A liner is one of the cheapest ways to make a sleeping bag warmer. 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 Extreme Liner sits in the middle of the Reactor range: warmer than the Lightweight, lighter and smaller than the Fleece. It cost us $130.
What’s in the box
A mummy-shaped liner in a soft, stretchy knit of thicker Thermolite Pro, 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 drawcord, and the fabric has HeiQ Fresh odour control.
What we like
- Adds up to 6°C of warmth
- Soft, stretchy Thermolite Pro
- Shoulder snaps, drawcord hood
- Drawcord footbox
- 350 g, packs fairly small
- HeiQ Fresh odour control
What we don’t
- No side zip like the Fleece
- Snug mummy shape
Quick Specs
| Spec | Details |
|---|---|
| Brand | Sea to Summit |
| Material | Thermolite Pro100% recycled polyester |
| Weight | 350 g |
| Packed size | 19 × 11.5 × 11.5 cm |
| Shape | Mummy |
| Temperature rating | Adds up to 6°C |
| Features | Shoulder snapsDrawcord hoodDrawcord footboxHeiQ Fresh odour control |
Warmth
The Thermolite Pro knit is thicker than the Reactor Lightweight’s, and Sea to Summit says it adds up to 6°C to your sleeping bag. That’s enough to take a three-season bag a good way into cooler nights, without the weight of a fleece liner. The knit is soft and stretchy.
Hood, snaps and footbox
You slide in from the top. Snaps at the shoulders open it up a little wider to get in, and the hood closes around your head with a drawcord. The footbox has its own drawcord with a toggle, so you can close it up or open it to cool your feet.
Weight and packing
At 350 g it’s not much lighter than the Fleece, but it packs smaller, to about 19 × 11.5 × 11.5 cm. It’s easy to air out too: on the trip we hung ours under the awning.
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 Lightweight is the lightest, and the Comfort Blend is a rectangular liner made to keep you cool.
This review Reactor Extreme |  Comfort Blend |  Reactor Fleece |  Reactor Lightweight | |
|---|---|---|---|---|
| Adds warmth | 6°C | – | 8°C | 4°C |
| Material | Thermolite Pro | Coolmax, Tencel | Grid fleece | Thermolite Pro |
| Shape | Mummy | Rectangular | Mummy | Mummy |
| Opening | Top, snaps | Top, no zip | Side zip | Top, snaps |
| Packed size | 19 × 11.5 × 11.5 cm | 15 × 7.5 × 12 cm | 20 × 13 × 13 cm | 18 × 10 × 10 cm |
| Weight | 350 g | 290 g | 379 g | 286 g |
| Price paid | $130 | $60 | $110 | $90 |
| Our score | 8.9 | 8.5 | 9.3 | 8.7 |
Conclusion: Sea to Summit Reactor Extreme LinerConclusion
The Sea to Summit Reactor Extreme Liner adds up to 6°C to a sleeping bag in a soft, stretchy Thermolite Pro knit, with shoulder snaps and drawcords at the hood and footbox. At 350 g it packs smaller than the Reactor Fleece, though it has no side zip and adds a bit less warmth. At $130 it cost us more than the Fleece, but it’s a good all-round liner for cooler nights when space counts.
A stretchy Thermolite Pro mummy liner that adds up to 6°C, with shoulder snaps and drawcords at the hood and footbox.
350 g, $130; no side zip.
- Hikers
- Car campers
- Rooftop tents
Yes, if you want warmth in a smaller pack. $130 buys a good chunk of extra warmth in a liner that packs smaller than the Fleece. If weight doesn't matter, the Reactor Fleece adds more for less.
