Rab Mythic 400 review: a premium sleeping bag for serious alpinists and fastpackers

Rab Mythic 400: first impressions

The Rab Mythic 400 is part of the British brand’s premium sleeping bag range aimed at alpinists and backpackers who desire low weight and impeccable quality. It weighs in at less than 25oz, yet is packed with 900 fill power (FP) goose down that has been treated with Nikwax Fluorocarbon-Free Hydrophobic Finish in a bid to improve durability in harsh conditions. The range-topping materials do come at a cost; with a list price of $650 (US) / £560 (UK), the Mythic 400 firmly in the high-end category.

The bag is made with a recycled Atmos fabric shell, which is lightweight, windproof and highly water-resistant. The fabric is made from a 10D recycled nylon, and is waterproofed with a fluorocarbon-free DWR (Durable Water Repellent) treatment. This commitment to sustainability is becoming a key focus for Rab, with the manufacturer claiming that 38% of the bag’s materials come from recycled sources. The move away from traditional PFAS (perfluoroalkyl and polyfluoroalkyl substances) in favor of more eco-friendly alternatives is something to be applauded.

The Mythic 400 has a trapezoidal baffle construction which is chevroned in shape, which Rab say is all in aid of keeping the down more lofted during use. This results in a usable temperature rating of 21°F (-6°C), putting the Mythic 400 firmly in the three-season category. On the left side of the bag, you’ll find a half-length YKK zipper, which, while saving weight, can feel restrictive in terms of ventilation and accessibility. Similarly, the tapered design is intended to maximize warmth, but it may feel constricting for larger users or side sleepers. It’s worth noting, too, that the Mythic 400 is still only available in one size and one color.

Expectations were very high for such a pricey sleeping bag from a prestigious brand such as Rab, and it’s fair to say that they were all met or exceeded. Rab has put together a fantastic blend of warmth and comfort, all in an incredibly packable product.

Meet the reviewer

Joe Baker

A lifelong outdoor enthusiast, Joe found a love for adventure early on, starting in Scouts as a young Beaver and progressing through the ranks. After years as a dedicated racing cyclist, Joe shifted focus to the mountains, diving into alpine trekking, hiking, and trail running. Now, he’s is constantly in pursuit of the perfect camp spots and the most breathtaking sunrises and sunsets. From remote mountain trails to rugged terrains, Joe is always planning the next remote escape.

Rab Mythic 400: in the mountains

What better way to test a sleeping bag marketed at alpinists, than by taking it to the home of alpinism? I deployed the Mythic 400 in multiple wild camps in Switzerland this summer – mostly in a tent – and several were at fairly high altitudes by European standards.

Sleeping temperatures ranged from just above freezing to around 50°F (10°C), and I never once felt the chill. The lofty 900FP down provided exceptional insulation, so much so that on a number of occasions I ended up using the Mythic more as a duvet than a sleeping bag despite wearing only thin pajamas to bed! An occasion where things did get slightly more chilly was a bivy camp at around 7,500 feet. It was dry enough that I didn’t have to worry about rain, but I appreciated the windproof Atmos fabric as it shielded me from the fairly brisk alpine air.

A note on size too – I was completely cocooned in my sleeping bag. I’m 6’0”, and fairly broad, and I was pretty comfortable in the bag. The one size is fairly roomy, but this could make it tight for those much larger than me, and potentially not as warm for those a little smaller. That said, I think Rab have done a good job at servicing as many people as possible for a specialist product where production numbers will be lower.

However, as temperatures climbed into the 40°F (4°C) range, the Mythic 400’s half-length zipper did become an issue. The short zipper prevents efficient venting, which could make the bag feel stuffy in warmer conditions. This must be caveated by the fact that this is a sleeping bag for extreme conditions. The only real snag, quite literally, was that the smaller YKK zipper did have a tendency to catch on the baffle that runs lengthways on the sleeping bag. It never caused the material any harm, but certainly something to be careful of.

Put the Mythic 400 in your pack, and you can pretty much forget about it. It compresses down to a modest 13.8in x 9.1 (35cm x 23cm) in the included compression sack. It’s a premium stuff sack too – a dry bag-style sack that’s totally waterproof to protect your pride and joy should the rain really start to come down, or you have a dreaded backpack spill.

As for durability, the Rab Mythic 400 held up well despite being subjected to rocky terrain during wild camps. While I did take care not to abuse the bag, the thin 10-denier shell fabric showed no significant wear after weeks of use. The DWR treatment kept moisture at bay, but like all ultralight gear, this bag is best used with a bit of caution to avoid snags or tears.

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