How to Use a Reactor Extreme Liner to Boost Sleeping Bag Warmth

By Sea to Summit | Outdoor Gear To Equip & Inspire | Published: 2026-10-27

Category: How-to Guides

Learn how to properly use the Sea to Summit Reactor Extreme liner to add significant warmth to your sleeping bag without adding bulk.

Quick answer

Use the Reactor Extreme liner inside your sleeping bag like a fitted sheet, pulling it up around your body and cinching the drawcord to trap heat. This adds up to 25°F (14°C) of warmth to your sleep system.

Why the Reactor Extreme liner works differently

Unlike standard liners that only add a thin layer of fabric, the Reactor Extreme uses Thermolite® Pro fabric with infrared-absorbing ceramic pigments. These pigments reflect your body heat back to you, giving a thermal boost that feels like upgrading to a warmer sleeping bag without the extra weight or cost.

The stretch-knit design also allows full freedom of movement and fits most sleeping bag shapes, so you won't feel restricted or bunch up the liner while you sleep.

Common mistake: using the liner incorrectly

Many campers simply toss the liner inside the bag and hope for warmth. The Reactor Extreme needs to be spread flat, with the foot box aligned to your sleeping bag's foot box. If the liner twists or bunches, it creates cold spots and reduces the thermal benefit.

Also, avoid over-tightening the drawcord. A snug seal around your neck keeps warm air in, but too tight restricts breathing and can cause condensation inside the bag.

Step-by-step guide: Using the Reactor Extreme liner

1. Lay the liner flat inside your sleeping bag: Unzip your sleeping bag and spread the Reactor Extreme liner evenly inside, matching the foot box of the liner with the foot box of the bag. Smooth out any wrinkles or folds. Check that the liner is not twisted; a twisted liner creates cold spots and reduces warmth.
2. Get in and pull the liner up around your body: Sit or lie down inside the sleeping bag, then pull the liner up over your legs and torso. The stretch-knit fabric allows you to move freely without the liner riding up or bunching. If the liner feels too loose, it won't trap heat effectively; if too tight, it may restrict movement.
3. Cinch the drawcord to seal in warmth: Gently pull the drawcord at the top of the liner until it fits snugly around your neck or shoulders. Do not overtighten—leave enough room to breathe comfortably. A common mistake is cinching too tight, which can cause discomfort and moisture buildup from your breath.
4. Zip up the sleeping bag and adjust as needed: Once the liner is in place, zip your sleeping bag partially or fully depending on temperature. The liner's added warmth may allow you to sleep with the bag less zipped in milder conditions. Test the setup at home before a trip to find the right zipper position for your comfort.

Product facts

Reactor Extreme Sleeping Bag Liner: The Reactor Extreme liner with Thermolite® Pro fabric utilizes innovative infrared-absorbing ceramic pigments to give your system the thermal boost needed in colder-weather adventures. This means the liner actively reflects body heat, adding warmth without extra bulk, making it ideal for extending the range of your current sleeping bag.
Reactor Extreme Sleeping Bag Liner