Get B3s. B2s on a warm day will be fine but on a cold day you'll be regretting B2s, especially if you are poorly acclimatised. Cold feet aren't fun and Mont Blanc is high enough and can be cold enough that you can do serious damage if your feet. There's a huge difference between the warmth of a pair of Mantas and the warmth provided by something like a Scarpa Phantom or Nepal Evo, and that will likely be the main reason the guide is specifying B3s; it will have little to do with stiffness.
One important factor is that the standard dummy at each test house is different. The test standard gives an idea of dimensions but just as two people who wear men's Medium people are not exactly the same shape, manikins are not exactly the same at each test house.
Hi all, I wouldn’t normally weigh in on a forum post in my ‘official capacity’ as working for Mountain Equipment, but as Bruce mentioned this thread to me I think there’s a few points here I can clear up. Sorry, it’s turned into a mammoth post.
EN 13537 is probably the best method we have to compare sleeping bags from different manufacturers. However, direct comparisons are still difficult because you are comparing numbers from a test that might not be representative of you. This is quite subtle and takes a while to get your head round. For example, if one sleeping bag performs really well on EN because it fits the manikin well – and perhaps not you – then it may well be less warm for you than a bag filled with much more down that fits the manikin poorly, even if it may fit you well. There are a lot of variables that affect the EN temperature rating and part of my role at ME is to investigate these and try to ensure our ratings are representative of real use and that the bags are as warm as possible for real users: we're not just trying to ace a test. The EN test does not apply to expedition sleeping bags (the Snowline is right on the limit of the test and the numbers the test produces for it and bags warmer than it are too conservative). Rab state that the EN test does not apply to bags with over 800 grams of fill, and while this isn’t strictly true, it does get round the problem of very conservative EN 13537 sleeping bag results on very warm bags. Having said all that, individuality has a much bigger effect than of the above: If I’ve got the flu and haven’t eaten for two days I’m unlikely to ever feel warm regardless of the EN rating of the bag; some people are just always cold, some people always warm.
As other posters have said, a good sleeping mat makes a big difference to how warm you are. I use a Neoair XTherm myself and if it's cold I notice a big difference between it and my other much thinner mats.
Regarding the EN test, the manikin has the same mat and clothing for every test, whether a -20 or +5 bag is being tested. He sleeps on a bit of foam of specific R-value. Regarding what the manikin wears, check him out below. He's not pretty, but don't say that to his face.
The US military invented the first process that I know of to treat down. That was in the '50s and was part of an attempt to replace down feathers - not many of them available in the US in times of war - with chicken feathers - lots of them available! They used a heavy metal treatment (chromium salts). A few other heavy metal treatments were also trialled, and silicones, fluorine chemistries and waxes (Nikwax) more recently.
Nikwax got ahead by offering an after-care product probably before anyone else to do the treatment at home, but as you say it's not easy for the product to get in past the face fabrics.
Absolutely, lots of wait-and-see to go. It may be a fad or it may be here to stay.