Thermolite Insulation FAQ: Reactor Liner, Endura Tights, UA Storm Fleece Pant and Nylon Breathability

I work in performance-fabric sourcing for cold-weather products. In 14 years, I've handled more than 300 rush material requests for apparel brands, outdoor manufacturers, and one very anxious product team trying to hit a February trade-show deadline. When the calendar shrinks, nobody wants a long introduction. So here are the seven direct answers that come up over and over when Thermolite shows up in a spec.

• What exactly is Thermolite?
• How does Thermolite compare with down and Primaloft?
• Does the Sea to Summit Reactor Thermolite liner really add warmth?
• Are Endura Thermolite tights warm enough for winter riding?
• Can a UA Storm fleece pant replace a hard shell?
• What exactly is a twill stretch woven fabric?
• So, is nylon a breathable fabric?

What exactly is Thermolite?

Thermolite is not one fabric. It's a family of thermal insulation materials made from engineered polyester fibers, and most versions use a hollow or channeled core to trap air. The air does the insulating; the polyester just holds it still.

That's why you'll see the name on everything from boot liners to sleeping bag liners to fleece-backed tights. The variety confuses buyers. When a supplier says Thermolite, the useful follow-up is: knit or batting? What weight? Continuous filament or cut staple? A lightweight hoodie and a winter cycling tight can both list Thermolite, but they solve completely different problems.

Thermolite is a trademark of The Lycra Company, which supplies the fiber technology to mills and brands. For anyone making a quick decision, it helps to remember that Thermolite in the spec is step one. The fabric construction and weight matter just as much as the fiber name.

How does Thermolite compare with down and Primaloft?

Down is still the lightweight champion whenever water is not part of the equation. Nothing beats it for warmth per gram in a dry sleeping bag or jacket. The problem is damp. Once down gets wet, it collapses, and all that warmth disappears.

That is the main reason synthetic insulation exists. Thermolite and Primaloft are both polyester-based, and both keep working far better than wet down. The difference is more about construction than a magic fiber. Primaloft often uses fine microfiber batting that mimics down clusters. Thermolite is broader: hollow-core staple fiber for sleeping bag liners and jacket batting, plus brushed knit and fleece forms for active gear.

So choose down when dry weight and pack size matter most. Choose a synthetic like Thermolite when you might get damp, when drying speed matters, or when the garment will face wet snow and sweat. There is no universal winner; there is only the right answer for the conditions you expect.

Does the Sea to Summit Reactor Thermolite liner really add warmth?

Short answer: yes, when it fits properly and is used as intended.

Sea to Summit's published product rating for the Thermolite Reactor liner is that it adds up to 8 °C (14.5 °F) to the warmth rating of a sleeping bag. The heavier Reactor Extreme is rated at up to 14 °C (25 °F). I checked those figures in January 2025; verify them on the current product page because specs and marketing copy do change.

The key phrase is 'added warmth', not a guaranteed final temperature. If a bag has a comfort limit near 5 °C, an 8 °C liner should shift the limit roughly to -3 °C. But real-world results depend on fit. If the liner is too loose, too small, or you move around a lot at night, drafts find the gaps. In my experience, the published number is achievable in a controlled test or a well-fitted mummy bag; in a roomy rectangular bag, the effective gain is closer to 3-5 °C.

That still makes a liner like this one of the most practical ways to add emergency warmth without buying a whole new bag. It also dries fast and adds a washable layer between you and the bag's interior.

Are Endura Thermolite tights warm enough for winter riding?

Warm enough depends on the rider, the ride length, and the wind. There is no single temperature at which any pair of tights is suddenly sufficient. A five-minute commute at low effort feels completely different from an hour-long club ride.

Endura Thermolite tights use Thermolite as the insulating layer in a tight-fitting cycling garment. The close fit helps trap a thin layer of warm air against the legs without the bulk of a baggy winter pant. They are typically a strong choice around 5-10 °C for moderate riding. When the temperature drops toward freezing or the pace picks up, windchill changes the equation. The insulation is not the problem; wind is. At cycling speed, the wind can cut right through a knitted fleece even when the air temperature is not that extreme.

That's when you add a windproof or soft-shell outer layer. If a rider tells me they plan to use Thermolite tights below 0 °C, I always ask what shell they own. The tights are the warmth layer; they are not the whole weather system.

Can a UA Storm fleece pant replace a hard shell?

No. And once you understand why, it's easier to plan the rest of your layering.

UA Storm is a durable water-repellent finish, not a waterproof membrane. It makes water bead up and roll off the surface when the fabric is new. That works well for light snow, a brief shower, or damp trail conditions. It is not enough for heavy, persistent rain. Eventually the DWR saturates and the fleece underneath starts wetting out.

A UA Storm fleece pant is really a do-it-all dry-cold pant. It gives you the warmth of a midweight fleece plus a bit of weather resistance on top. For high-output winter activities such as running or hiking, that is actually a great balance because you want moisture vapor to escape. A fully waterproof hard shell would trap heat and sweat.

So treat it as a winter outer pant for dry conditions, not as a rain pant. If you know you'll face sustained rain, carry a lightweight hard-shell overpant and save the Storm fleece for cold, crisp days.

What exactly is a twill stretch woven fabric?

A twill weave is the structure that makes denim look the way it does. The yarns cross in a way that creates a diagonal line across the fabric. That structure gives fabric better drape and often better tear resistance than a plain weave of similar weight.

A twill stretch woven fabric takes that construction and adds stretch. The stretch can come from elastane wrapped in the yarns, or from mechanically stretchy texturized synthetic yarns. The result is a fabric that moves with the body instead of fighting it. That's why you see twill stretches in cycling pants, soft-shell trousers, and fitted outerwear where mobility matters.

One caution: a twill stretch woven fabric is not automatically breathable. The weave affects hand feel, stretch, and abrasion, but airflow depends on yarn size, tightness, and finish. Some twill stretch fabrics are quite air-permeable; others are calendered tight enough to block wind. When a supplier describes a fabric this way, ask for an air-permeability number rather than assuming more airflow.

So, is nylon a breathable fabric?

The honest answer is: not by itself.

Nylon is a smooth, synthetic polymer fiber. Unlike wool or cotton, it does not absorb much moisture. A tightly woven nylon fabric can be extremely wind-resistant and, in some cases, almost completely airtight. That is why nylon is everywhere in rain shells. It is not naturally good at letting air or water vapor pass.

But the word 'nylon' covers so many constructions that the question is almost unanswerable without detail. A nylon mesh can be very breathable. A 20-denier nylon ripstop used for a wind shirt can be nearly impermeable. A nylon microfiber from a stretch woven might be somewhere between the two. Breathability is a property of the fabric construction, not of the polymer itself.

For technical fabrics, look for test data: air permeability is commonly measured by ASTM D737 in cubic feet per minute, and moisture-vapor resistance is measured under ISO 11092 as RET. A fabric can have decent airflow but poor vapor transfer, or the other way around. When someone asks me whether nylon is breathable, I ask which nylon and which test. In a layering system, the shell's breathability is often the bottleneck. You can wear the best moisture-managing base layer and a perfectly breathable Thermolite mid layer, but if your nylon outer shell traps everything inside, you will still end up damp.