Therm-a-Rest Parsec 0F/-18C Sleeping Bag review: a cozy bag that makes you excited about sleeping in the cold

The Therm-a-Rest Parsec 0F/-18C offers unrivalled warmth to weight, promising to keep you warm and cozy in temperatures down to around -10° C/14° F

Therm-a-Rest Parsec 0F/-18C sleeping bag
(Image: © Craig Taylor)

Advnture Verdict

The Therm-a-rest Parsec -18° Celsius/0° Fahrenheit sleeping bag is a comfy, packable and super-warm deep-winter down bag that can keep you warm in brutally cold temperatures. It comes with super lofty 800 fill power down in a soft and lightweight nylon shell, straps so you can better keep your sleeping pad in place overnight, and features one of the best warmth-to-weight ratios of any sleeping bag on the market right now.

Pros

  • +

    Very comfortable

  • +

    High-quality, lofty down

  • +

    Smooth zips

  • +

    Lofts up remarkably quickly

Cons

  • -

    Very expensive

You can trust Advnture Our expert reviewers spend days testing and comparing gear so you know how it will perform out in the real world. Find out more about how we test and compare products.

Meet the tester

Why I Love Camping – Finally time for a brew
Craig Taylor

Craig loves nothing more than pitching up in the backcountry, preferably while taking on a long-distance thru-hike. His adventures usually take place in the hills and mountains of Wales but he occasionally gets away to his beloved Alps. As one of our expert campers, Craig revels in testing camping equipment and knows a sturdy shelter from one that will give up the ghost when conditions become challenging.

Therm-a-Rest Parsec 0F/-18C sleeping bag: first impressions

The Therm-a-rest Parsec 0F/-18C sleeping bag is a deep-winter down bag that promises to keep you toasty warm at a chilly -10°C (14°F).

Specifications

• List price: $530 (US) / £550 (UK)
• Weight (reg): 1.09kg / 2lbs 6oz
• Length (reg): 203cm / 80in
• Fill: 800 fill power goose Nikwax hydrophobic down RDS
• Comfort rating: -10℃ / 14℉
• Limit: -18℃ / 0℉
• Compatibility: Backpacking, trekking, four-season camping

Weighing in at an impressive 1.09kg in the regular version and jam-packed with 0.74kgs of high-quality 800-fill power down, the Parsec 0F offers one of the best warmth-to-weight ratios of any sleeping bag on the market right now. 

What’s more, it packs down really small, lofts up quickly and is an incredibly cozy place to while away long winter nights in your tent. 

That being said, coming with a list price of $529.95 (US) / £550 (UK), this top-quality performance doesn’t come cheap. So is the investment worth it?

Therm-a-Rest Parsec 0F/-18C sleeping bag: in the wild

Therm-a-Rest Parsec 0F/-18C sleeping bag

Therm-a-Rest Parsec 0F/-18C sleeping bag on test in the Cairngorms National Park (Image credit: Craig Taylor)

To properly test the Therm-a-Rest Parsec 0F/-18C, we took it on a three-day wild camping trip through Scotland’s Cairngorms National Park. We camped in it in temperatures down to around -9°C (the comfort limit of the bag) and when the mercury stayed above zero all night, and lugged it up and down Scottish Munros to get a real feel for what it’s like using this sleeping bag in real conditions. So how did it perform? Well, let’s start by looking at its packability.

Packability

Therm-a-Rest Parsec 0F/-18C sleeping bag packed up

The Therm-a-Rest Parsec packs down to around the size of an American football (Image credit: Craig Taylor)

When it comes to warm winter sleeping bags, the most problematic thing about them is often their packed size. Most quickly fill up an entire 40L pack on their own, while the smaller ones tend to sacrifice warmth to get the package down to a manageable size. 

But not with the Parsec 0F. The bag can be compressed down to something resembling an American football, which is smaller than many two-season synthetic bags that I own. What’s more, weighing a tad over a kilogram, the bag is super light for the warmth it delivers. 

So much so that I’d have no qualms about taking this bag on trips when you don’t really need the warmth, but would like the comfort. It’s so light and packable that this bag will almost certainly become my go-to for any trips outside of high-summer adventures when the temperature isn’t set to dip below 12°C. Which, here in the UK, means that the Therm-a-Rest Parsec 0F will likely become the bag I use on around 80% of backpacking trips moving forward.

Materials

Therm-a-Rest Parsec 0F/-18C sleeping bag

The Therm-a-Rest Parsec has a DWR coating (Image credit: Craig Taylor)

The shell and liner of the Parsec 0F/-18C have been manufactured out of the same lightweight ripstop nylon material that’s comfortable on your skin and a joy to sleep in.

On the outside, Therm-a-Rest has added a handy DWR (durable water repellent) coating to help bead off any water droplets that hit the bag and to stop them from soaking into the fragile down feathers.

To provide lasting warmth in damp conditions, the down filler has also been treated with a similar hydrophobic treatment to prevent water from absorbing so quickly. Just don’t think that means you can use this in the rain: as with any down bag, the Parsec is going to lose most of its ability to keep you warm when it gets wet, so make sure you carry it in a waterproof dry bag when trekking through rainy conditions, and do your best to stop it getting wet overnight.

While we’re on the subject of down, the quality of the down filling deserves a special mention. The 800-fill power goose down is so gloriously lofty and comfortable to sleep in that you’ll never want to sleep in anything else ever again. It also lofts up quickly and doesn’t clump like in other bags. What’s more, the Parsec 0F comes with an additional quilted foot box in the bottom of the bag that provides extra warmth for your feet overnight – perfect on the coldest winter days.

Additional design features

Therm-a-Rest Parsec 0F/-18C sleeping bag

The Therm-a-Rest Parsec has straps for attaching it securely to a sleeping pad (Image credit: Craig Taylor)

Finally, I also really appreciate two additional design elements of the Parsec that become more enjoyable the more you use the bag. The first is the zip: it’s so smooth and easy to operate that you can do it firmly with one hand. The material rarely snags, even if you’re doing it up while lying in an awkward position, and the zipper always opens and closes with ease.

The bag also comes with sleeping pad straps on the back. And while these are small (they’re even tight when used in conjunction with one of Therm-a-Rest’s own sleeping pads), they keep your sleep system glued together, allowing you to roll around to your heart’s content in the night.

Is it worth the price?

Therm-a-Rest Parsec 0F/-18C sleeping bag in tent

As near to a true four-season sleeping bag as you’re likely to find – great in both warm and very cold conditions (Image credit: Craig Taylor)

Overall, the Therm-a-Rest Parsec 0F/-18C is, in a word, phenomenal. It’s decidedly high-end, with a performance that reflects the higher price tag. It’s packable, comfortable, lightweight and warm, and comes with a couple of additional features that truly make it stand out from the crowd, like sleeping pad straps, an additional quilted foot box, and the smoothest zip I’ve ever used.

And even though this thing is expensive, its weight and packability make it a truly versatile addition to your sleep system – suitable for camps in any weather outside of scorching summer evenings. In that sense, the Parsec 0F is, for me, a true four-season sleeping bag; one that I’ll turn to in any deep winter, spring or fall conditions, and I think the investment is worth it for those who have the money to spare.

Craig Taylor

Growing up just south of the glorious Brecon Beacons National Park, Craig spent his childhood walking uphill. As he got older, the hills got bigger, and his passion for spending quality time in the great outdoors only grew - falling in love with wild camping, long-distance hiking, bikepacking and fastpacking. Having recently returned to the UK after almost a decade in Germany, he now focuses on regular micro-adventures in nearby Snowdonia and the Brecon Beacons, as well as frequent trips to the Alps and beyond. You can follow his adventures over on komoot, or visit www.craigtaylor.co for more info.