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Reactor Extreme Liner (adds up to 25°F)

If you go to extremes, take an extreme with you

$69.95 – $74.95

Size Guide

Sleeping Bag Temperature Ratings Explained

The EN Rating scale is a scientific test to determine the warmth of a sleeping bag. The test produces a Lower rating (for male sleepers) and a Comfort rating (for female sleepers). Both are listed in the features table on the product page.

A unisex sleeping bag with a Lower limit of 14ºF means a man should be comfortable down to air temperatures of 14ºF. The same bag will also have a Comfort limit, in the realm of 27°F, so a woman would be comfortable down to an air temperature of 27ºF. These ratings require a sleeping mat with an R-Value appropriate for expected ground temperatures – without this, the sleeping bag itself is unlikely to keep the sleeper warm enough.

A variety of factors impact sleeping bag warmth - the above temperatures are merely guidelines, not guarantees. It’s important to think of a sleeping bag, an appropriately insulating sleeping pad and a liner as a sleep system. For example - if you plan on sleeping in nighttime air temperatures of 27ºF / -3°C, choose a bag with a gender-appropriate rating of 23°F / -5°C, plus a sleeping mat with an R-Value of 2.5 to 3, and consider a Reactor thermal liner.



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.

Does the Reactor Extreme Liner...?

  • 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.

How to Choose

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.

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 25°F / 14°C
Best Use
Backpacking, Camping & Winter Camping
Comes With
Ultra-Sil stuff sack
Thermolite polyester fabric
reactor-extreme-liner-adds-up-to-25-f-regular.svg reactor-extreme-liner-adds-up-to-25-f-long.svg
82 in / 208 cm
6.5 x 4.5 in / 17 x 11.5 cm
14 oz / 399 g
92.5 x 35.4in / 235 x 90cm
6.5 x 4.5 in / 17 x 11.5 cm
14.9oz / 422g


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