The effect of pressure changes on fire rated glass in high-altitude buildings

First, a quick refresher: What is fire rated glass?

Fire rated glass is specially engineered to resist the spread of fire and maintain structural integrity for a set amount of time, usually 30, 60 or 120 minutes. It is commonly used in doors, partitions and external facades. The material may incorporate multiple layers, special interlayers or fire-resistant coatings. In high-rise architecture, it plays a key role in compartmentalisation, visibility and passive fire protection. 

The invisible pressure: How air density changes with altitude

As you go higher in elevation, air pressure drops. At sea level, atmospheric pressure is about 101.3 kPa. But on the 80th floor of a skyscraper, it becomes less. This reduction in pressure may lead to a few things like:

• Lower air density
• Reduced oxygen concentration
• Changes in thermal convection patterns

All of these can influence how fire rated materials, including glass, respond to both everyday conditions and emergency situations like fire. 

So what happens to fire rated glass in thin air?

1. Changes in expansion and contraction

Fire rated glass, especially multi-laminated or gel-filled types, responds to heat and pressure through expansion. In lower-pressure environments, the balance between internal and external pressure can shift. This may result in:

• Altered expansion behaviour during a fire
• Potential delamination risks in laminated glass units
• Greater mechanical stress on the seals and framing systems

When pressure outside a sealed glass unit is lower than what’s inside, it can subtly deform the glazing over time or during temperature fluctuations. This is particularly critical in systems that rely on airtight construction or have cavity fills. 

2. Fire behaviour changes with altitude

Fires behave differently at elevation. Lower oxygen levels and reduced air pressure can actually lead to a slower rate of flame spread in open-air scenarios. However, in enclosed spaces like a high-rise corridor or sealed room, other factors come into play:

• Heat builds up more rapidly due to limited convective cooling. 
• Smoke stratification may occur faster, affecting visibility and egress.
• Fire rated glass may be exposed to more intense localised heat, pushing it closer to its critical failure point.

In these scenarios, the ability of fire rated glass to insulate against heat becomes even more important, not just in resisting flames but in delaying temperature transfer to the non-fire side. 

3. Framing system sensitivities

Even the strongest fire rated glass is only as reliable as its framing system. And in high-altitude environments, pressure fluctuations can affect:

• The integrity of the frame-glass seal
• Gasket performance and and compression levels
• The capacity of the frame to absorb thermal movement without cracking or buckling

That means it’s not just the glass that needs to be tested and certified, it is the entire assembly. Systems need to be designed and tested for both vertical load performance and pressure differential resistance. 

4. Prevention of spontaneous breakage

In some fire rated glass types, tiny inclusions or internal stresses can make the glass susceptible to spontaneous breakage, especially when pressure differences or temperature changes are abrupt. At elevation, this risk increases slightly due to the change in external load on the glass. 

This makes heat soaking and other quality-control processes even more essential during production, ensuring that imperfections don’t turn into major vulnerabilities later on.

5. Long-term durability and pressure cycling

Over time, repeated pressure cycling from weather changes, elevator shaft dynamics, or internal HVAC systems can cause micro-fatigue in glass units. This might not lead to immediate failure but it can shorten the overall service life of the glazing system.

Fire rated glass in high rise buildings must therefore be assessed not only for its fire performance, but also for its resilience over time. The higher the building, the more cycles it may experience over its lifespan, and the more important durability becomes. 

What designers and specifiers need to consider

When working on a building that reaches into the clouds, designers and specifiers should not assume that all fire rated glass will perform equally well. Here’s what needs to be taken into account:

• Choose fire rated glazing systems that are tested under differential pressure conditions
• Ensure the framing and anchoring systems are rated for high-altitude installations
• Consider pressure-balancing breather tubes in IGUs (insulated glass units), where applicable
• Work with manufacturers who understand the interplay between equally well. Here’s what needs to be taken into account:

Regulations may not be enough, at least not yet

Current fire safety codes are generally written based on ground-level conditions. While they address things like wind loads and seismic performance, they don’t always take pressure differential into account when evaluating fire-rated glazing.

This leaves a bit of a grey area. Architects and builders working on ultra-tall buildings should go beyond code minimums and request enhanced testing data from suppliers, particularly for projects above 300 metres in elevation.

Building safer cities, one window at a time

High-rise living and working isn’t slowing down anytime soon. From commercial skyscrapers to residential towers, buildings are reaching higher, and so are the demands on their materials. Fire rated glass is one of those materials that often goes unnoticed, but in an emergency, it plays a critical role in keeping people safe and fire contained.

And in high-altitude environments where pressure differences, heat behavior, and building movement come together in complex ways, a little extra attention goes a long way. Choosing the right fire rated glass system isn’t just about ticking a compliance box. It’s about understanding how science and safety intersect.