Sea to Summit Thermolite® Reactor Extreme Liner (Adds up to 25°F)

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  • Regular price $ 74.95

The THERMOLITE® Reactor™ Extreme liner is Sea to Summit’s warmest knitted sleeping bag liner. If your adventures take you to colder climates, the Reactor Extreme will add plenty of warmth to a sleeping bag. In warmer weather, it can be used a ‘stand alone’ sleeping bag. The hollow core fibers of the THERMOLITE® fabric work like hollow polar bear or caribou fur – along with the knitted structure, this traps plenty of insulating air. Super soft and amazingly stretchy, the Reactor Extreme uses a heavier version of THERMOLITE®  to provide extra warmth for an incredibly comfortable night’s sleep. Use one to build your cooler-weather sleep system and to help to keep your sleeping bag clean, too.

Learn more about which liner is right for you HERE.


  • Add 25°F / 15°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.
  • Work for backpacking? For most backpackers. The Reactor Extreme weighs 13.9 ozs – a reasonable trade off in weight/thermal performance for occasional cold-weather backpackers. Weight-conscious performance users may opt for a Spark Sp 0 / Flame Fm 0.
  • Need special care when being washed? No! The Reactor Extreme can be washed in a washing machine with normal supermarket laundry ‘soap’; just avoid fabric softeners as they will reduce the wicking performance of the fabric. You can dry it in a dryer, use cool or low heat.


The Reactor Extreme is Perfect For...

  • Backpackers whose sleep system will be used in colder climates
  • Campers whose sleeping bags don't keep them warm a the advertised temperature
  • Very warm-weather campers who don't need a full sleeping bag
  • Campers or backpackers looking to extend the performance of a (perhaps order) sleeping bag into shoulder seasons

Other Liner Options

  • The Reactor Fleece Liner will add even more warmth to your sleeping bag.
  • Performance-oriented, weight-focused end users may opt to use a Spark or Flame as a sleeping bag liner.


  • Stretchy, Thermolite® fabric is incredibly comfortable
  • 110g/m² fabric is warmer than the 80g/m² Thermolite® used in the standard Reactor
  • Works like your favorite cold-weather base layer to trap insulating air and increase warmth
  • Easy care – no special soap is needed
  • Significantly improves the thermal performance of your sleeping bag
  • Comfortable stretchy fabric
  • Keeps sleeping bag clean
  • Machine wash and air dry - Avoid fabric softeners & dryer sheets to maintain wicking performance
  • Packs into its own Ultra-Sil™stuff sack


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