Therm-a-Rest Parsec 20F/-6C Sleeping Bag Review

© UKC Gear

For three season use, nudging more than a toe into four seasons, this versatile sleeping bag has proved an excellent choice on the Scottish hills, says John McKenna.

Therm-a-Rest's Parsec sleeping bag range is designed to combine a good warmth:weight balance with a fit that doesn't compromise too far on comfort, giving you something more versatile than some narrower ultralight specialists. Recently updated with improved fabric and zip, this bag comes at three different temperature ratings: 0C, -6C and -18C, all comprising 800 fill power down. A refinement to the shape has also allowed Therm-a-Rest to shave extra weight, with up to 57g off the previous version. We've tested the -6C sleeping bag throughout the winter and cold spring nights when it was important to keep the weight down on long walk-ins or mountaineering trips in Scotland, but without compromising too far on insulation or overall comfort.

Waking up after a particularly cold night in the Parsec -6C  © UKC Gear
Waking up after a particularly cold night in the Parsec -6C

Weight and pack size

I've been using the Regular size (Small and Large also available). Including the stuff sack, the regular size Parsec comes in at 839g on the home scales (Therm-a-Rest quote 810g). There are similarly warm bags out there that come in at lower weight, but this one gives you a great weight-to-warmth ratio at a perhaps slightly more affordable price than some of the real ultralight specialists (all things are relative), and without sacrificing durability or a user-friendly fit.

Compresses down well   © UKC Gear
Compresses down well

Warmth and temperature rating

The Parsec 0C/32F is rated according to the standards of EN ISO 23537:

  • Comfort 0C
  • Transition/Limit -6C
  • Risk -24C

For more on understanding sleeping bag temperature ratings, see the article below:

Outside of the lab, a number of factors play into your perceived warmth that will vary from person to person, night to night, and the environment and conditions. It's worth noting that the perceived warmth of the bag in use will also depend on the quality and appropriateness of the sleeping mat being used to insulate you from the ground.

A few extra insulating layers necessary when temps dropped down to -13C!  © UKC Gear
A few extra insulating layers necessary when temps dropped down to -13C!

While it's billed as a 3-season model for 'early spring through late fall', in a UK context you could argue that the Parsec -6C comes close to a 4-season remit. Perhaps you could say it sits somewhere around 3.5 seasons. While most users in most situations should be happy below the 'comfort limit' of 0C, you would probably be pushing things at the -6 implied by the name, unless you were backing up the sleeping bag with extra warm clothing. As such it might not be the best for a particularly cold night like a winter summit camp.

Indeed I have slept in this on very cold nights, as low as -13C, and though I survived to tell the tale this was only viable because I was combining the sleeping bag with some thick insulated clothing. Although this is outside the bag's suggested operating temperature, it demonstrates its ability to keep you warm. This is great when doing a short trip when you will be carrying these extra layers anyway - say for winter climbing - and gives you that bit of extra versatility in the sleeping bag, compared to a warmer model (such as the Parsec -18) which might be overkill for most UK use. On warmer nights, closer to the suggested comfort rating, the bag has kept me warm and comfortable with no need of added extra bulk.

Fabric and construction

The inner fill is an 800-fill power Nikwax Hydrophobic Down. The higher the fill power the more loft the down provides to trap air, meaning greater insulating performance for a given weight, and this is pretty high-end stuff.

This packable high-quality down can be felt in the warmth of the bag, and you get a good even coverage across all the baffles too, with a generous 470g of fill in size Regular. Hydrophobic down keeps much of its insulating properties when wet, and dries faster, making this a good choice for the British climate, where gear can tend to get damp throughout the night.

The box baffles eliminate cold spots across the length of the bag and the zoned insulation means the weight is kept down in areas where extra insulation isn't as useful - namely the underside, where the fill will be compressed by your weight - while you get more down where it's most needed over the top side.

Compared to the previous version of the Parsec, the updated fabric is a lighter 100% recycled 20 denier Nylon RipStop with DWR. This feels durable for its weight, while the DWR furthers this bag's ability to perform in damp conditions.

Well shaped toe box area  © UKC Gear
Well shaped toe box area

Size and shape

In this model, three sizes are available: I've had the Regular size which is suitable for those up to 6ft. The Parsec comes in somewhere between that of a tight mummy fit and a looser-fitting bag. This offers a good warmth to the volume of the bag as there is less air to heat up than in a looser bag, while on the other hand it gives you a bit more space than an ultralight mummy, making it more comfy and less claustrophobic. Therm-a-Rest's Hyperion and Questar bags offer tighter and roomier fits respectively:

At the bottom end, the toe box is well-shaped to afford your feet a comfortable amount of room. Overall I've got on well with the fit, which has plenty of length for me and enough room to move around comfortably at night without being too loose for thermal efficiency.

A good cinchable hood  © UKC Gear
A good cinchable hood

The toe box comes with an inbuilt foot warmer pocket  © UKC Gear
The toe box comes with an inbuilt foot warmer pocket

Synergy link straps stop you rolling off the mat  © UKC Gear
Synergy link straps stop you rolling off the mat


To minimise heat loss, there is a neck area draft collar and a decent cinchable hood. These all feel very standard and sit comfortably whilst at rest. The 'SynergyLink Connectors' are elasticated straps that let you fix the sleeping bag to your camping mat. These have been updated in this version to further reduce weight and still work just as well as previous versions I have used. This is great for avoiding rolling off your mat during the night and also reduces heat loss by keeping the heavily insulated top of the bag orientated upwards at all times during your sleep. However they're more use for back sleepers than side sleepers, since the bag cannot roll over with you when fixed. Some on our gear team prefer not to use the connectors for this reason, and they're easily left at home if you like.

An external chest pocket allows you to store a phone or batteries during the night, although I generally prefer these internal in the sleeping back to ensure they are kept warm and easy to access when in the bag.

The 3/4-length YKK zipper feels truly anti-snag, meaning you are less likely to damage your bag when using it (few things are as annoying as catching your zip as you struggle to exit your bag for a night time pee). Not being able to open the bag fully means you can't use it as a quilt, or vent your feet if it gets warm in your tent, and while this isn't a big concern in colder weather it is something to bear in mind if planning to use it in typical spring or autumn conditions. On the other hand (or foot), when it's nippy outside the addition of an extra pocket at the bottom end gives you a particularly snug place to slide your feet.


Overall the Parsec 20F/-6C is a fantastic bag that performs very well in a number of major areas including warmth, weight, comfort, packability, and durability. Although not truly outstanding in many of these fields, the balance it manages to strike between them all results in something really versatile. A lot of different users will be able to enjoy this bag in a variety of circumstances. For myself, a good width and high-end fill have meant a warm and comfortable night's sleep without being remotely unhappy about some added weight or reduced packability over and above a more specialised (and likely more expensive) ultralight alternative. For three season use, nudging more than a toe into four seasons, it has proved an excellent choice. Its price tag is still going to be a stretch for many budgets, but for something of this high quality you could look on it as an investment in years of happy camping.

11 Apr

Great review!

11 Apr

I've just bought my first quilt, a Therm-a-Rest Vesper 20F -6C 572g. Not tested in the cold yet but I really like the huge extra freedom to move my arms around.

Remind me, is parsec a unit of distance or time? ;-)

I've had a look at those, they look great. I really like the idea of having a quilt and sleeping bag combo to keep your sleep system versatile for throughout the year. Those two in combination would have worked great on the night I had the parsec out in -13C. They look to work well together with Thermarest where you have the built-in loops to attach both to each other.

11 Apr

I've had one of those for a few years and I think the temperature rating is pretty optimistic. I still like it but it's not what it says and that makes it look like poor vfm. I keep meaning to get in touch with Thermarest about it and keep forgetting.

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