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Reactor Fleece Liner (adds up to 32°F)



Size Guide

The THERMOLITE® Reactor™ Fleece liner is Sea to Summit’s warmest sleeping bag liner. Add one to your sleeping bag before heading into really cold environments  or, use as a 'stand alone' sleeping bag when the weather is warmer. The hollow core THERMOLITE® fibers have the same hollow structure as polar bear – when made into a fleece, this traps a lot of insulating air. Luxuriously soft, the Reactor Fleece is perfect as part of a cold weather sleep system – plus it will help keep your sleeping bag clean.

Learn more about which liner is right for you HERE.

Does the Reactor Fleece Liner...?

  • Work for backpacking? It may. The 14.2oz weight is a very reasonable trade-off for its thermal performance, and the packed volume of 2.5 Liters is impressively small. Weight-conscious performance users may opt for a Spark Liner or Flame Liner.
  • Add 32°F / 18°C to any sleeping bag? Not necessarily. There are a lot of factors involved in the thermal performance of a sleep system, starting with the insulation value of your sleeping pad. Read more HERE.
  • Need special care when being washed? No! The Reactor Fleece Liner can be washed in a washing machine with normal supermarket laundry ‘soap’ and dried in a dryer, using cool or low heat. Fabric softeners will reduce wicking performance of the fabric. 

How to Choose

The Reactor Fleece is Perfect For...

  • Backpackers looking to add warmth to their sleeping bag for winter camping
  • Warm-weather campers who don’t need a full sleeping bag
  • Paddlers or desert users who want an easy-care summer sleeping bag they can easily wash the sand out of
  • Backpackers looking to extend the performance of a (perhaps older) sleeping bag for winter camping

Other Liner Options

  • The Reactor Extreme Liner is a slightly more compact, slightly lower-priced alternative for more moderate temperatures.
  • Performance-oriented, weight-focused end users may opt to use a  Spark or Flame sleeping bag liner.


  • Thermolite® fleece fabric is incredibly comfortable
  • Technical mummy shape with drawstring hood
  • Quarter zip makes it easy to get in and out
  • Easy care – machine wash; no special soap is needed. Avoid fabric softeners & dryer sheets to maintain wicking performance
  • Ideal for use as a sleeping bag in warmer weather or sandy environments
  • Significantly improves the thermal performance of your sleeping bag
  • Packs into its own Ultra-Sil™ stuff sack


Easy to wash and helps to keep your down bag cleaner for longer.

Technical Features


The Reactor Series Sleeping bag liners, the Venture, Traverse and Basecamp synthetic Sleeping Bags and the foot boxes of the Women’s Altitude and Journey down Sleeping Bags use hollow core THERMOLITE® fiber to add warmth. These fibers trap extra air for insulation (just like the hollow-cores of polar bear or caribou fur).

In the liners, the knitted THERMOLITE® fabric is soft and stretchy and comes in three different weights: 80g/m² fabric used in the Reactor and the underside of the Compact Plus liners; a 110g/m² fabric which is used in the Extreme, and the core and foot box of the Compact Plus liner; and the Fleece, the warmest of all the fabrics.

THERMOLITE® is also used in the Air Sprung Cell sleeping mats.

Tech Specs

Product Details

Added Warmth
Up to 32ºF / 18ºC
Best Use
Camping, Winter Camping & Travel
Comes With
Ultra-Sil stuff sack
Thermolite fleece
Zip Options
1/4th length zipper
6 ft 6 in x 28 in / 200 x 70 cm
9 x 5 in / 22 x 13 cm
14.8 oz / 420 g


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