Rab Mythic 600 Sleeping Bag Review


Our Rab Mythic 600 sleeping bag review explores a premium quality ultra lightweight down sleeping bag. It has the best warmth to weight ratio in the Rab range. We hope you find the review useful.


A Quality Brand With Great Heritage

Rab have always been at the cutting edge (no pun intended!) of sleeping bag manufacture.  At first this was Rab Carrington sewing up each bag in his small workshop in Sheffield. Gradually, as all good things do in the outdoor community,  word kept spreading. It really wasn’t long before his bags were THE ones to use.  Since then, over several decades of manufacture and innovation, the company have stayed on top of their game.  Their sleeping bags lead the world. Since those early days their range has broadened to include a wide range of specialist clothing. Even so, sleeping bags are still very much at the core of what they do.  

Their Mythic range of ultralight down bags continue that trend.  These are sleeping bags at the very pinnacle of what is available. I was delighted to be offered one of their 600 models to test and here are my findings.

900 fill power ethically sourced European goose down & Pertex Quantum fabrics


Insulation & Fabrics

A sleeping bag is really about 2 key things. Firstly, what insulation is used and how much of it. Secondly, what is used to keep that insulation in the right place.  In the case of the Mythic the insulation used is the highest quality 900 fill power ethically sourced European goose down.  The down is then treated with a hydrophobic treatment developed with industry leaders Nikwax. The inner and outer shell is manufactured from Pertex Quantum 7 denier fabric.

Additional Features

Aside from all that, Rab have added a 2 way three quarter length YKK 3 coil zip with a bonded zip guard which also as a draught excluder.  The Mythic features a tapered mummy shape with a generously sized angled box foot.  There is a hood with a draught collar and these cinch tight with drawcord closures. 

Rab have used a lightweight baffle construction to prevent down shift. The baffles are constructed with a newly developed chevron pattern chamber shape. This is designed to ensure the down stays centrally located over the core.  

The Mythic’s other features are designed for efficiency. The zip pullers are luminescent and the bag comes with a waterproof dry bag and large cotton storage bag. 

Temperature Ratings

Rab test the Mythic to the 2016 European EN13537:2016 test protocol which measures the insulating capacity of bags. The test allows consumers to compare different bags that have undertaken the same test. It offers a great way to understand the temperatures a particular bag can realistically be used in. The test considers 3 temperature zones.  

The comfort temperature is the outside temperature at which a woman could sleep comfortably. The Mythic 600 is rated at -5 degrees Celcius. The comfort limit temperature is a measure of the outside temperature at which a man can sleep for 8 hours without waking from the cold. The Mythic 600 has a rating of -12 degrees Celsius.  Finally, the extreme temperature is a measure of the lower limit of outside temperature a woman could survive in the bag for 6 hours without becoming hypothermic.  The 600 rating is -32 degrees Celsius.

Drawcord System

Rab Sleep Limit

Finally, Rab have also come up with a system to offer it’s own standardisation. This is called the Rab sleep limit.  This is based on in house testing and feedback from athletes and professionals with extensive knowledge of surviving in particular conditions.  The Mythic 600 rating is -16 degrees Celsius.  

Rab say that the Mythic 600 is designed to deliver maximum warmth for minimum weight. Here you have a bag with 600 grams of high quality down and yet a total weight of only 885 grams. It is fair to say that is mission accomplished!

In Use

Pack Size

Now to the crux of my Rab Mythic 600 sleeping bag review. After lots of technical info, it’s time to reveal my real world testing results. I also own a Rab Neutrino 600 and so have used this as a comparison.

The Mythic 600 packs up to a very compact size. It is noticeably smaller than the Neutrino.  This is impressive as that bag is also a very compact size when packed.  The included waterproof and dry bag style stuffsack makes it easy to squish plenty of air out before rolling the top for sealing.  You could certainly replace this with an expensive compression sack if you prefer. I don’t think you need too as the included bag works a treat.

900 Fill Power Down

Without going into loads more technical detail, suffice it to say that 900 fill power down is the business.  In simple terms fill power is a rating system. It represents the volume in cubic centimetres of a single gram of down when fully lofted. The higher the fill power, the warmer a bag will be for its weight.  What I have seen in the Mythic is that almost as soon as the bag is pulled from its stuff sack it pops up to a fluffy bundle of warmth in no time. Give the bag a few shakes and the job is done. The bag is ready for use.  

Pertex Quantum Fabrics

Of course, the down has a big part to play in this lofting process. The other big factor in how quickly a bag bulks up comes down to the shell fabrics. They need to be able to let air in to allow expansion.  So, a little more needs to be understood about that.  Pertex Quantum has been around for some time, but the 7 denier variant is the thinnest option they make. This is seriously light and very thin. 

It is also worth mentioning that it is a very slippy fabric.  The upshot of this for lofting is that it seems to allow air to pass through very effectively.  Any sleeping bag fabric has to strike a balance. It needs to allow the passage of air and still keep the down from escaping through the fabric. It must also allow a level of water resistance and be durable enough to survive the rigours of use. The video below explains about Pertex Quantum.


Moisture Resistance

So, Quantum 7 denier definitely allows air to efficiently get into the chambers for lofting. This, therefore, also means it will allow moisture laden air to pass through from a sleeping user.  This is important as down will soak up a certain amount of the moisture a body releases as water vapour. It is better if as much as possible of this can be allowed to escape.

The other side to this, of course, is that moisture can enter the down chambers from outside.  Hopefully you will protect your bag from being used when it could get rained or snowed on. Even so, if it is used in a snow shelter the fabric can easily contact the snow. The same might happen if you are rubbing your feet against the moisture laden fabric at the bottom of your tent wall. Getting the sleeping bag fabric wet is quite possible even when taking care. 

Well, I would say that the Quantum fabric offers a decent level of protection, but isn’t by any means waterproof.  It will wet out and allow moisture to penetrate into the down if it gets wet enough. However, that is only the same as most lightweight bags on the market.  Fortunately, and more on this later, the down of the Mythic is protected from moisture.


So, finally, having assessed that the Pertex Quantum fabric allows excellent lofting, is ultralight, has some water resistance and is down proof. How does it stand up to use?  Well, I’ve been very pleasantly surprised on this one.  Despite looking impossibly thin and like it will get scuffed, scraped, torn or be generally very prone to everyday damage, the fabric has actually stood up to everything I’ve thrown at it with aplomb.  I’ve been careful to protect and it certainly could be prone to damage against rough rock or sharp edges. Even so, I’ve really had no issues with wear and tear at all.  I’ve no idea how Pertex can make something this light and yet durable enough for this use, but it seems they have.

Nikwax Hydrophobic Down Treatment

Right, so where were we?  Oh yes, that down waterproofing.  Some time ago manufacturers started developing ways to protect down from moisture. What a potential game changer that is as down’s big nemesis is moisture. The down will soak it up and, if it gets damp or wet enough, the down will clump together. This makes it all but useless as an insulator.  

Rab worked with premium proofing manufacturer Nikwax to produce a waterproofing treatment. This is designed to stop the down soaking up as much water.  The treatment won’t make the down waterproof, but certainly far more water resistant.  It is a great innovation that certainly helps overcome the main shortcoming of our feathery insulator. It is also worth mentioning that all Nikwax products are PFC free. They are a brand massively focussed on environmentally sustainable products. 

Mythic 600 Baffles

Shape & Sizing

So, top marks for both the down and the shell materials.  What else?  Well, the next big thing will have to be the bag shape and features.  I find the Mythic a generously sized mummy bag. It is also really well shaped to be comfortable and roomy enough to allow additional layers to be used inside if needed.  The length is great for my 5’8” height and 40″ chest size. It would happily work with taller people too. The foot box is also superbly designed for comfort and performance. I say performance here because it can really affect insulation if the foot box isn’t shaped well.

Baffle Design

There are also those baffles. Sleeping bags need a system to keep the down where it should be and that is usually achieved by using a series of chambers.  For the Mythic range Rab have developed a downward angled chevron baffle pattern designed to minimise down shift.  It is hard to quantify exactly how much difference this makes, but I’ve certainly had no problem with the down shifting in the Mythic. 

The zip, which is only available in a left hand version, works really well. The generously sized snag resistant baffle flap also works a treat.  The zip looks quite thin and delicate, but it is a premium YKK model and is standing up well to use.  The hood is a generous size and the neck baffle seals well.  Put simply, it all just works really well.

Insulation Testing

My last consideration for this Rab Mythic 600 sleeping bag review is how well the Mythic copes with the cold and how my findings relate to the Rab information.  Any comparison between what I found and the findings of others is bound to be difficult. After all, everyone copes differently with the cold.  There are also factors such as how much a person has eaten, dampness levels, what a person wears and a host of other variables. All will affect the results.  

So,  I am simply going to say that I, a notoriously cold sleeper, found I was comfortable in the Mythic down to the lowest temperature I have used it in. This was -10 degrees Celsius.  Having said that, I really feel I would have been cold if the temperature were lower. This was sleeping on a winter rated sleeping mat and having been well fed and wearing lightweight Merino baselayers.  That’s all I can say really.  For you it might be different, but I still hopes that gives you a benchmark to work too.   


I’ve only ever used Rab sleeping bags and have no desire to stray.  I love the history of the company and Rab’s passion for innovation and continued excellence. Their products are absolutely top drawer.

That all said, the Mythic range is a significant step forward even for a brand that’s been producing sleeping bags for decades. The bag has certainly exceeded all my expectations and is a brilliant product. You are going to have to pay more for the Mythic, but if your budget allows you certainly won’t be disappointed. Rab describe the Mythic range as the ‘pinnacle of innovation’ and I have no reason to disagree.  The Mythic 600 costs £520 and full details are available on the Rab website here. Please also check out the Rab team discussing the development of the Mythic range in the short video below.

We hope you find this Rab Mythic 600 sleeping bag review useful. We have tested a number of Rab products and our reviews of their Alpha Freak Pull-On, Firewall Jacket, Xenon X Jacket and Kinetic Alpine Jacket might be of interest.