Sea to Summit Reactor Sleeping Bag Liner Review

Overview

The Sea to Summit Reactor is the brand’s best-selling sleeping bag liner, built from a 50/50 blend of Thermolite® Pro and Thermolite® EcoMade stretch-knit fabric. It’s aimed at backpackers and travelers who want to extend the warmth of an existing bag without buying a whole new sleep system. At 267g and packing down to roughly the size of an apple, it’s a practical add-on rather than a significant weight penalty — though if you’re counting every gram, it’s not weightless.

Key Specs

Spec	Detail
Weight	267 g (9.4 oz)
Thermal Boost	+14°F (+8°C)
Fabric	50% Thermolite® Pro / 50% Thermolite® EcoMade
Fabric Weight	80 g/m²
Shape	Mummy
Packed Size	7 × 15 cm (~2.8 × 5.9 in)
Sizes	Compact (198 × 80 cm) / Standard (216 × 80 cm)
Styles	Classic Mummy / Mummy with Drawcord
Odor Control	HeiQ Fresh FFL
Includes	Ultra-Sil stuff sack
Comparison	See how Reactor Sleeping Bag Liner compares to similar gear

Performance

Warmth

The headline claim is +14°F, and real-world results are — predictably — mixed. One REI reviewer found the 14-degree boost “maybe optimistic” but felt it performed well enough to turn a 41°F-rated sleeping bag into something closer to a 31°F bag during an early October wilderness overnight. That roughly 10°F real-world gain is a number that comes up repeatedly in community reviews, and it’s probably a more honest expectation than the marketing figure. Whether you get the full 14°F improvement? Not in most users’ experience — but that’s probably something that was already suspected.

A dissenting voice from Trailspace found the gains fell meaningfully short: the warmth the Reactor adds is only achieved when used with a top cover of some sort, due to the extreme breathability of the material — the fabric is more like an elaborate web of thread very loosely woven together than a piece of cloth. That’s a fair characterization of the fabric construction. The 80 g/m² weight is light, and the open-knit structure that makes it so breathable and comfortable also limits insulation at the margins.

Comfort & Fit

This is where the Reactor earns its reputation. Thermolite is a synthetic fabric that feels rather like fleece to touch but is lighter in weight and more packable. The stretch-knit fabric provides optimal comfort and ensures the Reactor fits any sleeping bag — that matters if you’re pairing it with a quilt or a non-mummy bag. The material does a good job at wicking sweat away while still being breathable enough to avoid overheating. Even though this sleeping bag liner is marketed towards those sleeping in cold conditions, users find it versatile enough to use in temperate climates as well.

There’s no side zipper — Sea to Summit omitted it to keep weight and packed volume down — but snaps at the shoulders provide a wider opening than some liners for entry and exit, and the drawcord footbox allows you to vent your feet or hitch up the liner and walk around camp.

The “walk to the bathroom in the dark” factor on the Drawcord version is a genuinely useful touch.

Odor & Hygiene

Sea to Summit liners are treated with HeiQ Fresh FFL, a biocide-free, highly effective odor control technology that uses a bio-based Amino Sugar Polymer to neutralize body odor.

In practice, this pairs nicely with the liner’s primary job of keeping your bag clean on multi-day trips.

A benefit of all liners is they help keep your sleeping bag fresher for longer, and it’s way easier to wash a small liner than a large bag on an expedition.

The Reactor is machine washable and air-dries quickly, which matters on longer trips.

Sustainability

The recycled Thermolite EcoMade fibers are made from 100% textile waste, are GRS-certified, and offer the same quality and durable lightweight warmth as their virgin polymer counterparts but with a lower carbon footprint.

The fact that half the fabric is reclaimed material without compromising on specs is a genuine plus.

Packability

The Reactor is compact and easy to store, coming in a little drawstring pouch — it takes up very little space, around the size of an apple.

It can even be tucked inside your sleeping bag so it’s less likely to be forgotten.

The 267g weight is real but not punishing — it’s the kind of thing most three-season backpackers can justify carrying when shoulder-season temps are in the forecast.

Pros & Cons

Pros

• Stretch-knit construction fits any sleeping bag shape without bunching
• Fleece-like feel against skin — genuinely comfortable to sleep in
• Compact and relatively light for a synthetic insulated liner
• HeiQ Fresh odor treatment is biocide-free and effective for multi-day use
• Thermolite EcoMade component uses GRS-certified recycled textile waste
• Machine washable and fast-drying
• Drawcord footbox version allows camp mobility without fully exiting the liner
• Delivers a real, if modest, warmth boost in practice

Cons

• The +14°F claim is optimistic; real-world gain is closer to 8–10°F for most users
• No side zipper — can feel confining for restless sleepers
• At 267g, it’s not an ultralight choice; a silk liner saves around 175g if warmth isn’t the priority
• Price (~$75) is on the higher end for a liner
• Open-weave fabric is potentially vulnerable to snagging on rough surfaces

Who Should Buy This

The Reactor is a strong fit for three-season backpackers who want a reliable margin of warmth on unexpectedly cold nights without buying a new bag. It’s also a smart travel companion for anyone doing hostels, guesthouses, or international hut trips where cleanliness matters and temperatures are unpredictable. Cold sleepers, in particular, will get more tangible benefit from the thermal boost than warm sleepers. If you’re a strict gram-counter already pushing a well-rated bag in its comfort zone, a lighter silk liner is the more appropriate call.

Verdict

The Sea to Summit Reactor does what a liner should — it extends your sleep system’s range, keeps your bag cleaner, and adds meaningful comfort — without costing you a separate stuff sack’s worth of sanity to pack. Just temper your expectations on that +14°F number; plan on 8–10°F in real use, and you won’t be disappointed. At 8/10, it’s the liner I’d recommend first to most three-season hikers shopping in this category.