If you’ve ever dug into how modern electronics keep cool, or how industrial machinery avoids overheating during long runs, you’ve probably run across the term “insulating film.” But here’s a question I get almost every week, from design engineers at startups to plant managers at manufacturing facilities: “What’s the thermal conductivity of an insulating film?” It’s not a trivial question, either—get this number wrong, and you could be looking at overheated circuits, shortened equipment lifespan, or even unexpected downtime that costs you thousands (if not tens of thousands) in lost production. As someone who’s worked as an insulating film supplier for over a decade, I’ve seen how much confusion surrounds this particular specification, so let’s break it down simply, without all the jargon that makes even technical specs feel like a foreign language. Insulating Film
First, let’s clear up a common myth. A lot of people assume that “insulating” means “thermal insulator,” but that’s not the whole story. Insulating films are designed to block electrical current, right? That’s their core job—they separate two conductive parts so electricity doesn’t arc, short out, or cause a safety hazard. But their thermal performance is a secondary, often overlooked, feature that makes or breaks their use in real-world applications. So when we talk about thermal conductivity here, we’re not talking about the kind of insulating film you’d use in your attic to keep your house warm—that’s a bulk, porous material meant to trap air. The films we supply are thin, flexible, engineered for precision applications: think backplane insulation in servers, motor winding insulation in electric vehicles, or even battery pack separators in consumer electronics. Their thermal conductivity is measured in watts per meter-kelvin (W/m·K), the standard unit for heat transfer across materials, and it’s a number that varies wildly depending on what the film is made of, how it’s processed, and even its thickness.
Let’s start with the baseline. Most general-purpose insulating films—like the common polyimide (PI) films used for low-stakes electronics, or polyethylene terephthalate (PET) films for simple industrial wraps—have a thermal conductivity of around 0.12 to 0.35 W/m·K. That’s pretty low, which makes sense for electrical insulation, but it also means they act as a small barrier to heat movement. If you’re wrapping a low-power LED strip for a home lamp, that’s totally fine—there’s not enough heat buildup to matter. But if you’re insulating the winding of an electric car traction motor, which can run at hundreds of degrees Fahrenheit while cranking out torque, a film with that kind of low thermal conductivity is a problem. The heat trapped between the windings can cause the insulation to degrade over time, leading to insulation breakdown and motor failure. That’s where high-performance insulating films come in, the ones we specialize in as a supplier.
Our high-performance thermal-grade insulating films, which are modified with ceramic fillers or proprietary polymer blends, bump that number up significantly. For example, our standard ceramic-filled polyimide film has a thermal conductivity of 1.2 to 1.8 W/m·K—five to ten times higher than basic PI. For even more demanding applications, like aerospace components or high-power industrial transformers, we offer a polyamide-imide (PAI) film with a thermal conductivity of up to 2.5 W/m·K. Wait, I know what you’re thinking: “2.5 W/m·K is still way lower than, say, copper, which is around 401 W/m·K, or aluminum at 205 W/m·K.” That’s true, but here’s the thing: we’re dealing with a film that’s often only 25 to 100 microns thick, not a solid metal block. The film’s job isn’t to conduct heat away—it’s to insulate electricity while letting a small amount of heat pass through to a heat sink or cooling system. If you tried to use a copper sheet for electrical insulation, that would be ridiculous—copper conducts electricity too, which defeats the whole purpose of an insulating film. So we’re always balancing two competing needs: high enough electrical resistance to prevent shorts, and high enough thermal conductivity to stop heat from building up in a tight space.
One of the most common mistakes I see customers make when evaluating thermal conductivity for insulating films is looking at a generic spec sheet and assuming it works for their application. For example, a customer might order a film rated at 1.5 W/m·K for a battery module, only to find that the actual heat transfer across the film is much lower than expected. Why? Because thermal conductivity can be affected by the film’s structure. For instance, some films are layered: a core insulating layer with a ceramic coating on one or both sides. The coating might have a high thermal conductivity, but if the layer is thin or the adhesion between the coating and core is poor, the overall thermal performance drops. Or take film thickness: thinner films tend to have slightly higher apparent thermal conductivity than thicker ones, because heat has a shorter distance to travel through the material, and there’s less chance of tiny air pockets or processing defects disrupting heat flow. That’s why when we test thermal conductivity for our films, we use standardized methods—ASTM D5470 for thin thermal insulating materials, to be exact—so customers can trust that the number on the spec sheet matches what they’ll get in production runs.
Another factor that’s easy to miss is temperature. Thermal conductivity isn’t a fixed number; it changes depending on how hot or cold the film is. Most polymer-based insulating films have thermal conductivity that increases slightly with temperature, up to a point. Once you hit the glass transition temperature of the polymer (the temperature where it stops being rigid and starts to soften), the thermal conductivity can drop off sharply as the material’s structure breaks down. For example, our standard polyimide film has a glass transition temperature of around 250°C, so at operating temperatures below that, its thermal conductivity stays consistent, but at temperatures above 280°C, it starts to degrade, and its thermal performance plummets. That’s why we always ask customers for their application’s operating temperature range before recommending a film—we don’t want them to pick a material that works at room temperature but fails once it’s installed in a high-voltage motor or a lithium-ion battery pack that runs at 150°C during fast charging.
I remember a case a few years back that really drove this point home. A startup making compact e-bike motors came to us because their existing insulating film was failing after only six months of use. They’d picked a cheap general-purpose PET film with a thermal conductivity of 0.2 W/m·K, thinking that any insulation would work. But their motor’s windings were running at 180°C, and the low thermal conductivity of the PET meant heat was trapped between the wires, causing the insulation to melt and short out. We recommended our ceramic-filled PI film, which had a thermal conductivity of 1.4 W/m·K and a continuous operating temperature of 220°C. Within three months of switching, they reported zero insulation failures, and their motor’s lifespan doubled. That’s the difference between picking a film based on a generic spec and understanding how thermal conductivity ties directly to real-world performance.
Now, let’s talk about how thermal conductivity is measured, because there’s a lot of inconsistency in the industry here. Some suppliers test their films under ideal lab conditions—room temperature, no ambient airflow, perfectly flat, no surface defects—while others test at actual operating conditions. As a supplier, we test every batch of our insulating films at the customer-relevant temperature, because that’s the number that matters. If you’re designing a server that runs at 80°C, you don’t care what the thermal conductivity is at 25°C. That’s why we include both room-temperature and operating-temperature thermal conductivity values on all our spec sheets, along with electrical insulation resistance, dielectric strength, and tensile strength. We don’t hide behind numbers that look good on paper but don’t reflect what the film will do in your equipment.
There’s also a common misconception that “higher thermal conductivity is always better.” That’s not true either. For low-power applications, a film with thermal conductivity on the lower end of the spectrum (0.2 to 0.5 W/m·K) is more than sufficient, and sometimes even preferable, because it has a higher electrical insulation resistance. For example, a simple household power adapter doesn’t need to dissipate much heat, so using a low-conductivity insulating film keeps costs down and doesn’t sacrifice performance. But for high-power applications—electric vehicles, wind turbine generators, power inverters—higher thermal conductivity is non-negotiable. These applications generate a lot of heat, so trapping that heat in the insulation leads to overheating, reduced efficiency, and safety risks.
Another point that’s important for customers to understand is that thermal conductivity isn’t the only metric that affects heat transfer. The way the film is installed matters too. If there are air gaps between the insulating film and the metal part it’s insulating, that air gap acts as a much better thermal insulator than the film itself, drastically reducing heat transfer. That’s why we offer installation guides for all our films, and we can even work with customers to develop custom pressure-sensitive adhesive coatings that eliminate air gaps when the film is applied. These adhesives are low outgassing (critical for aerospace and electronics applications) and don’t degrade the film’s electrical or thermal performance, so you get the full benefit of the film’s rated thermal conductivity.
As someone who’s been in this industry for long enough to see trends come and go, I can tell you that the demand for insulating films with precise thermal conductivity is only going to grow. The rise of electric vehicles, renewable energy systems, and compact consumer electronics means designers are pushing components to run hotter and fit into smaller spaces. That means the old one-size-fits-all insulating films are no longer enough. Customers need partners who can not only provide a film with the right thermal conductivity for their application, but also explain why that number matters, how it will perform under real operating conditions, and how to integrate it into their design correctly.
If you’re working on a project that uses insulating films, and you’re not sure what thermal conductivity you need, don’t guess. Too often, people make the mistake of picking the cheapest film or the one with the highest stated thermal conductivity without considering their specific operating conditions. We’ve helped hundreds of engineers across industries select the right film for their needs, whether that’s a 25-micron film for a flexible circuit or a thick 150-micron film for a high-voltage transformer. We test our films to strict industry standards, provide full transparency on performance specs, and work closely with every customer to answer their questions. Whether you’re prototyping a new product or ramping up full-scale production, we can help you avoid the costly mistakes that come from choosing the wrong insulating film. To discuss your project and get recommendations tailored to your needs, feel free to reach out to our team for a consultation. We’re here to make sure you get the right thermal performance, electrical insulation, and durability for your application.
Insulation Board References
- ASTM International. Standard Test Method for Thermal Transmission Properties of Thin Thermal Insulating Materials. ASTM D5470-17, 2017.
- Gupta, R. K. Polymer Film Technology: Properties and Applications. CRC Press, 2016.
- Zhang, Y., et al. Thermal Conductivity of Ceramic-Filled Polyimide Films for High-Temperature Electrical Insulation. Journal of Applied Polymer Science, vol. 136, no. 15, 2019, pp. 47321.
- IEEE Standards Association. Standard for Electrical Insulation of Rotating Electrical Machines. IEEE Std 117-2018, 2018.
Wenzhou Gisen Insulation Material Co., Ltd.
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