If you’ve ever installed a steel door or window in a climate that swings from blistering summer heat to freezing winter cold, you know the headache that comes with frames sticking in their tracks, doors that won’t latch, or gaps that let drafts in. For 10 years, I’ve been supplying 70SD steel door and window systems to residential builders, renovation specialists, and property managers across the Midwest and Pacific Northwest, and the question I get more than any other is about how these products hold up when temperatures shift. Today, I’m breaking down the expansion and contraction performance of 70SD steel doors and windows across different temperature ranges, from our lab tests to real-world install stories that prove why this product stands out. 70SD Steel Door and Window
First, let’s ground this in basic science, no overly stuffy textbook stuff—just what every supplier, installer, and homeowner needs to know. Steel is an alloy, and like all metals, it expands when heated and contracts when cooled. The rate at which it does this is measured by the coefficient of linear thermal expansion (CLTE), which tells you how much a material changes in length per degree of temperature change. For carbon steel, the standard CLTE is about 11.7 x 10^-6 per °C, or roughly 6.5 x 10^-6 per °F. That might sound tiny, but over a 7-foot door frame—one of the most common heights for exterior doors—small changes add up fast. A 50°F temperature swing, for example, can make a steel frame shift by about a quarter of an inch. If you’ve ever had a door that sticks in summer when the wood swells, or gaps open in winter, you can imagine how that plays out with steel.
Now, not all steel is the same, and that’s where our 70SD line stands apart. When we developed this product line 8 years ago, our team of engineers at the local testing lab ran dozens of tests to map its expansion and contraction across the full temperature range it would actually face in North America. We tested samples in chambers that swung from -40°F (a normal low in northern Minnesota or Alberta) to 160°F (a typical attic temperature in Arizona summer). The results were consistent, and that’s what made us confident to put our name on this product.
Let’s start with the cold side first, because that’s where a lot of steel door lines fail. In temperatures below 32°F, most basic steel doors contract a small but noticeable amount. What does that mean for a homeowner? It usually means gaps around the frame, drafts that drive up heating bills, and sometimes issues with locking mechanisms not engaging fully. In our lab tests, 70SD steel contracted at a rate of 0.0000112 inches per inch, per degree Fahrenheit below freezing. Compare that to a lower-grade steel door we tested at the same time, which came in at 0.000013 inches per inch per °F below freezing—so our line contracts about 14% less than that competing product. That might not sound like much, but for a 6-foot door frame, that’s a difference of about 0.005 inches over a 50°F cold swing. Enough to keep the weather out, which is exactly what you want.
Now, the hot side. In temperatures above 80°F, steel expands, and poor design can lead to big problems here. I’ve seen too many jobs where a contractor installs a basic steel door in Florida, and by mid-July, the frame has expanded so much that the door won’t close, or the window sash gets stuck in its track. Again, our 70SD line is engineered to mitigate this. At 100°F, 70SD expands at 0.0000115 inches per inch, per degree Fahrenheit above 80°F. For a standard 3-foot wide window, that’s only about 0.012 inches of expansion for a 60°F summer swing. Most installers don’t leave enough tolerance for that, but our design incorporates small, precision-cut gaps between the frame and the wall that account for exactly this movement. I recently got a call from a builder in Phoenix who installed 40 70SD windows last spring, and he told me he didn’t have a single call back about stuck windows or misaligned doors—something that used to happen with every other steel line he’d used in the past.
But here’s the part that matters most: this performance isn’t just lab-controlled. We’ve been tracking real-world data from hundreds of installations across 12 different climate zones for the past 5 years, and the results match our lab tests. Let’s take two examples: a home in Minneapolis, where temperatures swing from -20°F in winter to 95°F in summer, a 115°F total swing. Our calculations show that the total expansion/contraction for a 70SD door frame in that swing would be about 0.27 inches. That’s exactly what our installation guide recommends for tolerance, so no binding, no gaps, no drafts. Compare that to a basic steel door in the same home: total movement of about 0.31 inches, which is 0.04 inches more. Over time, that extra movement can wear on the seals, leading to air leakage and water intrusion during heavy rain.
Another example: a commercial building in Seattle, where temperatures rarely drop below 35°F or rise above 85°F, a 50°F total swing. For a 8-foot storefront window, our 70SD line moves about 0.13 inches over that swing. Because our frames are pre-punched with mounting holes that have slightly oversized clearances, that movement doesn’t pull the frame out of alignment or damage the sealant. I’ve had property managers in Seattle tell us they’ve gone 4 years without resealing their 70SD storefronts, whereas their old aluminum storefronts needed resealing every 2 years—because aluminum has a higher CLTE, around 13.3 x 10^-6 per °C, so it moves more than steel, leading to more stress on the sealants.
Wait, I should address a common myth here: some people think steel is too rigid, so it cracks or warps when it expands and contracts. That’s true for cheap, thin-gauge steel, but our 70SD line uses 18-gauge hot-rolled steel with a galvanized core that’s both durable and flexible enough to handle that movement without warping. We test every batch for impact resistance and thermal cycling—meaning we run 1,000 full hot-to-cold swings on each sample, equivalent to about 10 years of normal temperature changes. None of our test samples show any warping, cracking, or seal degradation after that many cycles.
I know what a lot of installers are thinking: “This sounds great, but what about thermal breaks?” That’s a fair question, because steel conducts heat and cold a lot faster than wood or vinyl. If you don’t insulate the frame, you can get condensation on the interior side, which leads to mold and rot. Our 70SD line has a proprietary polyamide thermal break that’s molded right into the frame, so it breaks the path for heat transfer. The thermal break doesn’t affect the expansion and contraction performance at all—we tested that separately, and the CLTE of the steel part of the frame remains consistent, so the movement is still exactly what we measure. The thermal break just stops the cold from traveling through the steel to your interior wall, which is a bonus that doesn’t hurt the performance we’re talking about today.
Let me also talk about what this means for homeowners, not just builders. When you’re looking for steel doors and windows, you want something that will last 20, 30 years, not 5. The expansion and contraction performance directly impacts how long your doors and windows work correctly, how much you pay for energy, and how comfortable your home is. If you have a door that sticks every summer and won’t latch every winter, you’re dealing with drafts that make your furnace or AC work harder—adding hundreds of dollars to your energy bills every year. With 70SD, that gap is controlled, so your home stays more consistent in temperature, year-round.
I recently worked with a retired couple in Des Moines who had old aluminum windows that would stick in the winter and rattle in the summer. They replaced them with 70SD windows and doors two years ago, and their energy bill went down by 18% that first year. They told me the biggest difference was that they no longer had to caulk the gaps every fall and paint the sticking frames every spring—something they’d done every year for 15 years. That’s the real win of getting expansion and contraction right: less maintenance, lower costs, more comfort.
Now, let’s talk about how we test this, because you shouldn’t just take my word for it. We partner with independent testing labs to verify all our thermal expansion data, so it’s not just internal numbers. Each year, we send 10 random samples of 70SD frames to a third-party lab to run thermal cycling and CLTE testing. The results have been within 2% of our in-house tests for the past 5 years, which is exactly what we aim for—consistent performance, every time.
One thing I want to make clear: not all steel door and window lines are created equal. Some manufacturers cut corners by using thinner steel, lower-quality alloys, or no thermal breaks, which leads to worse expansion and contraction performance. When you’re choosing a steel product, ask for the CLTE rating of the steel they use, and ask for third-party test data on thermal cycling. If a supplier can’t show you that, that’s a red flag.
As someone who’s been in this business for a decade, I’ve seen too many installers and homeowners get burned by products that look good on paper but fail when the temperature changes. That’s why we designed 70SD to solve that exact problem. We don’t just sell doors and windows—we sell products that work when you need them, no matter what the thermometer says.
If you’re a builder looking for a steel door and window line that performs consistently across all climates, or a homeowner renovating and tired of drafty, misaligned doors and windows, we’d love to talk through your project. Whether you have a single-family home in a cold climate, a commercial building in a hot one, or anything in between, we can help you find the right 70SD products for your needs. We offer custom sizing, professional installation support, and competitive pricing, so you can get a product that will last for decades without the hassle of constant repairs.
50 System References
ASTM E831, Standard Test Method for Linear Thermal Expansion of Solid Materials by Thermomechanical Analysis
Steel Construction Manual, 15th Edition, American Institute of Steel Construction
National Fenestration Rating Council (NFRC) 100, Procedures for Determining Fenestration Product U-Factors
International Residential Code (IRC) 2021, Section R312, Windows and Doors
Building Science Corporation, Thermal Bridging in Envelopes: Causes, Effects, and Mitigation Strategies
PDA Steel Door Window Systems
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