The Mythic 400 is 100% designed to be as light and packable as possible while trying not to sacrificing much in the way of warmth. A few of its design characteristics do limit its overall versatility. However, for those looking for a 20F bag that is among the lightest and most compressible made the Mythic is certainly worth considering.
Designed for trips where weight and packed volume are at a premium, we found every aspect of this model's design to be geared toward that purpose.
The Mythic 400 uses 14.1 ounces of top-quality 900 fill down treated with Nikwax Hydrophobic treatment for insulation. Those 14.1 ounces are equal to 400g which is where it gets its name. Rab gives this model a 19.5° degree rating, and overall we felt it offered a similar level of warmth or was slightly warmer than most of other 20° F bags.
With that said, we still feel the theoretically "20° F-rated" Western Mountaineering UltraLite, which offers 16 ounces of 800+ fill down and tighter dimensions, is still noticeably warmer. The Marmot Phase 20 also tested consistently to be ever-so-slightly warmer. When directly compared to the Phase 20, the EN ISO testing numbers line up to share what our testers felt. The Mythic has a comfort rating of 30° F and a lower Limit of 21° F; while the Phase offers an EN comfort rating of 29.5° F and a Lower Limit 18.5° F. We did find the Mythic warmer than all the other 20° F bags in our review.
While our testing team loved almost everything about this model, we found its hood just so-so for comfort, something that surprised us considering the attention to detail in the rest of this bag.
Impressively lightweight with solid warm is one of the primary reasons you buy this model. The Mythic 400 is an impressive 1 lbs 7 oz (650g), which makes it one of the lightest 20° F models available being marginally lighter than the Marmot Phase 20 (just over an ounce) and lighter than all the other 20° F bags (with the exception of the unique The North Face The One) and a number of 25-30° F rated models.
The Mythic minimizes its overall weight in several ways, with the three most notable being its 1) zipper, 2) down quality, and 3) fabrics. The quarter-length side zipper employs a very low gauge zipper (YKK #3) that is in line with the smallest gauge zippers we have seen on a sleeping bag. It uses top-quality 900 fill down to obtain maximum loft with a minimal amount of fill. Lastly, the Pertex Quantum 7D interior and exterior fabric are ultra-thin.
Unlike a lot of other lightweight models, the Mythic's torso dimensions are actually larger than average, not only making this bag a little more comfortable to sleep in but it also facilitates adding layers on colder-than-20F nights.
We found the Mythic surprisingly comfortable for such a low weight bag. This comes mostly from its larger-than-average upper torso dimensions, which allows the user to turn over and move their arms around more freely than other lightweight options. For example, this model has a torso circumference of 72.8-inches compared to its closest competition the Marmot Phase 20, which is 60-inches, or the Western Mountaineering UltraLite, which features 59-inches.
Our entire review team noted this model's comfortable and nicely articulated vaulted foot box.
Other things our testers liked about this model's comfort is its above-average angled foot box, which helps create a roomy feel around our feet. While the Mythic offers larger-than-average torso dimensions, it is slightly smaller than average for its foot and leg dimensions. The toebox design compensates well for this.
We also loved this model's silky feeling interior fabric that was less clammy-feeling than most and felt exceptionally nice against our skin. The only things we felt were so-so (but certainly not deal-breakers) are that we could feel the elastic hood cinch when it was tightened directly on our foreheads and this model is only available in left zip only.
This model's super low-gauge zipper snagged frequently on the wafer-thin 7D fabric. While not a deal breaker considering this model's impressively low weight, it was annoying never-the-less.
This bag is quite compressible and among the smallest 20° F bags currently on the market, and even packs tighter than several 25° F models. This is where a majority of factors that contributed to this model being lightweight like thin 7D fabric, quarter-length zipper, and 900 fill down also help its packability. The only thing we didn't love was its stuff sack, which is light and very water resistant but difficult for minimizing its packed volume.
Consider purchasing a compression sack if you want to fully minimize this bag's packed size.
For any trip where weight and packed volume are of the utmost importance, this model performs fantastically, although it's best used on chilly nights as opposed to warmer ones. This model is great for cold-weather backpacking, colder sleepers (aka people who get cold easily) in mild-weather backpacking, spring ski-tours, along with spring and summer-time mountaineering.
Its quarter-length zipper means it's difficult to increase airflow and dump heat on warmer nights, limiting this model's overall versatility across temperature ranges. Conversely, for colder-than-20F trips, its roomy torso dimensions make adding layers to boost the temperature rating easy and effective.
This model sports the straight-up thinnest fabric in our review, a tiny 1/3-length zipper, and some of the highest quality down. While we loved how light the design made it, we feel that the 1/3 length zipper does reduce some of this bag's overall versatility.
Features and design
This model is geared to be as light as possible, which might mean some features won't be your favorite. As we mentioned, this model uses the smallest gauge zipper in our review that only unzips the top quarter of the bag. This design no-doubt limits its overall versatility but saves a few ounces and reduces packed volume in the process.
We thought it was a nice bonus that this model uses Nikwax Hydrophobic Down, which helps reduce water absorption and quickens drying times compared to untreated down. Overall, this isn't a feature-laden bag, but what you lose in features, you also lose weight!
Not a ton of features on this bag, but we appreciate the application of Nikwax Hydrophobic Down treatment.
The Mythic 400 is perfect for any trip requiring a 20° F bag where the weight and packed volume are of the utmost priority. We used this bag for summertime mountaineering trips and spring ski tours and think its fantastic for cool weather backpacking or for folks who get cold easily. It's easy to layer up for slightly-colder-than-20° F tempts but it wasn't very easy to stay cool and keep from sweating on warmer nights.
At $500, this is mostly in-line with other high-end 20° F models, like the Marmot Phase 20 ($460) and the Western Mountaineering Ultralite ($500). While you can spend significantly less on a 20° F model with options like The North Face Cats Meow, with sleeping bags you do get what you pay for. Several other 20° F models might be twice the packed volume and double the weight of this model.
What the Mythic might lose in overall versatility (mainly when using in warmer temps) it certainly makes up for in weight and small packed size.
Impressive low weight and packed size measurements are why you buy this bag over a majority of others. What makes the Rab Mythic 400 standout over other weight-conscious 20° F models is its wide torso area, providing plenty of arm room and facilitating easy tossing-and-turning. Its only disadvantage is part of its advantage: several of its lightweight aspects of construction make venting more difficult and zipping up more cumbersome than average. However, as long as you either have another bag for warmer trips or don't plan many adventures with overnight temps much above 30-40F degrees where this model won't feel too warm, it should be a strong consideration for anyone looking for a light and compact 20° F model.