Thermal Ratings vs Fire Ratings and Why Both Matter
David Vermeulen, North America Sales Director, Technical Glass Products
Fire-resistance-rated glazing blocks the spread of fire, smoke, radiant and conductive heat.
These products are often specified for building interiors. However, it’s not uncommon to find these products specified for building exteriors, as well. For exterior use, these assemblies must also be thermally rated to promote energy efficiency by managing seasonal heat loss and solar heat gain.
While fire-rated glass can occasionally offer superior thermal performance compared to non-fire-rated glass, this efficiency is an incidental benefit not a direct result of its fire rating. Although a correlation between fire resistance and general heat insulation seems logical, the relationship between fire protection and daily thermal insulation is more complex than it appears. To understand why, it is important to first know what does a fire rating mean for glass.
A fire rating measures how a glazing system performs in a standard fire test. For fire-resistance-rated assemblies, the thermal component tracks extreme heat spikes, measuring temperature increases of hundreds of degrees over a relatively short duration.
Thermal ratings, by contrast, measure gradual temperature changes that impact the built environment over time. The quantification of thermal performance is quite different than fire ratings.
How is fire rating different from thermal performance?
On a pragmatic level, thermal ratings primarily apply to exterior glazing applications while fire ratings are more for interior ones—though there are instances when an exterior assembly may need a fire rating.
From a performance perspective, the key distinction when considering fire rating vs. thermal rating lies in how the two ratings conceptualize temperature. Put generally, fire ratings treat temperature as an experienced, macro-level phenomenon, and thermal ratings measure it as the cumulative transfer of energy at a small scale.
To go further, thermal performance ratings measure a material’s ability to slow the rate at which two different environments reach equilibrium through diffusion. In glazing, this efficiency is captured by a U-value—a metric of steady-state heat loss under everyday conditions. Unlike fire ratings, which test structural survival during extreme, rapid heat surges, U-values measure standard, small-scale energy movement using watts per square meter Kelvin.
How thermal ratings translate to occupant comfort
U-values are the reciprocal of the total sum of R-values across all its components. This means systems with higher R-values will have lower U-values, both of which represent superior daily energy performance. However, as mentioned earlier, high thermal performance does not automatically equal fire protection.
To illustrate this trade-off: an uninsulated 60-minute fire-resistance-rated lite of Pilkington Pyrostop® can achieve U-values as low as 0.37, balancing solid daily energy efficiency with life safety protection (making them ideal for mild to slightly cold climates). In contrast, Pilkington Profilit™, a non-rated architectural glazing material, can provide U-values down to 0.19. While Pilkington Profilit is the better insulator for cold climates, it lacks the essential fire protection of Pilkington Pyrostop.
It is important to note that fire-resistance-rated glazing can be insulated to lower its U-value, but this would take coordination during the design and engineering phases.
When glazing products with lower U-values are paired with other strategies to resist heat transfer, they can contribute to buildings that maintain a comfortable indoor environment more efficiently. This is because HVAC systems will not need to expend as much energy to slow the rate of equilibrium, since the building envelope will better insulate its interior.
Solar heat gain and thermal bridging impact energy efficiency
While resisting heat transfer is incredibly important for energy efficiency, it is not the only metric for glazing.
All glass assemblies have Solar Heat Gain Coefficient (SHGC) performance ratings. Measured on a scale between 0 and 1, this number represents the amount of solar radiation transmitted by a glazing system. As an example, a glazing material with a Solar Transmittance (SHGC) performance rating of 0.29 effectively blocks 71% of solar radiation.
The radiation permitted to transmit is then released internally, warming interior surfaces, raising the indoor temperature. The SHGC works with the U-value to more comprehensively describe how a glass assembly will contribute to the thermal performance of a building.
Finally, glazing assemblies are more than glass—they also incorporate frames. Any framing system will contain thermal bridges, or localized paths of high conductivity where heat flows much faster than through the insulated glass or surrounding wall.
Whether through its material or design, some framing systems allow fewer instances of thermal bridging than others. Selecting framing materials and designs that mitigate thermal bridging can support a more efficient and sustainable building design.
Bringing it all together
Finding balance is a hallmark of contemporary design, whether that is for comfortable interiors or safer, more secure environments.
While thermal ratings and fire ratings measure different (but somewhat related) performance capabilities, it is important to note that it is rare for project teams to select one metric over another entirely. Instead of fire rating vs. thermal rating, building professionals are encouraged to find systems that offer code-driven fire ratings (when needed) and achieve adequate U-values, to engineer strategies for minimizing heat transfer, or both.
Taking time to plan buildings more intentionally can help create designs that optimize interiors for safety, comfort and long-term sustainability. Project teams can start the planning process more efficiently by requesting presentations, product resources and more from Technical Glass Products.
Roux Center for the Environment, Bowdoin College: © Jeff Goldberg /Esto Images
David Vermeulen is North America Sales Director for Technical Glass Products and also the author of the Fired Up blog on Glass Magazine.
