Fiberglass sleeve is one of the most common ways to insulate and protect wires, leads and cables in motors, transformers, heaters and furnaces. The name covers a whole family, though. A plain glass braid, a PVC coated sleeve for a control panel, a silicone coated sleeve for a Class H motor lead and an aluminized heat shield are all sold as fiberglass sleeving, and they are not interchangeable.
This guide explains the main types of fiberglass sleeve, what each coating changes, and how to choose the right one.
How Fiberglass Sleeve Is Made
Every fiberglass sleeve starts the same way. Continuous glass yarn, usually E-glass, is braided into a flexible tube. Glass does not burn or rot, and it keeps its strength at temperatures that destroy most plastics. Our fiberglass yarn is the raw material for this braid.
An open braid has small gaps between the yarns, so on its own it gives heat and abrasion protection but limited dielectric strength. Manufacturers either use the braid as it is, heat-treat it, or coat it with a resin or rubber that seals those gaps. The coating decides most of what the finished sleeve can do:
- its temperature rating
- its breakdown voltage
- how flexible it stays, hot and cold
- how it copes with oil, solvents and impregnating varnish
Uncoated Fiberglass Braided Sleeve
Uncoated sleeve is the bare braid with nothing added. It is chosen for heat rather than voltage. With no organic coating to soften, smoke or burn, it handles far higher temperatures than any coated grade.
Typical uses include protecting thermocouple and heater leads near furnaces and ovens, bundling high-temperature wiring, and wrapping parts that sit close to a heat source. Its limits are electrical and practical: the open weave gives little dielectric strength on its own, cut ends fray unless they are sealed, and loose glass fibres can irritate the skin. See our uncoated fiberglass braided sleeves.
Heat-Treated (HTG) Sleeve
Heat-treated sleeve is uncoated braid that has been baked to remove the sizing and binders left from yarn manufacture. What remains is clean glass that does not smoke or give off fumes the first time it gets hot. It is used in furnaces, glass plants, steel works and other places where fumes or residue would cause problems. See our HTG heat-treated sleeve.
Coated Fiberglass Sleeves
Coated sleeves are the ones used to insulate live conductors. The coating fills the braid, adds dielectric strength and stops the ends from fraying. Four coatings cover most needs.
PVC Coated
PVC coating gives a flexible, tough sleeve with good dielectric strength at a moderate temperature rating, typically around Class B (130°C). It is widely used in control panels, appliances, lighting and small motors. It is not meant for continuous high heat. See our PVC coated fiberglass sleeve.
Acrylic Coated
Acrylic coated sleeve is typically rated around Class F (155°C). It has good cut-through resistance and works well with most impregnating varnishes, which is why it is common on motor and transformer leads that go through dip or VPI. See our acrylic coated fiberglass sleeves.
Silicone Coated
Silicone rubber coating stays soft and flexible at high temperatures, with ratings typically around 180 to 200°C. It suits Class H motor leads, heaters, lighting fixtures and appliances where wires move or vibrate. It is softer than acrylic, so it resists cutting and abrasion less well. Our post on silicone vs acrylic fiberglass sleeve compares the two in detail, and you can find our silicone coated fiberglass sleeves here.
Polyurethane Coated and Varnished
PU coated fiberglass sleeves are flexible and resist abrasion and many chemicals. Varnished fiberglass sleeving is coated with insulating varnish and remains a dependable choice for coil leads.
Coated sleeves are graded by breakdown voltage as well as temperature, commonly from about 1.5 kV up to around 7 kV. Check both figures on the datasheet before you order.
Heat Reflective Sleeve and Fire Sleeve
These two protect hoses, pipes and cables from heat outside them, rather than insulating a conductor.
Heat reflective sleeve combines a fiberglass base with an aluminized outer layer. The shiny surface reflects radiant heat from exhausts, furnaces and engines, keeping the cable or hose underneath much cooler. See our heat reflective sleeving.
Fire sleeve is a thick fiberglass sleeve coated with silicone rubber. It shields hydraulic hoses and cables from brief flame and molten metal splash in foundries, steel plants and heavy industry. See our fire sleeve and our comparison of fire sleeve vs therma sleeve.
Fiberglass Sleeve Types Compared
| Type | Main strength | Typical temperature | Insulates a live conductor? | Typical uses |
| Uncoated braided | Very high heat resistance | Several hundred °C | Limited on its own | Furnace and heater leads, thermocouples |
| Heat-treated (HTG) | Clean glass, no fumes | Several hundred °C | Limited on its own | Furnaces, glass and steel plants |
| PVC coated | Flexible and tough | Around 130°C | Yes | Panels, appliances, small motors |
| Acrylic coated | Varnish compatible, cut resistant | Around 155°C | Yes | Motor and transformer leads |
| Silicone coated | Flexible when hot | Around 180 to 200°C | Yes | Class H motors, heaters |
| Heat reflective | Reflects radiant heat | Depends on construction | No | Hoses and cables near exhausts |
| Fire sleeve | Splash and flame protection | Brief high exposure | No | Hydraulic hoses in foundries |
Ratings vary between manufacturers and grades. Treat these as typical values and confirm them on the datasheet.
How to Choose the Right Fiberglass Sleeve
- Find the hottest continuous temperature. Match the sleeve to the insulation class of the machine, not just the average running temperature.
- Decide whether it insulates a live conductor. If it does, use a coated sleeve with enough breakdown voltage. Uncoated, heat reflective and fire sleeves are for heat and mechanical protection.
- Check the process. If the winding will be varnished or vacuum impregnated, acrylic or varnished sleeves are the usual choice. Confirm compatibility with your resin.
- Think about movement. Leads that flex or vibrate last longer in silicone coated sleeve.
- Size it correctly. Braided sleeve does not shrink, so pick an inner diameter just above the outer diameter of the wire.
If you need a sleeve that grips tightly or seals out moisture, a heat shrink sleeve may suit better. Our guide on how to choose a heat shrink sleeve and the comparison of fiberglass sleeve vs heat shrink explain when. For other options, see fiberglass sleeve vs Nomex sleeve and braided sleeve vs extruded tubing.
Picking the Right Type
Use uncoated or heat-treated sleeve for raw heat, PVC coated for general wiring, acrylic for varnished motor and transformer leads, and silicone for flexible Class H work. Heat reflective and fire sleeves shield hoses and cables from heat around them.
Harnawa manufactures and supplies the full range of fiberglass insulation sleeves, along with Teflon insulation sleeves, Kevlar sleeves and polyester expandable braided sleeves. Contact us with your temperature, voltage and wire size, and we will suggest the right sleeve.
Frequently Asked Questions
What are the different types of fiberglass sleeve?
The main types are uncoated braided sleeve, heat-treated (HTG) sleeve, coated sleeves (PVC, acrylic, silicone, polyurethane and varnished), heat reflective sleeve and fire sleeve.
What is the temperature rating of fiberglass sleeve?
It depends on the type. Coated sleeves are typically rated around 130°C for PVC, 155°C for acrylic and 180 to 200°C for silicone. Uncoated and heat-treated sleeves handle several hundred degrees. Always check the datasheet.
Which fiberglass sleeve is best for motor leads?
Acrylic coated sleeve is a common choice for leads that go through varnish or VPI. Silicone coated sleeve suits Class H motors and leads that flex or vibrate.
Is uncoated fiberglass sleeve an electrical insulator?
Glass itself insulates, but an open braid has gaps and gives limited dielectric strength. For live conductors, use a coated sleeve with a stated breakdown voltage.
What is the difference between PVC coated and silicone coated fiberglass sleeve?
PVC coated sleeve is typically rated around 130°C and suits general wiring. Silicone coated sleeve stays flexible up to around 180 to 200°C, which makes it suitable for Class H work and moving leads.
What is heat reflective sleeve used for?
It protects hoses, pipes and cables from radiant heat near exhausts, engines and furnaces. Its aluminized surface reflects heat away. It is not designed to insulate a live conductor.
