Thermarest Corus

Review: Therm-A-Rest Parsec & Corus Sleeping System

Therm-a-Rest recently released a series of new sleeping bags and backcountry quilts designed to be compatible with one another. With the ability to be mixed and matched you can create a expandable sleeping system suitable for almost any trip or season. Much like a sleeping bag – over bag combination but more finessed.

Thermarest Parsec and Corus
The combined Therm-a-Rest Parsec sleeping bag and Corus quilt. Together they’re rated to -15C.

In this review we’re focusing on one such combination: the Parsec sleeping bag and Corus quilt. This bag and quilt, like many others in Therm-A-Rest’s line, can be mixed and matched to make an extremely versatile sleeping system. On warm summer nights the Corus quilt can be used alone as can the Parsec on cooler nights. Or, on even colder nights, they can be combined for additional warmth. A great idea in concept but of course the devil’s in the details. How the system attaches together, the quality of the materials, the cut and layout, etc. When testing out this system it quickly became obvious that Therm-A-Rest got a lot of things right and have made a great multi-season sleeping system.

Specs – Parsec Sleeping Bag (Regular):

  • Temp Range:
    • Comfort: 0⁰C / 32⁰F
    • Transition*: -6⁰C / 20⁰F
    • Risk: -24⁰C / -12⁰F
  • Fill: 800-fill Nikwax Hydrophobic Down
  • Fill Distribution: 40% on the bottom & 60% on the top
  • Outer Fabric: 20D DWR treated Polyester Ripstop
  • Interior Fabric: 20D Polyester Taffeta with ThermaCapture reflective coating
  • Weight: 890g (500g of down)
  • Shape: Tapered
  • RDS Certified Down

Specs – Corus Down Quilt (Regular):

  • Temp Range:
    • Comfort: 2⁰C / 45⁰F
    • Transition*: 7⁰C / 35⁰F
  • Fill: 650-fill Nikwax Hydrophobic Down
  • RDS Certified Down
  • Sizes: Regular or Large (regular tested)

Specs – Combined (Bag & Quilt):

  • Temp Range:
    • Comfort: Not provided
    • Transition*: -15⁰C / 5⁰F

* Basically the realistic performance limit of the bag, any colder and you’ll be shivering yourself to sleep.

Thermarest parsec and corus
Left: the Parsec and Corus in their storage bags. Right: same in their transport sacs. I also included the Universal Sheet and of course the Nalgene’s there for size comparison.

Nikwax Hydrophobic Down:

For anyone not familiar with Nikwax hydrophobic down I wanted to include a quick introduction to the product. Both the Parsec bag and the Corus quilt utilize ethically sourced down with the Nikwax treatment. This treatment makes the down resistant to wetting which destroys the down’s loft and therefore the insulation value. Of course this doesn’t mean that the down is waterproof or that it maintains its R value when wet! It means that the down won’t attract and absorb water from adjacent fabrics or materials keeping it much more dry in we areas. It also means that water vapor from sweat or damp cloths have much less impact on the down. A great feature in a sleeping bag as they always seem to get dew or frost on them when in the outdoors not to mention the occasional tent bottom puddle. I don’t hesitate to take Nikwax treated down products into wetter environments where I would normally just consider synthetics.

NikWax Treated Down
A demonstration by Nikwax at an OR show a few years ago when they first introduced their new Hydrophobic Down.

Parsec Sleeping Bag:

Part of Therm-A-Rest’s new Fast and Light line, the Parsec is one of the best sleeping bags I’ve tested in a long while! Therm-a-Rest packed some very functional and cool features into a well-made and thought out sleeping bag as a stand-alone unit. Of course it becomes more versatile when matched with a compatible quilt.

Thermarest parsec Sleeping Bag
Testing out the Parsec sleeping bag on an alpine traverse this summer.


Foot Warming Pocket – One feature that I really like is the foot warming pocket. It’s basically a small down sleeve that I could slip my feet into to warm them up or to keep them warmer on cool nights. It feels so great to slide my feet into this down pocket when crawling into the bag!

Mat Attachment Straps  The Parsec comes with two straps for connecting the bag to a sleeping mat so you won’t slip off at night. I tested the system on a variety of mats from different brands and found that they all worked together reasonably well. So no compatibility issues are likely to pop up. The straps did tend to bunch up a bit but it didn’t seem to impact their performance at all. The tiny little clips, 6 per strap, can be a bit tedious to do-up so I ended up just leaving the straps attached to the bag and would slide the mat into place.

2-Way Zipper – The Parsec has a full-length, two-way zipper. I’m a big fan of such zippers as they allow me to stick a leg out of the bag when I’m hot without unzipping the entire thing.

Dual Zone Fill – 60% of the bag’s fill is located on the top of the bag where it’s in contact with the cold air while the remaining 40% is positioned on the bottom next to the sleeping mat. This seems to strike a good balance between warmth and weight. I’ve found that some bags with even less insulation along the bottom often feel quite cold.

Cinch-able Hood – The hood has a drawstring on the right hand side for cinching it up on cold nights.

Pocket – A small zippered pocket is located on the outside of the Parsec. Great for storing the mat attachment straps and anything else you might want with the bag like ear plugs.

Draft Collar Standard on basically all mountaineering type sleeping bags the draft collar limits the amount of heat that escapes around the neck.

Temperature Rating:

The Parsec hit’s a bit of a sweet spot for me with the -6⁰C temperature rating. I find it works for the majority of my mountain camping over the spring, summer and fall. And it’s an honest -6⁰C, not at all overstated like some bags seem to be! I can easily sleep in the bag down to the Transition Range of -6⁰C before needed to wear anything but my boxers so long as I’m on a good quality mat. If I added a few layers of clothing it made for a great spring ski touring bag.

Thermarest Parsec Sleeping Bag

Fit & Feel:

The interior fabric on the Parsec feels quite nice next to skin, not at all clammy or plastic-bag-esq. I would often sleep in just my boxers on warmer nights. The bag heats up quickly and the added Toe-asis compartment is fantastic for cold feet. The shape of the sleeping bag offers a very functional balance between warmth and comfort/room. It has a fairly wide top with room for my arms and a few pieces of kit that I want to keep warm but only a bit of extra width over a standard sleeping mat. Not as warm as a tight mummy-bag fit but much more comfortable. Lengthwise, the Regular easily fit my 5′-10″ frame with room to spare.


Pros: Versatile, well thought out, comfortable, warm

Cons: Mat straps bunch up and the tiny attachments clips are a bit finicky

Overall: I’ve been incredibly happy with the Parsec Sleeping bag and would highly recommend it. It falls into a versatility sweet spot, warm enough for late season ski touring trips right through the summer alpine season and into the fall. It’s comfortable to sleep in, packs down reasonably small and has all the features that I really want. It’s been my go-to bag since I received it.

Corus Down Quilt:

This innovative and cool product from Therm-a-Rest serves the same purpose as an over-bag in a very lightweight and compact package. It can be used as a stand-alone unit for summer camping and bivi’s or be attached to a compatible sleeping bag, like the Parsec, for increased warmth in the winter or high elevation camping. It’s also a fantastic option for hut based trips where a proper sleeping bag can be overkill.

Thermarest Corus


Attachment Clips – the Corus has 8 snap-loops that are designed to aid in attaching the quilt to a mat or compatible sleeping bag. With 3 along each side of the quilt and 2 at the bottom not all of them are needed all the time. I often found that I would only use one side and the bottom on warm nights allowing me to throw back the quilt if I got hot. On colder nights however I stitched them right up.

Side Baffles – Basically an extra flap of insulated material that runs along the sides of the quilt to help seal out cold air. When attached to a mat the inner flap can be tucked in along the body while the outer flap seals along the edge of the mat. A great design that makes up for the gaps in the mat attachment system.

Elasticized Foot Box – The bottom of the quilt is elasticized much like a fitted sheet for a mattress. This creates a great seal along the bottom of a mat for warm feet and keeps the bottom of the quilt firmly attached.

Neck Snap – At the head of the Corus is a small snap that allows the stretchy end of the quilt to be secured around the neck. Sort of like a sleeping bag just without the hood. A nice feature if you end up pushing the temperature limits.

Pocket – Like on the Parsec, Therm-A-Rest has included a small pocket on the Corus but with a snap closure items can easily fall out. A small zipper would have made it a much more secure.

Temperature Rating:

I found that the +7⁰C rating was accurate so long as I was sleeping on a lightly insulated mat. If I went with a super light, un-insulated mat on cooler nights the mat would feel a bit cold making it more difficult to sleep. The sewn through baffles on the Corus are not as warm as a baffled box construction but use less material and are simpler to construct. The down is fairly loosely packed into each baffle and so can leave gaps where only the fabric separates the inside from the night. That being said I never found the sewn seams to be overly cool when using the quilt by itself.

Thermarest Corus


As mentioned above, the Corus quilt can be used as a standalone system or combined with a compatible sleeping bag when things get really cold. There are two different set-up options when the quilt is used alone and another when used over a sleeping bag. Each are described below:

Thermarest parsec and corusSolo Option 1: Loop Kit. By utilizing the included Loop Kit the Corus can be attached to almost any sleeping mat. The kit consists of 8 stick-on patches that are applied directly to a sleeping mat, one for each snap-loop on the quilt. The result is a very lightweight and secure attachment method that can be customized for your mat. Of course the downside is that your mat now has a bunch of Velcro-like patches on it whether you’re using the Corus or not. It also means that you’re sleeping directly on the mat.

Thermarest CorusSolo Option 2: Universal Sheet The other option for connecting the Corus to a mat is with the Therm-A-Rest’s Universal Sheet. Sold separately, the sheet comes with attachment points for the quilt’s snap-loops and provides a much nicer surface to sleep on than the mat. The downside of course is that it’s one more thing to buy and carry. Also the loops on the sheet are on the top edge which doesn’t provide the best seal. I would have thought they’d be placed along the bottom edge to pull the side of the quilt down to seal along the edge of the mat.

Thermarest parsec and corusCombined with a Sleeping Bag: Therm-a-Rest has stitched loops into the majority of their sleeping bags to make them compatible with their quilts. Much like when the quilt is used with the Universal Sheet. The foot box on the quilt is looped under the toe of the mat and over the sleeping bag. It’s then secured in place with the snap-loops. The sleeping bag is in turn attached to the mat so everything stays connected. A neat little package thought attaching the quilt to the bag is gloves-off work so can be hard on the fingers when it’s really cold out.

Fit & Feel:

The Corus is quite roomy. It extends side-to-side on the mat without issue and space to spare. Lengthwise it would extend up to my eyes but not over my head (I’m 5′-10″ tall) unless I was really pulling on the fabric. Of course I was testing the Regular, the L is 4″ longer. Like the Parsec bag the Corus fabric feels nice next to skin and heats up quickly. Personally I’m not a huge fan of sleeping directly on a mat unless I leave my base layer on so prefer the sheet option which makes for a quite comfortable package.


Pros: Versatile, pack-able, lightweight

Cons: Snap instead of a zipper on the pocket, sheet snap loops are on the top so don’t pull the quilt edge down.

Overall: Great for camping on warm summer nights, sitting around outside when it’s cooler or sleeping in huts. Everyone who’s seen the Corus during testing wanted one.

Combined Quilt & Bag:


Attaching the Corus quilt to the Parsec sleeping bag and in turn attaching them to a sleeping mat can make the set-up process seem a little cumbersome. It can also be hard on the fingers as it can’t be done while wearing gloves. That being said, it’s not really any more difficult than stuffing a sleeping bag into an over-bag. Also, depending on the nature of the trip, it may only need to happen a few times. I found that I could cheat a little and not detach the sleeping mat straps or the quilt when moving camps which  facilitated a quicker and easier set-up. Once set-up, the quilt, mat and sleeping bag all stay together as a nice neat unit.Thermarest Parsec and Corus

Fit & Feel:

When the quilt is attached to the sleeping bag it covers the zipper. A good feature from a warmth perspective but it can make getting in and out of the bag a little more challenging. It’s also a bit of process to find and un-clip a few snap-loops if you’re too hot. A hard tug on the quilt will cause the snaps to release though this may end up tearing off the loops over the long term. I found that not attaching the top two side clips on the zipper side provided great access and would allow me to flip the quilt on or off my body if I got hot. So basically the Corus/Parsec combo is a bit more work than a single bag rated to -15⁰C but similar to an overbag. However it’s more versatile as not all/any of the snaps need to be used.

The addition of the Corus over the Parsec sleeping bag definitely ups the warmth. I haven’t yet tested it to -15⁰C but have been out on some -10⁰C nights and couldn’t keep the quilt done up completely as I got way too hot. Of course I was also in a tent so the inside temps would have been a bit warmer. Given Therm-a-Rest’s accuracy with their other temperature ratings I don’t doubt that it’s accurate.


Pros: Warm, Stays together as a package very well, lots of set-up/attachment options

Cons: Snap loops can be hard to find to open or close when in the bag

Overall: A very versatile system that basically offers 3 sleeping bags for the price of two. The combined package has some of the issues of a typical overbag system but is a little lighter with more attachment versatility.

Black Sheep Adventure Sports was provided with a sample to review but of course this did not influence us in any way.