Sometimes you just want a little extra warmth for very little weight. We took four Sea to Summit liners on our big trip from Sydney to Adelaide, Melbourne and back, and tried them all. The Sea to Summit Reactor Liner, now sold as the Reactor Lightweight, is the lightest of the three Reactors: 286 g on our scale, and it adds up to 4°C to your sleeping bag. It cost us $90.
What’s in the box
A mummy-shaped liner in a soft, stretchy Thermolite Pro knit, made from 100% recycled polyester, with snaps at the shoulders, a hood with a drawcord and a drawcord footbox. It packs into a grey stuff sack with a yellow label, and the fabric has HeiQ Fresh odour control. On the box Sea to Summit rates its warmth boost as “Cosy”, the first step of its scale.
What we like
- Light: 286 g on our scale
- Packs to 18 × 10 × 10 cm
- Adds up to 4°C of warmth
- Soft, stretchy Thermolite Pro
- Shoulder snaps, drawcord hood
- HeiQ Fresh odour control
What we don’t
- Least warmth of the Reactors
- No side zip like the Fleece
- Snug mummy shape
Quick Specs
| Spec | Details |
|---|---|
| Brand | Sea to Summit |
| Material | Thermolite Pro100% recycled polyester |
| Weight | 286 g |
| Packed size | 18 × 10 × 10 cm |
| Shape | Mummy |
| Temperature rating | Adds up to 4°C |
| Features | Shoulder snapsDrawcord hoodDrawcord footboxHeiQ Fresh odour control |
Warmth
The Reactor’s knit is the thinnest of the three Reactors, and Sea to Summit says it adds up to 4°C. That’s enough to make a sleeping bag a bit cosier on a cool night, and it keeps the inside of the bag clean. If you want real extra warmth, the Extreme or the Fleece add more; the Naturehike ZY20 is lighter again, but adds very little warmth.
Hood, snaps and footbox
It works like the Reactor Extreme: you slide in from the top, with snaps at the shoulders and a drawcord to close the hood around your head. The footbox has its own drawcord with a toggle, to close it up or open it when your feet get warm.
Weight and packing
This is where it shines. Ours weighed 286 g in its stuff sack, and it packs to about 18 × 10 × 10 cm, the smallest of the three Reactors. It’s an easy one to throw in a pack for a hike as well as the car.
Vs the other liners
We’ve reviewed all four Sea to Summit liners we took on the trip. The Reactor Fleece is the warmest and our favourite, the Reactor Extreme sits in the middle, and the Comfort Blend is a rectangular liner made to keep you cool.
This review Reactor Lightweight |  Comfort Blend |  Reactor Extreme |  Reactor Fleece | |
|---|---|---|---|---|
| Adds warmth | 4°C | – | 6°C | 8°C |
| Material | Thermolite Pro | Coolmax, Tencel | Thermolite Pro | Grid fleece |
| Shape | Mummy | Rectangular | Mummy | Mummy |
| Opening | Top, snaps | Top, no zip | Top, snaps | Side zip |
| Packed size | 18 × 10 × 10 cm | 15 × 7.5 × 12 cm | 19 × 11.5 × 11.5 cm | 20 × 13 × 13 cm |
| Weight | 286 g | 290 g | 350 g | 379 g |
| Price paid | $90 | $60 | $130 | $110 |
| Our score | 8.7 | 8.5 | 8.9 | 9.3 |
Conclusion: Sea to Summit Reactor Lightweight LinerConclusion
The Sea to Summit Reactor Lightweight Liner is the lightest Reactor: a soft, stretchy Thermolite Pro mummy liner that adds up to 4°C, weighs 286 g in its stuff sack and packs to about 18 × 10 × 10 cm. It adds less warmth than the Extreme or Fleece and has no side zip, but it’s the easy pick when weight and space count.
The lightest Reactor: a stretchy Thermolite Pro mummy liner that adds up to 4°C, at 286 g in its stuff sack.
Less warmth than the Extreme or Fleece, and no side zip.
- Hikers
- Travellers
- Campers
Yes, if weight matters. For $90 it adds a few degrees and keeps your bag clean for less than 300 g. For more warmth, the Reactor Extreme or Fleece are better.
