Wild CampingTherm-a-RestSleeping Bag
Therm-a-Rest Parsec –18C sleeping bag review: high-quality warmth for the coldest UK adventures
Tested in the Cairngorms at –9°C, this exceptional deep-winter bag delivers one of the best warmth-to-weight ratios we have seen — if you can stomach the premium price.
✓ Top PickTherm-a-Rest Parsec –18C
Weight1.09 kgFill power800-fillTemp rating–10°C comfort
The good✓ Outstanding warmth-to-weight ratio
✓ Lofts up almost immediately
✓ Hydrophobic down resists moisture
✓ Very packable for a deep-winter bag
✓ Toe-asis foot warmer pocket is excellent
✓ Robust recycled ripstop nylon shell
✓ Includes stuff sack and storage bag
The not-so-good✗ Very expensive (£450–550)
✗ Pad attachment straps too small for most mats
At a glance
| Brand | Therm-a-Rest |
| Model | Parsec 0F/–18C |
| RRP | £450–550 (commonly on offer) |
| Weight | 1.09 kg (regular) |
| Fill power | 800-fill power hydrophobic down |
| Down weight | 750 g |
| Comfort temp | –10°C |
| Limit temp | –18°C |
| Shell | 100% recycled nylon ripstop, DWR treated |
| Extras | Toe-asis foot warmer |
Overview
The Therm-a-Rest Parsec –18C is the warmest bag in the brand’s Parsec line — a deep-winter down sleeping bag built to keep you comfortable in conditions that most campers would sensibly avoid. Filled with 750 grams of 800-fill power hydrophobic goose down, it offers exceptional warmth-to-weight performance, packing down to roughly the size of a rugby ball at just 1.09 kg. That makes it the lightest and most compressible deep-winter bag we have tested at GBAC.
At £450–550 depending on where you shop, it sits firmly at the premium end of the 4-season sleeping bag market. But in a category where alternatives are either heavier, less packable or similarly priced, it proves itself competitive.
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How we tested
We tested the Parsec –18C across several trips including relaxed wildcamps in the Chilterns and a couple of nights on Dartmoor. The most significant test was a three-day backpacking trip through the Cairngorms in winter, where overnight temperatures dropped to –9°C — almost exactly the bag’s stated comfort limit. We slept in it in a bothy at around 5°C and on exposed pitches in seriously cold conditions, giving us a thorough picture of its real-world performance range.
Features and performance
Warmth and packability
The Parsec’s warmth is exceptional. At –9°C, we slept comfortably wearing Helly Hansen Lifa baselayers and thick wool socks — and even started the night in a Patagonia Micro Puff before removing it when we got too warm. The 800-fill power down lofts back into shape almost instantly after compression, and even after leaving it packed tightly for three days the down showed no clumping.
“The lightest and most compressible deep-winter bag we have ever tested at GBAC — and one that actually delivers on its warmth claims.”
Build and features
The outer shell is 100% recycled nylon ripstop with a DWR coating — it kept the down dry during a light shower when the bag was accidentally left outside, beading water off the surface for around five minutes. The Toe-asis foot warmer pocket is a genuinely useful addition: a secondary enclosed foot box that creates measurably more warmth at the extremities in the coldest conditions, and which quickly became one of our favourite features.
The pad attachment straps are the one significant design shortcoming. Intended to keep the bag from sliding off your mat, they were too small to fit around most pads — including Therm-a-Rest’s own NeoAir XTherm. Adjustable straps in a future iteration would make the system genuinely useful.
Our verdictPremium, proven and worth every penny for serious winter campers.The Therm-a-Rest Parsec –18C is an exceptional deep-winter sleeping bag. It delivers one of the best warmth-to-weight ratios on the market, packs down smaller than anything else we have tested in its class, and comes with several smart design touches that make a real difference on cold nights.For anyone who regularly camps in serious UK winter conditions — multi-day backpacking in the Scottish hills, exposed winter wild camps or similar — it is a worthy and durable investment. If you only occasionally venture out in cold weather, a more affordable alternative will serve you better. But for those who need it, the Parsec is outstanding.
FAQs
How warm is the Therm-a-Rest Parsec –18C in real-world conditions?Very warm. We tested it at –9°C in the Cairngorms and slept comfortably wearing baselayers and a thin fleece — close to the bag’s stated comfort limit. For most UK winter wild camping it is more than adequate.
How comfortable is the Therm-a-Rest Parsec –18C to sleep in?Very comfortable. The 800-fill down lofts immediately, the DWR-treated shell is smooth and pleasant, and the hood drapes naturally. The Toe-asis foot pocket adds notable warmth at the extremities on the coldest nights.
How packable is the Therm-a-Rest Parsec –18C?Impressively so for a deep-winter bag. At 1.09kg it packs down to roughly the size of a rugby ball — the lightest and most compressible 4-season bag we have tested at GBAC.
How does the Therm-a-Rest Parsec –18C perform in damp conditions?Better than most. The hydrophobic down coating resists moisture absorption and dries significantly faster than untreated down — a genuine advantage in the inevitably damp conditions of UK winter camping.
Is the Therm-a-Rest Parsec –18C worth the money?If you need a deep-winter bag and will use it regularly, yes. At £450–550 it is at the top of the market, but the warmth-to-weight ratio, build quality and thoughtful features justify the cost for committed winter campers.