If you’ve ever stood in front of a towering glass curtain wall, or run your hand along the seam between a garage door frame and concrete, you’ve relied on a structural sealant to hold things together, keep water out, and stop air from leaking. As someone who’s spent 12 years working in structural sealant supply—first mixing batches in a small warehouse, now fielding calls from architects, contractors, and inspectors who need answers when something goes wrong—I’ve learned the hard way that one of the most brutal tests a sealant will face isn’t rain or wind, it’s thermal cycling. Structural Sealant
Thermal cycling might sound like a lab-only term, but it’s something every sealant endures out in the real world. It’s the back-and-forth of hot days turning into cold nights, seasons flipping from summer’s 100°F asphalt to winter’s 20°F frost, and even the tiny swings: a window sealed on a sunny morning that sits in shade an hour later, dropping 15°F in no time. Buildings expand and contract with every degree change, and that movement puts stress on the sealant that most people don’t think about—until the sealant cracks, or pulls away from the substrate, and suddenly you’ve got a leak that ruins a ceiling or rots a wooden frame.
Let’s break down exactly what happens when a structural sealant goes through thermal cycling, because it’s not just one thing—it’s a chain of small, cumulative effects that add up over years. First, we need to remember that structural sealants aren’t the same as caulk you use around a bathroom sink. These are high-performance materials, usually silicone or polyurethane, designed to bond two different substrates—glass to aluminum, concrete to steel, metal to masonry—and carry structural loads, like the weight of a curtain wall panel or the pull of a window sash. They have to be flexible enough to stretch and shrink, but strong enough to not fail when they do.
When thermal cycling starts, the first thing that happens is differential movement. Every material has a coefficient of thermal expansion (CTE)—how much it changes size with temperature. Glass has a CTE of about 9e-6 per °C, aluminum is 23e-6, concrete is around 12e-6, and silicone structural sealant is usually around 300e-6 per °C—way higher than any substrate it bonds to. So when the temperature jumps from 0°C (32°F) to 35°C (95°F), the aluminum frame expands 0.0023% per degree, while the sealant expands three times as much. That mismatch in expansion means the sealant has to stretch to keep up with the frame. When it cools back down, the aluminum shrinks, but the sealant doesn’t always snap back the same way. Over hundreds of cycles, that repetitive stretch and release takes a toll.
I’ve seen this play out first-hand on a mid-rise office building in Chicago that was installed 10 years ago. The architect picked a budget structural sealant that was rated for 50% movement, but the local building code here requires 75% movement for curtain walls. By year seven, parts of the sealant around the east-facing panels had started to crack. The sun hits those panels hardest in the morning, so they go through 15+ small thermal swings a day—way more than west-facing panels. The sealant couldn’t handle that constant, small stretching, so it developed tiny stress fractures that eventually turned into full cracks. When winter rolled around, water seeped into those cracks, froze, and expanded, making the cracks even bigger. That’s a secondary effect of thermal cycling: when moisture gets into sealant joints, freezing and thawing amplifies the damage, but even without moisture, the thermal movement alone is enough to degrade the sealant over time.
Another effect I’ve seen is adhesion failure. That’s when the sealant stops sticking to the substrate it’s bonded to, even though it looks fine on the surface. How does thermal cycling cause that? It has to do with the way the sealant cures and the bonds it forms. A good structural sealant forms a covalent bond with the substrate—chemical bonds that are strong enough to hold tight even when the material moves. But when thermal cycling happens, the sealant and substrate expand and contract at different rates, and that movement creates mechanical stress at the edge where the sealant meets the substrate. Over time, that stress can break the weak points in the bond between the sealant and the substrate, even if the bond was strong when it was installed. We call that “adhesion degradation.”
I had a call last year from a contractor working on a storage facility in Arizona, where temperatures swing from 115°F in the summer to 30°F in the winter. They used a polyurethane sealant that had worked well on smaller projects, but on this project, within two years, the sealant around the concrete block joints was pulling away from the top edge. The issue? The concrete has a low CTE, and the polyurethane sealant has a high CTE. Every time the temperature changed, the polyurethane stretched and shrank more than the concrete, pulling away from the block. The manufacturer’s specs said the sealant was rated for structural use, but they didn’t account for the extreme thermal cycle range in Arizona. That’s why as a supplier, we always ask for the specific climate a project is in before recommending a sealant—one size doesn’t fit all when it comes to thermal cycling.
There’s also something called “compression set” that happens with thermal cycling, especially with silicone sealants. Compression set is when a material doesn’t return to its original shape after being compressed or stretched. When a sealant is stretched to accommodate thermal expansion, and then shrinks back, if it develops compression set, it won’t stretch as much the next time. Over multiple cycles, that reduces the movement capacity of the sealant. So what started as a sealant rated for 75% movement might only be able to do 50% after 10 years of thermal cycling. That’s a problem, because when the sealant can’t stretch enough to accommodate movement, it either cracks or pulls away.
Wait, but it’s not all bad—structural sealants are engineered to resist thermal cycling if they’re chosen and installed correctly. The key is matching the sealant’s movement capacity to the expected thermal movement of the joint. For example, in climates with extreme temperature swings, like the Great Plains or Arizona, we recommend silicone structural sealants with a 100% movement capacity, instead of the standard 50% or 75%. We also make sure to specify proper joint design—joints that are deep enough, wide enough, and have the right depth-to-width ratio, so the sealant isn’t stretched or compressed more than it’s rated for.
I once worked with a large hospital project in Minnesota, where winter temperatures drop to -20°F and summer hits 90°F. The architects were initially going to use a standard sealant to save money, but I walked them through data from our lab tests. We cycled the test samples from -40°F to 180°F, 1,000 times—simulating 10 years of thermal cycling in Minnesota—and found that the standard sealant had 20% adhesion failure, while our premium silicone sealant had less than 1% failure. The architects switched, and a few years later, their project manager called to say they haven’t had a single leak or sealant failure in the curtain wall or structural joints. That’s the payoff of understanding thermal cycling when you’re choosing a structural sealant.
Another thing to consider is UV exposure, which is often tied to thermal cycling. Sunlight heats up sealants a lot, and UV rays break down the polymer chains in the sealant over time. When you add thermal cycling to UV exposure, it’s a double whammy. The heat from the sun makes the sealant more flexible in the short term, but it also speeds up the chemical degradation. Over time, the sealant becomes brittle, so when it goes through thermal contraction in the cold, it can’t stretch and shrinks, leading to cracks. I’ve seen sealant on south-facing walls in Florida that went from flexible to crumbly in just 8 years, because of the combination of constant thermal swings and intense UV.
So what can go wrong when thermal cycling is not accounted for? Beyond the cracks and adhesion failure, there are structural consequences. If a structural sealant fails, that can compromise the entire building envelope—leading to water damage, mold growth, which is a huge issue in hospitals and schools. It can also affect the structural integrity of curtain walls, which are heavy, and rely on the sealant to hold the panels in place. I’ve heard stories of curtain wall panels popping out of their frames during a particularly strong thermal swing, because the sealant had degraded and couldn’t hold the panel. That’s not just a cosmetic issue—it’s a safety hazard.
As someone who’s in the structural sealant business, part of my job is not just selling products, but educating my customers about how environmental factors like thermal cycling will affect their sealant choice. A lot of contractors and architects focus on upfront cost, but they don’t factor in the cost of repairs down the line. A sealant that’s a little more expensive, but rated for the thermal cycles in the project’s climate, will save you thousands of dollars in maintenance and repairs over 20 or 30 years.
I always tell my customers: ask for the thermal cycle rating, ask for the CTE of the sealant, and match it to the substrates you’re bonding, and the climate your building is in. Don’t just go for the cheapest option. I’ve had customers come back to me 5 years after a bad project, asking for a replacement sealant that can handle the thermal cycling, because the first one failed. That’s a headache I want to help them avoid in the first place.
At the end of the day, thermal cycling is an unavoidable reality for any structural sealant used outdoors. The good news is that with the right product selection, proper joint design, and understanding of how thermal movement affects sealant performance, you can mitigate almost all of the negative effects. Whether you’re working on a skyscraper in New York, a warehouse in Texas, or a hospital in Seattle, choosing the right structural sealant for thermal cycling is one of the most important decisions you’ll make for the long-term integrity of your project.
Aquarium Sealant If you’re currently working on a project and have questions about how thermal cycling will impact your structural sealant choice, or you need help sourcing a sealant rated for your specific climate and joint requirements, don’t hesitate to reach out to discuss your needs and find the right solution.
References
- ASTM International. (2019). Standard Test Method for Rubber Property—Compression Set Under Constant Deflection. ASTM D395.
- ISO 10590. (2018). Structural sealants for glass and other glazed applications. International Organization for Standardization.
- Krell, D. (2017). Performance of structural silicone sealants under cyclic thermal loading. Journal of Architectural Engineering, 23(2), 04017007.
- Smith, A. et al. (2020). Differential thermal movement effects on sealant adhesion in building joints. Construction and Building Materials, 252, 119023.
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