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Most backpackers and campers are familiar with Sea to Summit’s Reactor range of thermal sleeping bag liners: they have become incredibly popular over the past few years for adding warmth to sleeping bags. Just how much warmth requires some explanation – you may care to start by reading about the thermal performance of liners and how they work as part of a sleep system

But – what if you want to add even more warmth? No worries – Sea to Summit’s design team has turned the volume up to eleven with the Thermolite® Reactor Fleece Liner.

Here’s what makes it such a great addition to your gear collection:


The Thermolite® Reactor Fleece Liner is – as the name suggests – a polyester fleece material. However, that fleece is made with hollow-core Thermolite® fibers which trap extra air. This provides significant warmth without the weight or bulk normally associated with fleece materials. A technical hood with a drawcord closure and a shaped foot box enhance the thermal performance of this liner, while a one-third length zipper allows for easy entry and some ventilation. Rather than a specific temperature rating, we simply provide a guideline: the Thermolite® Reactor Fleece will add a season to your existing sleeping bag (make sure that the other components of your sleep system, particularly the sleeping mat, are up to the task – take a look at the blog post mentioned above).


The Reactor Fleece Liner tips the scales at a mere 13.4ozs / 379g. Because of its warmth to weight ratio, the Reactor Fleece Liner is an obvious choice to boost the performance of your sleeping bag in winter. You may also care to include it in your emergency gear for cold weather trips – there’s a blog post about this HERE.


The Reactor Fleece Liner comes in its own Ultra-Sil stuff sack. Its uncompressed packed size is only 8” (20cm) long x 5” (13cm) diameter.


Unlike some pieces of winter gear, the Thermolite® Reactor Fleece Liner doesn’t go into hibernation in the summer. It works really well as a stand-alone sleeping bag in air temperatures down to around 50°F / 10°C. If you have warm base layers, you may even find it comfortable at temperatures lower than this. The only thing you need to be aware of is that moving air will pass through the fleece material easily; so you need to be inside a shelter or use a tarp or poncho to keep you away from drafts.

The great part about using a Thermolite® Reactor Fleece Liner in summer is that if you are sleeping out in sandy or dusty conditions (for instance on a paddle trip, or mountain biking in the desert), you can take the Reactor Fleece Liner home and just throw it in the washing machine: it is much easier to take care of than a down sleeping bag.

Warm, light, compact, versatile – oh, and really comfortable, too.

This one really does go to eleven.

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