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Aerogel, Firewall Barrier Sheet, Mica, Glass-Fiber Paper, or Silicone Foam: Selecting an EV Thermal-Runaway Barrier
Thermal-runaway barrier selection starts with an uncomfortable truth: there is no single "fire material." There are materials that slow heat conduction, materials that survive flame and ejecta, materials that also insulate electrically, and materials that also cushion a breathing cell stack, and the families that lead each duty are different.
This guide compares the five families the H-O thermal-runaway application page covers, aerogel blanket, EV firewall barrier sheet, mica barrier sheet, glass-fiber paper, and high-temperature silicone foam, by the location and job each one wins.
Test methods are named by designation (UL 2596 for torch-and-grit, UL 9540A and GB 38031 at system and vehicle level, ASTM E162 and E662 for flammability characterization, UL 94 for classifications); no rating value is claimed for any material, because those results belong to tested constructions and assemblies.
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Name the job before the material: isolation, flame, or both
The parent page's first decision is the one that sorts everything: is the barrier's job thermal isolation, flame protection, or both? Thermal isolation slows the conduction of heat from one cell to the next, so a single-cell event has less chance of heating its neighbors past their own runaway threshold; the relevant property is low through-thickness thermal conductivity at the in-service thickness and compression.
Fire protection resists flame, hot gas, and ejected particles on a vent path, module wall, or pack lid; the relevant property is flame and erosion resistance at very high temperature. A low-conduction insulator is not automatically a good flame shield, and a flame-proof sheet is not automatically a good insulator, so the job comes first, and many real designs laminate two materials to do both.
The five families, by the decision each one wins
Aerogel blanket: the thinnest path to low conduction
Aerogel blanket gives among the lowest thermal conductivity of any flexible insulation, which makes it the lead where conduction must be slowed inside a very thin cell-to-cell gap. Its caveat is mechanical: aerogel can be friable, shedding dust when cut and handled, so it is often supplied faced, and H-O commonly converts it with a facing film, edge encapsulation, or lamination that captures the friable layer between cleaner skins.
Its thermal performance is thickness- and compression-dependent, so the gap and the compression range belong on the drawing next to the footprint.
EV firewall barrier sheet: purpose-built for propagation duty
The EV firewall barrier family is engineered specifically as a propagation barrier: the sheet a designer reaches for when the governing requirement is cell-to-cell or module-to-module propagation resistance rather than steady-state insulation. The broader EV battery thermal and fire barrier category collects the related purpose-built constructions.
These materials are characterized under the propagation-relevant methods (UL 2596 at the material level; UL 9540A and GB 38031 at system and vehicle level), and the demonstrated result always belongs to the tested construction and assembly, which is why the data sheet and your own assembly test, not a category page, carry the numbers.
Mica barrier sheet: very high temperature plus an electrical role
Mica barrier sheet is the choice when very high temperature and an electrical-isolation role meet on the same part: vent paths, module walls facing direct exposure, busbar and terminal surroundings. As an inorganic, non-combustible mineral sheet it holds up where polymers are consumed, and it serves a high-temperature electrical barrier function in addition to the flame role.
It is rigid and brittle, so H-O usually laser- or waterjet-cuts it rather than die-cutting, and any dielectric duty is confirmed against the grade's own electrical data rather than assumed from the thermal role.
Glass-fiber paper: the thin inorganic layer
Glass-fiber paper covers the thin inorganic insulating or flame layer: interleaves, facings, and supplementary plies inside a laminated barrier. It is the material that lets a stack-up add an inorganic, high-temperature layer without the thickness or rigidity of a board, and it commonly partners with aerogel or a firewall sheet in multi-layer constructions.
High-temperature silicone foam: when the barrier must also cushion
Silicone foam earns the slot when the cell-to-cell layer must compress and recover across the breathing range while contributing thermal and flame behavior. A rigid or friable insulator cannot follow a swelling cell; a high-temperature silicone foam can, which is why combined cushion-plus-barrier layers lean on it, sometimes laminated with one of the inorganic layers above. Where the mechanical duty dominates outright, the part becomes a compression pad and belongs on the compression and cushioning page.
Barrier Families by Job and Location
| Family | Slows conduction | Flame-face / ejecta duty | Electrical role | Compresses with the stack | Typical location |
|---|---|---|---|---|---|
| Aerogel blanket | Leads (thin gaps) | Often faced | Confirm per grade | Limited; friable | Cell-to-cell isolation layers |
| EV firewall barrier sheet | Good | Purpose-built | Confirm per grade | Construction-dependent | Propagation barriers, module walls |
| Mica barrier sheet | Moderate | Non-combustible, very high temp | High-temp electrical barrier | Rigid, brittle | Vent paths, busbar zones, lids |
| Glass-fiber paper | Thin-layer duty | Inorganic layer | Confirm per grade | Conformable paper | Interleaves, facings, liners |
| High-temp silicone foam | Moderate | Flame-rated grades | Not the dielectric lead | Compresses and recovers | Cushion-plus-barrier cell layers |
Illustrative and relative only; confirm each grade and construction against its current technical data sheet and the relevant method (UL 2596, UL 94, ASTM E162 / E662). System and vehicle outcomes (UL 9540A, GB 38031) belong to the tested cell, module, pack, or system.
Compression: the property the bench test hides
A cell-to-cell barrier does its thermal job at the thickness it actually occupies in service, and that thickness changes as cells breathe and swell. A compressible barrier compressed to a different thickness has a different effective thermal performance, so the question is never how the material behaves at free thickness on the bench, but how it behaves at the in-service compressed thickness across the stack's life.
That is why the gap range and the compression the barrier sees belong on the drawing, and why the parent page treats "thickness, compression, and gap" as a selection decision of its own rank, not a detail.
Converting and the friability question
Several of these materials need converting strategies as much as cutting. Aerogel and some inorganic papers can shed fibers or particles when cut, a cleanliness concern on a battery line, so H-O die-cuts and laser-cuts them with a facing film, encapsulates the cut edge, or laminates the friable layer into a multi-layer part captured between facings. Mica's rigidity points it at laser and waterjet cutting.
Multi-layer barrier stacks (an insulating core with a flame-facing skin, or a cushion layer bonded to an inorganic ply) are laminated and supplied as one die-cut part, and kitted where the pack uses several barrier geometries. Describe where the barrier lives and any cleanliness requirement, and the converting approach follows, confirmed against what the material manufacturer's data sheet allows.
How H-O Products helps
H-O Products is a precision die-cut converter, not a raw-material producer. We do not make the aerogel, firewall sheet, mica, paper, or foam; we buy blanket, sheet, paper, and roll stock from the material manufacturers and convert it to your drawing: die-cut, kiss-cut, laser- and waterjet-cut, laminated, edge-encapsulated, slit, and kitted, with material traceability and lot-level data-sheet records under our ISO 9001:2015 certified quality management system.
Materials are evaluated against and support compliance with the cited methods through the manufacturer's data sheet; H-O does not independently certify a material to them, and the propagation result demonstrated under UL 9540A or GB 38031 belongs to your tested cell, module, pack, or system.
Frequently asked questions
How do I choose between aerogel and a firewall barrier sheet for a cell-to-cell layer?
Start with the gap, the compression, and the governing requirement. Aerogel blanket leads where conduction must be slowed in a very thin gap, and it is often faced because it can be friable. A firewall barrier sheet is purpose-built as a propagation barrier and leads where a propagation requirement governs. High-temperature silicone foam takes the slot when the layer must also compress and recover with cell breathing. Many designs laminate more than one, and the grade behavior is confirmed on the data sheet for your gap.
When is mica the answer instead of glass-fiber paper?
When very high temperature and an electrical-isolation role meet on the same part, typically a vent path, an exposed module wall, or a busbar or terminal area. Mica is inorganic, non-combustible, and serves a high-temperature electrical barrier role, but it is rigid and brittle, so it is usually laser- or waterjet-cut. Glass-fiber paper is the thin, conformable inorganic layer for interleaves and facings. They are partners more often than rivals.
Can a thermal barrier double as busbar insulation?
Sometimes, but the electrical role must be confirmed, never assumed from the thermal role. Mica and glass-fiber paper both serve high-temperature electrical-isolation functions, but dielectric behavior, creepage, and clearance are governed by their own properties and methods, and an excellent flame barrier may or may not meet a given dielectric requirement at your thickness and voltage. Call the electrical requirement out explicitly so the grade is checked against both duties.
Does any of this "pass" UL 9540A or GB 38031 at the material level?
No. UL 9540A evaluates how thermal runaway propagates through a battery energy storage system, and GB 38031 includes a thermal-propagation requirement for EV traction batteries; both apply to a cell, module, pack, or system, not to a raw barrier material. A converted barrier can help your assembly meet a propagation requirement, but the demonstrated result belongs to the tested assembly under your specific design.
Get a barrier material recommendation
Tell us the location, the job (isolation, flame, or both), the gap and compression range, the temperature exposure, and any electrical role, and we will recommend a family, a grade direction, and a converting approach.
Samples typically ship in 3 to 5 business days for common die-cut configurations; standard production runs about two weeks; special orders are quoted to the job. MOQ varies by material and part.
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Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- UL 2596 (thermal and mechanical performance of battery enclosure materials: the torch-and-grit method).
- UL 9540A (thermal-runaway propagation in energy storage systems) and GB 38031 (EV traction battery safety, incl. thermal propagation), both at assembly level.
- ASTM E162 and ASTM E662 (surface flammability and smoke characterization); UL 94 (flammability classifications); UN 38.3 (transport, pack level).
- Manufacturer technical data sheets: aerogel blanket grades, EV firewall barrier sheet, mica barrier sheet, glass-fiber paper, high-temperature silicone foam.
This article summarizes barrier-material selection principles for EV thermal-runaway and fire-protection duty using the families and published methods named above. Comparisons are qualitative; no thermal conductivity, temperature limit, or test outcome is stated for any grade, and grade-level behavior is confirmed on the manufacturer's current technical data sheet at your thickness and compression.
Propagation results belong to the tested cell, module, pack, or system. All parts are custom manufactured to order; minimum production run quantities vary by material and part. H-O is a die-cut converter, not a raw-material producer.