Most specifications are wrong regarding Insulated Glass Units in Skylights. Here is why.

Most specifications are wrong regarding Insulated Glass Units in Skylights. Here is why.

Skylights, the beautiful architectural features that invite natural light into our buildings, are more than just aesthetically pleasing designs. They are symbols of innovation, efficiency, and artistic brilliance. At the heart of this innovation lies the use of Insulated Glass Units (IGUs), which enhance the functionality and appeal of skylights. Today, we are going to explore the critical role of IGUs in skylights, delving into their various aspects, compositions, historical evolution, and importance in today's modern architecture.

A little bit of history

Laminated glass was invented in 1903 by the French chemist Édouard Bénédictus, inspired by a laboratory accident: a glass flask had become coated with the plastic cellulose nitrate, and when dropped it shattered but did not break into pieces. In 1909 Bénédictus filed a patent, after hearing about a car accident where two women were severely injured by glass debris. In 1911, he formed the Société du Verre Triplex, which fabricated a glass-plastic composite to reduce injuries in car accidents. Production of Triplex glass was slow and painstaking, so it was expensive; this is why it was not immediately widely adopted by automobile manufacturers, but laminated glass was widely used in the eyepieces of gas masks during World War I. In 1912, the process was licensed to the English Triplex Safety Glass Company. Subsequently, in the United States, both Libbey-Owens-Ford and Du Pont with Pittsburgh Plate Glass produced Triplex glass.

The type of material used for lamination was rather fragile at low temperatures, therefore more or less efficient. On the other hand, around 1927, a Canadian created polyvinyl butyral, commonly called PVB. The latter is still today the most used interlayer in the field of architecture and in the automotive field. It was not until about 30 years later, in the 1960s, that laminated glass became a standard in the automotive industry. Certain glass suppliers such as Laminated Glass corporation began producing laminated glass for the construction industry in the 1970s. We can therefore wonder what these glasses were intended for since there was no standard for glass design at that time in Canada and the United States. It must be said that the properties of glass had been well known since the 1960s, because thousands of glasses had been tested to determine their properties. These tests revealed two important characteristics of the glass. The first being that glass is a fragile material with elastic behavior until breaking and the second being that glass has a very variable resistance. In Figure 2 you can see the spread of results from 740 trials.

The resistance of the glass was therefore established based on a probability of breakage of 8/1000. This therefore means that if we respect the maximum stress established according to this principle, by carrying out a test on 1000 glasses, there should be around 8 which would break. This turns out to be acceptable for window glass subject to wind loads, and it is on this basis that the American standard was established. But what about an application above a traffic area. Is the possibility of broken glass falling to the ground acceptable? To this possibility of breakage under loads combination, we must add the possibility of spontaneous breakage caused by defects inside the glasses (nickel sulphide inclusions). It is well known in the industry that tempered glass is more prone to spontaneous breakage. This is why some suppliers today offer secondary heat treatment to reduce this risk: Heat soak test.

So, if we summarize, we have a fragile material with variable resistance and presenting risks of spontaneous breakage. It should also be mentioned that despite popular belief, tempered glass will not necessarily break into a multitude of small pieces (see figure 2.7). There could therefore be pieces of a certain mass which could fall to the ground if there is no protection. This is also why monolithic glass railings are placed at a certain distance from the end of a concrete slab. In the event of breakage, the glass clumps comes into contact with the end of the slab and fragments. This is why a risk study was and remains necessary to this day when designing glass elements involving people's safety.

Moreover, the concept of redundancy has been known since the 80s and even before. This concept requires that the design of structural glass be done assuming that one or more glasses can break and that its loads can be taken up by other elements. It is also according to this fundamental principle that the CGSB-12.20 standard was established.

In fact, as early as in the 19th century, engineers and architects had begun working on solutions to prevent glass debris from falling on users of buildings with glass skylights. This is why metal nets were installed on the building named ''Manufactures and Liberal Arts Building at the World's Columbian Exposition'' in 1893. It is a building in Chicago on which there was glass at the roof level. After several glasses fell, the authorities installed metal nets to protect and secure users. Metal nets had drawbacks, however, as they could sometimes drop some glass particles in addition to having corrosion problems. This is why wired glass (with a metal mesh inside) was developed in the late 1800s. In fact, some states in the United States (Pennsylvania and New York) began to require glass armed at the level of skylights from the end of the 1800s, around 1899 to be precise.

Introduction to modern Insulated Glass Units (IGUs)

Insulated glass units, a cornerstone of modern glazing systems, are made of two or more panes of glass separated by a spacer and sealed at the edges. This setup creates a space filled with air or gas between the panes, providing superior insulation against heat and noise. In the case of IGUs specifically designed for skylights, in Canada, according to CAN/CGSB-12.20-M89, the inner glass pane must be laminated.

Key Interlayer Materials

The most popular interlayer for skylights are Polyvinyl Butyral (PVB) and SentryGlas Plus (SGP). Let's take a closer look:

Polyvinyl Butyral (PVB)

A popular choice for applications when the edges are not exposed to the weather. Some kinds of PVB blocks over 99% of UV rays and offers an acoustic version to improve STC ratings. This interlayer is soft and malleable. On the other hand, It’s Young’s Modulus is low and greatly affected by temperature.

SentryGlas Plus (SGP) Interlayer

SGP was initially developed for hurricane glazing. Made of ionoplast polymer technology, it outperforms PVB in terms of mechanical properties (Young’s Modulus) and provides superior tear strength and moisture resistance. It's a safer option for point-fixed glass in overhead glazing applications and is more suitable for outdoor applications. This interlayer is much more rigid than pvb and less affected by temperature. On the other hand, this interlayer is more expensive than pvb.

Applications of IGUs with laminated glazing

IGUs have widespread exterior applications such as:

  • Sloped insulated glazing
  • Decorative insulating glass
  • Greenhouse and Solarium
  • Skylights, and awnings

Special attention to the thickness and type of glass is crucial, as described in standard CAN/CGSB-12.20-M89. Choices in the laminated glass can impact safety considerations, especially in areas occupied by people.

The Environmental Impact of IGUs in Skylights

By reducing heat transfer, IGUs significantly contribute to maintaining a consistent indoor temperature. This results in lower energy consumption for heating or cooling, reducing the carbon footprint of a building.

Aesthetic Appeal: Design Possibilities with IGUs

IGUs in skylights offer a plethora of design possibilities. Various interlayer options and customization potentials allow architects and designers to create visually stunning structures that blend with the building's design theme.

Possible Compositions of Glazings in Skylights

Single Glazing

Simple and cost-effective but offers minimal insulation. Usually reserved for canopy. The glass will have to be laminated.

Double Glazing

Consisting of two panes with air or gas in between, it's commonly used in both commercial and residential skylights. The inner glass pane will have to be laminated and the outside glass pane will usually be tempered to resist impacts.

Triple Glazing

Three panes with superior thermal performance, suitable for colder climates or high-end projects. The inner glass pane will also have to be laminated and the outside glass pane will usually be tempered to resist impacts.

Specialized Glass

Tinted glass, reflective coatings, and low-emissivity (low-E) coatings are examples of specialized glass, allowing architects to control solar heat gain and aesthetics. The same rules as mentioned above will apply to those specialized glass.

How to determine the interlayer and the thermal treatment

The snow loads on a roof or on a portion of a roof will depend on different factors. The location of the building has a great impact, but the geometry of the roof has an even greater impact. According to figure from NBC (see below), when there is different height in the configuration of the roof, the snow will form a snowdrift, so if there is a skylight near the upper portion of the roof, the snow loads can sometimes be around 3 times the standard snow loads and even more. The designer should always refer to structural drawings for snow loads. The overloads should be mentioned on the roof plan view and if not mentioned, an engineer should calculate it for the design of the skylight.

Other loads also must be considered. If the skylight as a great width it might be possible that the maintenance team will have to walk on the glass to clean it up. If so, this crucial information must be mentioned in the specifications because it will probably affect the thickness of the outer glass pane.

So the question is: Now that we have the loads, how do we determine the thickness and the thermal treatment of the glass for an IGU in a skylight?

Ok, so now that we have the loads, what do we do? In the past, you had to hire an engineer specialized in glass calculation to analyse your project and propose a glazing composition which is safe according to the Canadian standard CAN/CGSB-12.20-M89, but this might not be the case for so long. Did you know that our team at LSVM is working on an app which will allow you, the architects, the glaziers, etc. to determine the glazing composition used in skylights during the pre-design phase of the project? You will still need an engineer to revise and stamp shop drawings, but you will be able to make preliminary validation to adjust pricing during the quotation process. This will allow you to optimize the glass thickness and avoid bad surprise later in the project.

Now, you remember when I said that most specifications were wrong regarding insulated glass units used in skylights. I guess you remember and it’s probably why you’ve continued to read so far. In fact, I said that not only to have a nice title, but because it’s 100% true. Most specifications we see are as follows:

  • Exterior glass: 6mm thick glass fully tempered
  • Spacer: 13mm with argon
  • Interior glass: 2 times 6mm thick glass fully tempered with pvb

What’s wrong with this? In fact, the thickness of the glass has to be determined according to the loads and dimensions. We all know this fact, so the point is not about the thickness. If you remember well, we discussed previously about the fact that pvb was soft, malleable, and more affected by temperature. As fully tempered glass will usually breaks in small pieces equivalent to the thickness of the glass, when you laminate it with pvb, you get a malleable assembly when broken (see picture below).

We call it the wet towel effect. As the pvb is not stiff enough, when combined with fully tempered glass, the post-breakage behavior is not safe.

So, what are the solutions? Well, there is usually two of them:

First option, you combine heat strengthened glass with pvb. For those less familiar, heat strengthened glass is kind of a semi-tempered glass. It receives a thermal treatment, but less severe than a fully tempered glass. The result is a glass which is less resistant than fully tempered one, but twice as much as annealed glass. And the break pattern is similar to the one of annealed glass. So, when broken, as the glass shards are way bigger than the one of fully tempered glass, the glass will have a much better post-breakage resistance, so less chance of detaching from the frame.

The second option is to use fully tempered glass combined with SGP (SentryGlas). As the Young’s Modulus of the SGP is way higher than the one of pvb, with the glass broken, the SGP will be able to sustain the tension generated by the loads on the glass. This will result in a very good post-breakage resistance, even better than the one of the first option.

Conclusion: The Revolutions and Innovations in Skylights

Insulated glass units have indeed revolutionized skylight design and construction. With innovative interlayer materials like PVB and SGP it offers enhanced safety, energy efficiency, and design flexibility.

Skylights, through time, have symbolized human creativity, the drive for comfort, and the pursuit of beauty. Whether it's the historical charm of ornate glasswork or the modern sophistication of energy-efficient glazing, skylights continue to illuminate our lives, narrating a tale of innovation as timeless as the sun itself.

Understanding the diverse properties and applications of these materials is essential for making informed decisions in skylight design and construction. Whether for residential or commercial purposes, the thoughtful implementation of IGUs contributes to creating spaces that are not just functional but also visually captivating, adding a touch of sky-kissed elegance to our surroundings.

Now, if you’re interested to know when our app for IGUs in skylights will be available, communicate with us today.

If you have not seen our recent post regarding IGUs acting as guardrail, click here.


© Les Solutions de Verre et Mur-rideau inc., 2023. All rights reserved. Any unauthorized reproduction of this article or its contents is prohibited. This article is based on our interpretation of the codes and standards. This article and its contents do not constitute professional opinion and are provided for informational purposes only. This is an overview and does not necessarily cover all technical aspects and/or special cases. LSVM, MCi VSA or the author of the article cannot be held responsible for decisions taken in connection with this article. Reuse of this article or its contents for professional purposes, including engineering purposes, is prohibited. Please note that each project has specific conditions and must be validated by an engineer. Also note that this is valid for all articles published previously and subsequently.
Back to blog