The Therm-a-Rest Parsec 0F sleeping bag is a super warm, cosy and perfectly put-together sleeping bag. Offering unbeatable warmth for its weight and coming with a handful of additional smart features, such as straps for your pad and an enclosed foot box for warming up, the bag is a versatile heating machine for use in the UK’s coldest conditions. But only if you’re happy to part with some serious money. 

Therm-a-Rest Parsec 0F sleeping bag

Ideal for: Wildcamping, 4-season backpacking, winter adventures

Not suitable for: Campsite camping in summer, ultralight adventures

Incredibly warm, flawlessly designed and adept at keeping you comfortable in anything the UK will ever throw at you, the Therm-a-Rest Parsec 0F is one of the most impressive sleeping bags we’ve ever tested. It delivers market-leading warmth to weight, is decidedly packable and easy to carry, and comes with some extra features that make this bag such a joy to use. But none of that comes cheap.

The Good

Amazing warmth to weight

Lofts up quickly

Hydrophobic down

Very packable

800-fill power down is exceptionally lofty

Great extra features

Comes with stuff sack and a lofty storage bag

The Bad

Very expensive

The Therm-a-Rest Parsec 0F sleeping bag review

The Therm-a-Rest Parsec 0F is a deep-winter down bag designed to keep you warm in brutally cold temperatures. The warmest in the brand’s high-quality Parsec line, the point of the Parsec 0F is to offer 4-season warmth in a lightweight and easy-to-transport package, promising to keep you warm down to -18° Celsius. And, as the bag only weighs 1.09 kg for the regular version, it seems like Therm-a-Rest has delivered on their original brief, with the Parsec 0F offering one of the best warmth-to-weight ratios we’ve ever seen in a sleeping bag.

The warmth the Parsec delivers comes from 750 grams of high-quality 800-fill power hydrophobic goose down. Treated to perform when wet, Therm-a-Rest claims the hydrophobic feathers absorb up to 90% less water than untreated feathers and dry up to 5 times faster. As these feathers have been optimally spread through the bag to deliver more warmth where you need it and less where you don’t (such as along the underside of the bag), the Parsec has been fully optimised to be as warm as possible — all while being as light and as packable as a bag of this calibre could ever be.

What does fill power actually mean? 

OK, so it’s all well and good to tout the ‘fill power’ of the Therm-a-Rest Parsec 0F. But what does that actually mean? Well, ‘fill power’ is simply a marker to understand the quality of the down used versus the volume. It’s a number that’s based on the amount the down in the sleeping bag lofts back up after being compressed, with higher-quality down lofting up more than lower-quality down. In general, the higher the fill power, the less down you need to deliver the same effect — making for lighter and more packable down-filled products.

For the most part, 400-fill power is the lowest most brands use, rising to 650, 700 and 800-fill power in the lightest and most packable products. Then, at the top end, you’ve 900 and 1000-fill power down, which is only used in the priciest and most premium products due to the costs associated with sourcing this material. These are the loftiest feathers of them all and are included in bags like the super-premium (and super pricey) Rab Mythic 600.

The Therm-a-Rest Parces 0F comes with an RRP of £550, although it can commonly be picked up at a range of eCommerce sites for around £450 - £500 (such as at Elite Mountain Supplies or Ultralight Outdoor Gear at the time of writing). Naturally, then, this puts it at the top of the spectrum when you look at deep winter bags of this class, with bags such as the OEX Leviathan EV 900 or the Simonds Makalu II coming in several hundred pounds cheaper than the Therm-a-Rest. So, what do we think? Is the Therm-a-Rest Parsec -18 down sleeping bag worth the extra cost?

Our experience using the Therm-a-Rest 0F sleeping bag

To test the Therm-a-Rest Parsec -18° Celsius sleeping bag, we went out on numerous trips through the winter of 2023/2024 to properly put this bag through its paces. Aside from a handful of relaxed wildcamps in the Chiltern Hills and a couple of nights out on Dartmoor when the mercury plummeted, the most notable testing trip we took it on was a three-day backpacking trip through the Cairngorms National Park in January 2024. On this trip, we got to sleep in it in temperatures down to -9° Celsius (which is almost bang-on the comfort rating of the bag), slept in it in a bothy when the temperature remained decidedly above freezing all night, and lugged it up and down windy Scottish Munros to get a feel for how it performs in real-world backpacking conditions. So, how did the Parsec do?

All in, we think the Parsec is an incredibly impressive piece of kit. It kept us super warm and comfortable when the temperature outside the tent was anything but, yet it somehow kept us comfortable even when camping in a bothy in temperatures of around 5-6° Celsius. Light and packable, it was also a joy to carry, being barely heavier than the sleeping pad we paired this thing with – the equally impressive Sea to Summit Ether Light XT Extreme. And thanks to the robust stuff sack Therm-a-Rest provides, we were able to squash it down to the size of a rugby ball, which easily fit in our pack’s bottom compartment, alongside a pillow and the aforementioned Sea to Summit sleeping pad. In this sense, the Parsec 0F is easily the lightest and most compressible deep winter bag we’ve ever tested at the GBAC, being around 300 grams lighter than both the OEX Leviathan EV 900 and the Simonds MAKALU II.

Warmth

Warmth-wise, the Therm-a-Rest -18 comes with a comfort rating of -10 °Celsius and a limit rating of -18° Celsius. Therefore, despite it being communicated boldly as a part of the bag’s name, you’re probably not going to want to take this thing down to -18° C – or not without some additional warm layers, at least. We found, however, that the comfort rating is pretty much bang on, having slept in it in temperatures down to -9° C (1 degree off the bag’s advertised comfort limit). When doing so, we paired the bag with the super warm Sea to Summit Ether Light XT Extreme and wore a pair of baselayers and a thin fleece mid-layer inside. In fact, we even wore a Patagonia Micro Puff hoody to start the night, but quickly took this off as we got too warm before falling asleep.

Delivering that warmth is over 750 grams of 800-fill power down. This is some of the highest quality down you’ll ever find in a sleeping bag, and it lofts up in a matter of seconds. In fact, even after compressing the bag down as small as it would go with the help of a drybag (and leaving it for 3 full days), the bag came back to life in less than an hour, and we noticed that none of the down clumped in the baffles, as is often the case when you leave down compressed too tightly for too long (as was the case with more budget Simonds MAKALU II).

For extra warmth, the Parsec 0F also comes with an extra foot box inside the bag for warming up your feet in the coldest conditions. Referred to as the ‘Toe-asis Foot Warmer Pocket’, it’s basically an additional pouch for your feet that works much like down booties do. Due to the extra warmth this delivers, we found that we would only use this in the coldest conditions, and it quickly became one of our favourite things about the bag when camping in temperatures below freezing.

Durability

The Therm-a-Rest Parsec 0F is created from a 100% recycled nylon RipStop material, which we found to be plenty durable. On the outer side of the bag, Therm-a-Rest has even added a DWR (durable water repellent) coating to prevent water from immediately soaking into the bag. And on a camp in the Chiltern Hills where we tested the Robens Pioneer 4EX, we even got to try this out when we forgot our bag was hanging on a tree outside as it started to rain. Luckily, the DWR coating meant that the rain just beaded on the outside of the bag and very little (if any) soaked into the down. That being said, even though the DWR protected the bag for about 5 minutes in a light shower, we wouldn’t expect the bag to put up with anything more than that — so don’t read the above to mean that the Therm-a-Rest Parsec -18 is waterproof in any way, shape or form.

Fit

The regular version of the Therm-a-Rest Parsec 0F, which we tested here, is designed to fit people up to 183 cm tall (which is pretty much bang-on 6 feet). We are 177 cm in height and found that the bag was long enough to sleep comfortably while still offering enough room to move around in and not feel overly restricted.

Extra features

As an added bonus, the Therm-a-Rest Parsec also comes with additional straps around the back of the bag, through which you can insert your sleeping pad. This allows you to roll around in the night while still keeping your sleep system together – which is another smart consideration when camping in the coldest conditions. Unfortunately, however, we found the straps were much too small for most pads, and barely even fit around Therm-a-Rest’s own Neoair XTherm. In fact, they were so tight around the Sea to Summit Ether Light XT Extreme that we only opted to use them on one of the 3 nights we camped using this system. In the next iteration, we’d love to see Therm-a-Rest make these straps adjustable so you can insert pads of differing sizes; something which we feel would make the bag even more versatile still.

Price

Finally, let’s talk about the elephant in the room: The price. When bought for full price, the Therm-a-Rest Parsec 0F will cost you £550, which makes this the most expensive sleeping bag we’ve ever tested at the GBAC. That being said, however, warm and packable sleeping bags are always going to be some of the most expensive items of gear in your backpack. Take the Rab Neutrino 600, for example: This bag costs £500 when bought directly from Rab, yet comes with a much lower comfort rating (-5°C) and only weighs around 90 grams less than the Therm-a-Rest. Or take the Sea to Summit Spark -18: A bag with the same limit rating and a remarkably similar weight, yet costs £100 more than the Therm-a-Rest (although it does come with slightly loftier 850-fill power down). And while this review isn’t about either of those bags (nor have we tried them at the time of writing), the comparison is merely to illustrate that the Therm-a-Rest Parsec 0F is still relatively competitive, despite its high price tag. For that reason, we consider it a solid option if you’re looking for a premium bag from a super reliable brand, and think the bag is well worth the price, especially if bought on offer.

If you’re looking for a cheaper alternative, however, we don’t think you’ll find anything better than the Simonds MAKALU II from Decathlon. This comes filled with the same 800-fill power down that you get in the Therm-a-Rest and delivers almost identical comfort and limit ratings. Granted, the MAKALU II is over 300 grams heavier than the Parsec -18 but in our experience, it packs down just as small (the difference is barely noticeable) and costs less than half the price of the Parsec at an astonishing £239.99.

Therm-a-Rest Parsec 0F sleeping bag FAQs

Conclusion

All in, we really rate the Therm-a-Rest Parsec 0F. It delivers one of the best warmth-to-weight ratios of any sleeping bag on the market right now, and it does it all in a considered, well-designed and beautifully luxurious package. Featuring buttery-smooth zips, some smart extra design features and some of the highest-quality down we’ve ever tried, we’ve enjoyed every night we’ve spent in this bag, and are looking forward to many more.