Hey everyone, Sam here, and I’m the go-to guy for all things 70SD25 EI steel fire windows over here—yeah, that’s our bread and butter, no cap. Lately, I’ve been getting the same question on repeat, and it’s such a basic one but it’s super important, especially if you’re specifying fire-rated glazing for a building project or just curious about how these bad boys work. The question? Is the glass in the 70SD25 EI Steel Fire Window double-glazed? Let’s cut the BS and get into it, because I see a lot of misinfo online that’s making people overcomplicate this, and that’s the last thing anyone needs when dealing with fire safety. 70SD25 EI Steel Fire Window
First off, let’s get one thing straight: EI-rated fire windows aren’t just regular windows with a fire sticker slapped on. EI stands for two things—E is integrity, which basically means the glass and frame don’t burn through or let flames/smoke seep out, and I is insulation, so the other side of the window doesn’t get too hot and start a fire. The 70SD25 model is our mid-range workhorse, used in all sorts of spots—office hallways, stairwells, commercial storage areas, even some residential ground floors where fire safety is non-negotiable. I talk to contractors, architects, and facility managers every single day, and half the time the confusion about double-glazing comes from mixing up general fire window specs with model-specific stuff.
Let’s start with what double-glazed actually is, for anyone who’s not super in the know. Regular double-glazed windows have two panes of glass with a air gap between them—usually like 12mm to 20mm, sometimes filled with argon gas for better insulation. Regular double-glazed is mostly for noise reduction and energy efficiency, right? Now, fire-rated glass is a whole different ballgame because it has to stand up to extreme heat for a set amount of time—EI 25 means it can handle 25 minutes of standard fire test conditions without failing. That glass isn’t just your average clear float glass; it’s usually a type of intumescent fire glass, or sometimes laminated fire glass, that swells up when it gets hot to form a thick, heat-resistant barrier.
So back to the 70SD25. Here’s the thing: we don’t use standard double-glazed units in this window. Wait, hold on—don’t get mad at me for saying that, because it’s not that we’re cutting corners. The fire-rated glass we use for the 70SD25 is a single-pane unit that’s engineered specifically for 25-minute EI ratings, and if you tried to turn that into a double-glazed unit, it would fail the fire test every single time. Let me break that down. When you make a double-glazed unit, you have two separate panes separated by a spacer, right? To make that seal airtight (or gas-tight), you have to use a structural adhesive or a sealant that goes around the edge of the two panes. Now, fire-rated glass is tested under really high heat—like 1000°C plus. When that happens, the sealant in a regular double-glazed unit is going to break down, melt, or burn through long before the 25 minutes are up. That would create a gap where flames and smoke can get through, which totally kills the EI rating.
But wait a second—some of our customers ask us, “What about that extra insulation? Double-glazed would help with that, right?” And that’s a valid question! Because I is about insulation, so the other side of the window stays cool. So how do we handle that if we can’t do standard double-glazing? Oh, easy—we use a different type of double-glazed unit, but it’s not the kind you have in your living room. For the 70SD25, we use what’s called a “thermally broken single-pane fire glass” with a custom air gap, but it’s not two separate panes. Let me explain that tech stuff in plain terms. The fire glass we use is a single thick pane (usually 12mm or 15mm, depending on the exact rating) that’s sealed on the inside of the window frame, and we leave a small, controlled gap—like 8mm, but sometimes adjustable depending on the climate—between that fire glass and the inner trim of the window. That gap is what gives us the extra insulation without the standard double-glazed seal. It keeps the noise down and helps with thermal performance, so it’s not like you’re getting a drafty window, which is a huge win for commercial buildings where energy bills matter.
I’ve seen other companies get this wrong. Some will slap a regular double-glazed unit into a fire window, mark it as EI-rated, and then when it fails a real fire test (which contractors have to do on-site sometimes), they act surprised. That’s why we test every 70SD25 model three times a year at independent labs, right? Not just to check the fire rating, but to check how the thermal gap works. Last quarter we had a contractor come to our warehouse to test a sample, and he was shocked when we pulled the window apart to show him the gap—he thought it was standard double-glazed, so that’s why I try to be open about this stuff. No smoke and mirrors here.
Another common confusion is between different EI ratings. The 70SD25 is EI 25, so if you were looking at a 70SD60 (our 60-minute fire window), that one might have a slightly different setup, but no—even that one doesn’t use standard double-glazed. The only time we do something closer to residential double-glazed is with our non-fire-rated windows, which makes sense. The 70SD25 is strictly for fire applications, so every part is built to meet that strict test standard. Let’s also talk about the frame, because that’s part of why the glass setup works. The 70SD25 steel frame is thermally broken too, which means it has a strip of non-conductive material that stops heat from transferring through the frame. So the small air gap we have with the fire glass works with the frame’s thermal break to keep the inner side of the window at a safe temperature, which is exactly what the I rating requires.
I get that for people who aren’t in the construction world, this all sounds super jargon-heavy. Let’s put it in real terms: if you’re building a hotel stairwell and you need a window that’ll keep flames from spreading if there’s a fire on another floor, the 70SD25 is the one, and the glass in it isn’t the double-glazed unit you have in your bedroom that makes it quiet at night. But that doesn’t mean it’s worse—it means it’s built for a completely different job. I’ve had a few customers tell me they tried to use cheap double-glazed fire windows from another supplier, and they failed a third-party test, so they switched to ours. That’s the stuff that makes me proud, because we test everything to the letter, no cutting corners.
Wait, also, let’s address a myth I’ve seen online: some people say double-glazed fire windows can work if you use special fire-resistant sealant. Yeah, you can get sealant that’s fire-rated, but it only works for like 10 minutes, right? The 70SD25 is 25 minutes, so that’s not enough. For our EI 25 rating, we need the seal between the glass and frame to hold up for the full 25 minutes, and the only way we’ve found to do that is with the single-pane fire glass plus the small air gap. We tried the special fire-rated sealant on a test model a couple years ago, and it started breaking down at 18 minutes—total waste of time. So that’s why we stick with what works, not what sounds cool.
Another thing: why does this even matter for someone specifying a fire window? If you pick a window that has standard double-glazed, it might say EI-rated on the spec sheet, but when a real fire happens, that seal will fail, and the window won’t do its job. That’s not just a minor issue—That’s a safety hazard. So knowing that the 70SD25’s glass is a single-pane fire glass with a small insulation gap, not regular double-glazed, is super important for making sure your project meets local fire codes. Most building codes require that fire windows are tested to the exact standard, so faking the double-glazed setup can get you in big trouble during inspections.
Let me also mention that we’ve updated the 70SD25 model a couple times over the last two years based on customer feedback. A lot of architects were asking for better acoustic performance, so we tweaked that air gap from 8mm to 10mm for most installations, which brings the sound reduction up by 3dB—hardly noticeable, but enough to make it better for offices where noise is a problem. We didn’t have to mess with the glass or the seal at all, so the fire rating stayed exactly the same. That’s the kind of thing we focus on—improving the practical stuff without compromising safety.
I know a lot of suppliers will try to confuse you with tech terms to make their product sound better, but we keep it real. When someone asks if the 70SD25’s glass is double-glazed, I just say “No, and here’s why that’s a good thing for fire safety.” That’s the answer, plain and simple. If you still have questions, or you need specs, or you want to place an order for a project, just reach out. We work with contractors, architects, and even property managers on small jobs, so no job is too big or too small. We’ll send you a sample, walk you through the test data, whatever you need to make sure you’re picking the right fire window for your space.
At the end of the day, fire safety isn’t something to mess around with. The 70SD25 EI Steel Fire Window is built to do exactly what it says it does: keep people safe for 25 minutes if there’s a fire. The glass setup is designed specifically for that, not for energy efficiency or noise, even though it’s got those side benefits. So next time you’re spec’ing fire windows for a project, you’ll know the difference between standard double-glazed and the 70SD25’s glass—no more confusion. Hit us up if you need anything, we’re here to help.
50 Steel Fire Door and Partiton References
National Fire Protection Association (NFPA) 80: Standard for Fire Doors and Other Opening Protectives, 2022 Edition
ASTM E119: Standard Test Methods for Fire Tests of Building Construction and Materials, 2021 Edition
Underwriters Laboratories (UL) 10B: Standard for Safety for Fire Tests of Door Assemblies, 2020 Edition
Steel Fire Window Manufacturers Association (SFWMA): Technical Guidelines for EI-Rated Steel Fire Windows, 2023
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