through the non-thermally broken aluminium). What does this mean? Two things: important heat losses in winter and very important heat gains in summer, both along the whole working day. As a result, a) services must have been designed to cover peak loads, at an important extracost, and b) energy consumption along the year will be clearly higher than if designed otherwise. The energy performance of this curtain wall is much better than the Seagram or the Lever House from the 50s, of course, but it's nothing extraordinary nor any example of energy efficiency in buildings. I will skip the glare issue here, but I bet not all Bank of America clerks are happy about their transparent facade when they try to read their computer screen at the office.

But then, who is wrong? Wasn't it an example of environmental commitment? According to LEED, yes it is. According to some of us, there is much room for improvement - both in the design of this facade and in the way LEED points are measured and obtained. You can find more on the matter at this interesting webpage, written by Steve Mouzon: One Bryant Park and the LEED problem. I completely agree with his point of view about LEED: the US Green Building Council has made a lot for achieving better buildings and deserves our praise, but it's time for a change in the way LEED points are given. One sole change to begin with, please: all Gold and Platinum pre-qualified buildings should measure their energy output once they are built and occupied, and compare real life results against simulations, if they want to receive the final medal.

Time to come back to our old Frank Gehry and his slender residential Beekman tower. The developer here has not opted for a LEED certification (at least that I know). Compared to One Bryant Park, though, the project is quite reasonable in terms of facade energy performance.

The stainless-steel folds that now drape all but the top few floors of the Beekman Tower have already created a new landmark on Lower Manhattan. “I designed this building for New York,” says Gehry. “I’m a deeply rooted contextualist regardless of what anybody says. I stair-stepped the building like a New York skyscraper. It fits in without pandering to, or copying, its neighbors”.

To produce the tower’s wavy skin in a cost-efficient process, the facade concept is based on a flat, unitized curtain wall with a back-ventilated rain-screen cladding attached to its front. Permasteelisa (once again) was selected as the facade contractor. You will read lots of papers about the computer design process, Rhino, Catia, etc. This is not our stuff right now, we are just onto sustainable performance today.

Let's have a look at a the facade plan section. The folds of the facade become something as bay windows for the apartments, providing top and lateral shadows along the day. The residents will feel they are living within thick walls, at least that's the impression one gets from the plan section. This is a good feeling, don't you think?

The amount of opaque surfaces in this facade is much bigger than at One Bryant Park. All columns, partitions and parapets are clad with 16-gauge stainless steel face sheets, hiding a thick mineral wool insulation behind. The curtain wall elements are thermally broken. I haven't found any data about the glass yet, but I bet it's a low-e double glass unit without any additional coating.

The external wall looks really well from a nearby position. OK, stainless steel is not cheap, and these flumsy shapes are not easy to do. Even though, if we conducted a life cycle analysis of this facade, I wouldn't be surprised to find out that the low energy transmission - both during winter and summer - plus a low maintenance operation cost can offset the extra construction cost in a few years, making this facade more sustainable in the long term than the Bank of America's one. LEED permitting, of course.

Who knows? Maybe Gehry is more aligned with the real spirit of New York facades than Cook+Fox: a spirit that favours tall, vertical windows, stepped-back volumes and decorated external walls. There's nothing wrong about it, after all...