The insulation value of a sleeping bag depends on three things, the amount of down, the quality of that down and the way the bag is constructed – which needs to have baffles large enough so that the down can loft freely. There are lots of other minor factors like over filling to top compared to the bottom which gets crushed, zip and shoulder baffles, hood design etc.. but the basic three are still the most important.
With this in mind designers constructing a sleeping bag designed to be the lightest weight possible for a given warmth can’t save weight by reducing the amount of down in the bag because that would produce a lower performing bag – the only way they can save weight is in the shell of the sleeping bag.
A quick analysis of other RAB bags shows that the shell of a Neutrino 200 bag is 420grams (that’s the total weight less the fill weight of the down), a figure typical of ‘standard’ performance sleeping bags. RAB’s Infinity range were designed to be the lightweight choice and the shell of the Inifinity 300 is just 350grams, saving 70grams on the Neutrino bag. Performing the same calculation on the Mythic yield a shell weight of 286grams.
Reducing the weight of the sleeping bag ‘shell’ is exactly what RAB have done with the Mythic sleeping bag range, and they’ve managed to do it with very little compromise on features. I’m very glad they didn’t give in to the temptation of a ‘sewn through’ construction which can cause cold spots, the Mythic bags have a full Trapezoidal baffle construction which not only means the down has ample volume in which to loft – but the stitch lines are offset between the inside and outside of the bag to ensure no cold spots can occur.
The foot-box is angled to match your feet, there’s a cinchable shoulder baffle and a decent sized hood that can be brought in close to the face. The short zip is a compromise, it ends about a third of the way down the bag. The Mythic bag is no smaller than other performance bags in the RAB range so they’ve also resisted tweaking the sizing to achieve a lighter weight.
The Mythic bags are filled with the highest quality Hydrophobic down and achieve excellent EN test results given their low weight, they achieve these results through quality of down and quality of construction.
I’ve used the Mythic 200 for quite a few nights and it lives up to its rating, I tend to sleep cold but had no problems keeping warm in temperatures around 4-6 degrees without any other clothing, on a decent sleeping mat.
The zip is interesting – it comes apart at the bottom, I’m pretty sure this is aimed at multi-day climbers because you could zip up your sleeping bag without unclipping your harness if you were sleeping on a ledge. I undid the zip to attend to a call of nature and the parts separated at the bottom, it was a frustrating struggle to get them together and zipped up again.
I liked the cordlocks on the hood and shoulder baffle, these are easy to use, there is an anti-snag baffle along the main zip which worked about as well as on other bags. Space-wise I found it perfectly adequate, you don’t want your bag to be too spacious as this allows the air to circulate and heat to escape. There aren’t many bags around achieving such good results at such low weight, and some that do compromise more on features than the Mythic bags, we like them.