Sea To Summit Thermolite Reactor Liner review: feels like a fleece but without the weight

Add warmth without weight with the fleecy Sea To Summit Thermolite Reactor Liner

Sea To Summit Thermolite Reactor Liner
(Image: © Sea To Summit)

Advnture Verdict

Push the temperature of your sleeping bag by adding Sea to Summit’s Thermolite cosy Reactor, which has the snug, soft feel of fleece but at a far lower weight.


  • +

    Reliable added warmth

  • +

    Fleecy feel

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  • -

    Too warm for summer weather

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Sea To Summit Thermolite Reactor Liner: first impressions

The Sea To Summit Thermolite Reactor Liner – the brand’s bestselling liner – is made from lightweight Thermolite fabric, which traps in body heat fast – if you couple this with your best sleeping bag, you’ll have a guaranteed toasty sleep system that suits most temperatures.

Thermolite is a synthetic fabric that feels rather like fleece to touch but is lighter in weight and more packable. So you get winter-ready warmth without too much bulk and at 248g, barely any added weight.

This is perfect if you’re packing light for hike-camp adventures or a wild camping and want to push the temperature you’re cosy at – and all with the quite luxuriously comfy feeling of fleece against the skin.


• RRP: $63 (US) / £50 (UK)
• Weight: 248g / 9oz
• Pack size: 15cm x 11cm / 6in x 4in
• Material: Thermolite
• Shape: Mummy
• Colors: Black / Yellow
• Compatibility: Ideal for adding warmth to wild camps without significant weight

Sea To Summit Thermolite Reactor Liner: in the field

Sea To Summit Thermolite Reactor Liner

Packs up small and light (Image credit: Sea To Summit)

A lot of brands talk big about how much extra heat their sleeping bag liners can add to your sleep system, but with the Thermolite Reactor this was no hollow boast. It was one of few on test for our best sleeping bag liners buying guide that we found significantly improved how much warmth was trapped inside our sleeping bag. 

This mummy shape fits nicely into tapered sleeping bags, too. We tested this liner in early spring and found it definitely helped stave off the chill, but if you need something that offers a bigger hit of heat for cold winter nights you could upgrade to the Reactor Extreme (£60 / $70), an even warmer version claiming to offer a boost of 15°C.

Sian Lewis

An award-winning travel and outdoors journalist, presenter and blogger, Sian regularly writes for The Independent, Evening Standard, BBC Countryfile, Coast, Outdoor Enthusiast and Sunday Times Travel. Life as a hiking, camping, wild-swimming adventure-writer has taken her around the world, exploring Bolivian jungles, kayaking in Greenland, diving with turtles in Australia, climbing mountains in Africa and, in Thailand, learning the hard way that peeing on a jellyfish sting doesn’t help. Her blog,, champions accessible adventures.