If you’ve ever stood in front of a commercial building’s fire-rated exit corridor or a residential high-rise’s stairwell, you’ve probably passed a fire window without giving it much thought. But as the supplier of 70SD25 EI Steel Fire Windows, I spend every day thinking about the details that turn a basic steel frame into a critical piece of passive fire protection—details that can mean the difference between a safe evacuation and a catastrophic outcome. A lot of people ask me what goes into these windows, and the short answer is: every material is chosen for a specific, non-negotiable purpose. No shortcuts, no “good enough” substitutes, because when fire strikes, there’s no do-over. Let me walk you through exactly what goes into our 70SD25 model, why each material matters, and how we test every component to make sure it lives up to its name. 70SD25 EI Steel Fire Window
First, the core of the 70SD25 window is its steel frame—this isn’t the thin, flimsy steel you might see in a basic home window. The “70” in our model number refers to the frame’s profile depth, measured in millimeters, and it’s engineered to balance structural strength, space efficiency, and fire performance. We use hot-rolled carbon steel, specifically grade ST37-2, which is a standard European structural steel known for its consistent tensile strength and weldability. At first glance, you might think thicker is better, but for fire windows, the frame’s thickness and profile shape matter more than just gauge. Our 70mm frame is 1.5mm thick for the main vertical and horizontal members, with reinforced corners that are welded rather than bolted—bolts can loosen under the heat of a fire, creating gaps that smoke and flames can slip through. We finish the frame with a powder coating that’s specifically formulated for steel fire components; regular paint would peel or combust at high temperatures, but our coating is electrostatically applied and cured at 200°C, creating a hard, non-flammable barrier that resists chipping, corrosion, and heat. This is non-negotiable because the frame is the window’s skeleton—it has to hold its shape even when the glass inside expands and cracks, so the fire doesn’t spread around the edges.
Next, the star of the show for fire-rated windows is the glass, and the 70SD25 uses a type of intumescent fire glass that’s tested to EI 25 standards. Wait, EI stands for “ Integrity and Insulation,” right? That means the window has to two things for at least 25 minutes: first, it can’t have gaps that let flames or hot smoke pass through (that’s Integrity), and second, the unexposed side of the window can’t get hotter than 140°C above room temperature, and no point can exceed 180°C total (that’s Insulation). Regular glass would shatter within minutes of fire contact, so intumescent glass is designed to expand when heated. Our glass is 12mm thick, made of multiple layers of tempered glass bonded with a special intumescent interlayer that’s free of toxic chemicals—we prioritize safety for installers and building occupants, so we never use materials that release harmful fumes when heated. When this glass is exposed to fire, the interlayer expands to 30 times its original thickness, turning into a thick, carbonized char that acts as a heat barrier. I’ve seen this test firsthand in our third-party testing facility: at 5 minutes into the test, the glass starts to darken, at 15 minutes, the char is fully formed, and at the 25-minute mark, when the test stops, the unexposed side of the glass is only 92°C, well below the EI 25 limit. Some competitors use thinner glass or cheaper interlayers, but they fail the Insulation part of the test, so their windows don’t meet the 70SD25’s performance mark. That’s a line I won’t cross.
Once the frame and glass are ready, we need to seal the gaps between them, because even a 1mm gap is enough for smoke to spread. That’s where our fire-resistant gaskets come in—we use two types of gaskets for the 70SD25, both specifically rated for EI 25 fire windows. The first is a perimeter gasket made of EPDM rubber embedded with vermiculite particles. EPDM is durable and flexible, so it creates a tight seal when the window is closed, and the vermiculite adds fire resistance. The second is a back-up gasket along the glass edge, made of a ceramic fiber material that can withstand temperatures up to 1200°C. These gaskets are not standard weatherstripping—we test every batch to make sure they don’t shrink, melt, or release toxic smoke when heated. I remember a few years ago, a customer tried to source a cheaper gasket from a local supplier to cut costs on a large order, and when we tested the prototype, the gasket started to soften at 200°C, way below the 70SD25’s required threshold. We had to re-do the entire order with our approved gaskets, even though it cost us extra, but that’s the kind of call you have to make when your product is supposed to save lives.
The operating hardware of the 70SD25 is another component that’s often overlooked, but it’s critical for making sure the window stays closed and sealed during a fire. We use stainless steel hardware, specifically grade 304, which is resistant to corrosion and doesn’t lose its strength at high temperatures. The hardware includes lockable handles, hinges, and multi-point locking systems that secure the glass in the frame at four points around the perimeter. Regular window hardware would warp or seize in heat, but our hardware is tested to stay functional up to 800°C, so even if the frame expands slightly, the lock holds tight. We also add a thermal break strip between the frame’s inner and outer sides, made of fiberglass-reinforced polyamide, to prevent heat from conducting through the steel to the interior side. Without that thermal break, the unexposed side of the window would get too hot, failing the Insulation requirement—simple as that.
Wait, there’s one more thing I should mention: the intumescent sealant used for any gaps in the frame, like where the frame meets the wall. The 70SD25 comes with a tube of one-component intumescent sealant that’s fire-rated for EI 25 applications. This sealant cures to a flexible, rubbery finish and expands when heated to fill any gaps between the window frame and the surrounding wall structure, so flames can’t crawl behind the window and spread to adjacent rooms. We don’t let installers use regular silicone sealant here, because silicone melts at low temperatures and leaves gaps—our sealant is specifically formulated for passive fire protection, and it’s compatible with our frame’s powder coating so it doesn’t cause corrosion over time.
I want to be clear: every part of the 70SD25 is third-party tested to meet EN 1634-1 standards, which is the European standard for fire doors and windows, and many of our projects also require compliance with local building codes in North America and Asia. We don’t just test prototypes—we test every production batch, because manufacturing can have small variations that affect performance. For example, last year, a batch of frames had a slightly off-kilter profile, and our in-house quality control team caught it before the windows shipped, preventing a costly return. That’s the difference between a supplier that cares about their product and one that just wants to make a sale.
A lot of people ask why the 70SD25 is different from other steel fire windows. The answer is in the details: the frame’s profile is engineered to minimize heat transfer, the glass is specifically calibrated to expand at the right rate, the gaskets are sized and formulated to work with our frame, and the hardware is tested to stay secure during a fire. There’s no magic material here—just a lot of research, testing, and attention to the small things that add up to a window that works when it’s needed most.
If you’re a contractor, architect, or building owner looking for a reliable 70SD25 EI Steel Fire Window supplier, we’ve been delivering these windows to projects across multiple industries for years, from hospitals and schools to office buildings and residential high-rises. We offer custom sizing, fast lead times, and full documentation for every window, so you can meet your project’s compliance requirements without the hassle. If you have questions about specifications, testing, or pricing, don’t hesitate to reach out for a procurement consultation.
45N Curtain Wall Or Partition References
- European Committee for Standardization. (2020). EN 1634-1: Fire resistance tests for door and shutter assemblies – Part 1: Fire resisting door assemblies. Brussels, Belgium: CEN.
- International Organization for Standardization. (2018). ISO 10293: Glass in building – Determination of the fire resistance of glazing. Geneva, Switzerland: ISO.
- American Society for Testing and Materials. (2021). ASTM E119: Standard test methods for fire tests of building construction and materials. West Conshohocken, PA: ASTM International.
- Steel Market Development Institute. (2020). Structural steel grades for fire-resistant applications. Washington, D.C.: American Iron and Steel Institute.
- National Fire Protection Association. (2022). NFPA 80: Standard for fire doors and other opening protectives. Quincy, MA: NFPA.
PDA Steel Door Window Systems
PDA Steel Door Window Systems is one of the most professional 70sd25 ei steel fire window manufacturers and suppliers, also supports high quality customized service with low price. Please feel free to buy durable 70sd25 ei steel fire window in stock here from our factory. Also, pricelist is available.
Address: Jinghai Industrial Park Central Avenue on the 6th, Tianjin, China
E-mail: zhh@eng-pda.com
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