Review: Rab Mythic sleeping bag & Alpine bivi

It’s been a few weeks now since I got back from Patagonia and I have finally managed to find some time to reflect on the experience and of course the equipment. While we unfortunately did not get the weather window we had hoped for (welcome to Patagonia!), we still managed to get out in the mountains and put several pieces of equipment to the test.

I am very fortunate to be in a position where I have access to a vast array of top end kit to take out and put through its paces. The only downside of this situation is that sometimes even excellent products get overlooked because something else light and shiny grabbed my attention first. But at the end of the day the best gear always stands out above the rest and this is certainly true of Rab’s Mythic Sleeping bags.

The Mythic range of bags are high performance super light down bags for weight conscious adventures in demanding environments. In terms of warmth to weight ratio these bags are as good as you can get. Rab achieve this by making the bag from Pertex Quantum, an extremely light material that is both highly compressible and durable. The bags are then filled with 900 fill power hydrophobic European goose down. Rab fill their sleeping bags in the UK so that the down clusters do not get damaged by being compressed in a shipping container for a couple months on their way from the far east. This has a big impact on the lofting performance of the down and is really something worth considering when buying down insulation.

The finished product is very impressive from the moment you get your hands on it. At just 660g it feels so light and yet you also get the sense that the Pertex outer will be tough enough for the job. Durability is the obvious trade off with lightweight kit and as a result, lack of durability is definitely the pitfall of many ultralight products. In this case though, I wasn’t disappointed. When I first got my Mythic 400 I was prepared to manage my expectations for the durability of this bag, because it’s so amazingly light. For alpine climbing the weight saving seemed worth it to me, even if it did turn out to be a bit delicate. But in practice I was pleasantly surprised to find this wasn’t the case. I have now spent over 20 nights in the bag and it looks as good as new. For such a light bag I feel it is very well made and if carefully looked after, will last a long time.

As well as being light and durable, a good sleeping bag needs to be warm and user friendly. In terms of warmth, I was very satisfied with how warm the bag was. The high quality goose down Rab use has a very impressive loft, trapping head and keeping you warm through the night. I choose the 400 with the idea that it would be warm enough for summer alpine climbing, a light option for extreme winter alpine routes and suitable for warmer Scottish winter days. Overall I am been very happy how the bag has performed so far in summer alpine and Scottish winter conditions.

The stated ‘Rab Limit’ is -7 C which intuitively seems about right to me. I have not yet used the bag in temperatures quite that cold, but judging by my experience so far I am quite confident I could sleep well in the bag around -7 C, but perhaps not too much colder. All of that being said it’s worth pointing out that several factors affect how warm you sleep and it can vary a lot from person to person. I don’t consider myself an exceptionally warm or cold sleeper, but it’s a tough thing to measure as you have nothing to compare too.

The final thing to consider is the user friendliness of the bag. You might assume sleeping bag designs are basically all the same, but in reality small features like closure systems and neck baffles can make all the difference for a good night sleep. I was again really happy with how the draw cords around the hood work on this bag and found it easy to adjust them during the night by feel alone. I found the insulated neck baffles were quite effective and helped keep me significantly warmer in the coldest parts of the night.

Perhaps my only critique, of what is otherwise a great sleeping bag, is the half length zipper. I fully understand the benefits it brings in terms of weight savings, but for me it’s not worth the sacrifice in functionality. I personally find a full zip makes a sleeping bag  much easier to get into and out of especially in bivi bags and small tents. Only having a half zip also makes it impossible to share one bag over two people. Although that might seem like a rather extreme weight saving measure, it’s not as uncommon as you might think and can be useful in emergencies. Ultimately having the shorter zip reduces weight and traps heat more efficiently so it’s hard to say which is really best in the end.

On the topic of bivi bags Rab were kind enough to give us two Rab eVent Alpine Bivi’s to test in Patagonia. As we never got a suitable weather window for any climbing involving a bivi, at the moment the bags remain untested. Having closely inspected the bag I am convinced they are up to the task and will be the ideal complement to the Mythic giving you a functional sleep system with significant weather protection and minimal weight. At the next opportunity I get I will test the bivi bags and report back with an update.

Until then here are a few initial thoughts on the Rab Alpine bivi. At the top end of the Rab’s bivi bag range the Alpine Bivi is a classic simple design made with an eVent waterproof breathable membrane to keep you dry. Condensation in your bivi bag is your worst enemy and will ultimately result in a wet sleeping bag that does not loft, greatly reducing its performance. Nikwax hydrophobic treated down will certainly your bag stay drier for longer but even with this, the performance of the sleeping bag will eventually decline over time. To minimise bivi condensation Rab use an eVent membrane which allows water vapour to escape, while still being waterproof.

Unlike other waterproof membranes and coatings, the eVent membrane does not require a significant temperature difference between the inside and outside of the bivi bag to expel moisture. This subtle difference is perhaps more critical for a bivi bag than any other waterproof garment. When you are active and you wear a waterproof jacket you usually generate plenty of body heat to drive out moisture. In this case almost any membrane can perform well. Once you are inside your sleeping bag within your bivi, it will trap all the heat you produce (as this is how it works to keeps you warm). The result is that minimal heat is lost from the sleeping bag out into the bivi, which means hardly any heat is present to drive moisture out through the bivi’s membrane.

So if you want to get the most performance out of you down sleeping bag make sure you have a bivi bag to match. Overall with its tapered shape and eVent membrane I’m confident the Rab Alpine bivi will be the perfect companion for the outstanding Mythic sleeping bag. In the end only time will tell and I look forward to testing!

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