Sleeping bags vex me. The lack of certainty in assessing their true performance offends my vaguely obsessive-compulsive sensibilities. Yet, nothing in life is certain - apart of course from death and taxes - and the purportedly flawed EN13537 rating system is as good as any in giving some assessment of performance. That a company I trust and admire - Alpkit - have not put their bags through that test due to cost speaks as to one of major flaws with that system. That said, no other more effective system has yet been postulated so it provides some uniformity at least.
None of this is news, but for the assistance of those who do not know, EN 13537 testing starts from the premise that women sleep colder than men given identical conditions. Consequently, sleeping bags are rated thus:
EN 'Comfort' Rating (for Women): The lowest outside air temperature at which a 'standard' woman can sleep comfortably.
EN 'Lower Limit' or 'Transition' Rating (for Men): The lowest outside air temperature at which a 'standard' man can sleep comfortably.
EN ratings analysis is based on the user wearing a base layer and a hat, and using an insulating sleeping pad under the bag, which is appropriate to the conditions.
An EN 'Extreme' or 'Survival' rating describes the temperature at which a 'standard' female user would be kept alive. I would ignore this rating lest you be lulled into a false sense of security.
Sleepings bags vex me too because they are expensive. I am fortunate in that hillwalking and mountaineering, and independent travel, are my only vices, if a deep love of the outdoors can be categorised in such a pejorative way. I am therefore in a position to afford to spend what I would want to on kit. That said, were I to supplement my collection with a true All-Season bag, the PHD Hispar 400 (-9C, 770g, £368) or the 500 (-15C, 890g, £408) would be my choices. I toyed with the Western Mountaineering Ultralite (-7C, 820g, £315) but the PHD bags look better on paper. Either way, this analysis provides me with one inexorable conclusion - in order to bed myself down in deeply sub-zero conditions, I would need to spend £315-408 on a bag I might use twice a year.
My Western Mountaineering Summerlite is rated down to 0C and weighs 575g. I have used it repeatedly around that temperature and in base layer/longjohns only, I am fine. Camped at Angle Tarn, at 2C, I used the Nano-Puff (60g Primaloft One fill insulation) in the middle of the night but only for comfort rather than necessity. Thus, I am of the view that for temperatures down to a maximum of -5C, with a decent insulating layer, hat, long-johns and a winter mat such as the BA IAC, the Summerlite will do. I suspect I'll be a bit cold, but I'll be reasonably happy. Of course, I bear in mind that the loft of both the bag and the PHD Yukon I am awaiting will likely be less than the sum of both separately as they will be both be slightly compressed by the other.
So it was that I stumbled, as I so often do with these things, on the Sea to Summit Reactor Thermolite Liner. There are three in the range - Reactor, Reactor Plus and Reactor Extreme. All are made from ADVANSA Thermolite, created by Dupont, which is a hollow-core fibre with insulating and wicking properties. It feels rather like thin merino wool, akin to something like a merino wool base layer, which is very pleasant - although it is a polyester weave. It is breathable (it is virtually see-through, it's so thin) which has its advantages as a layer for a sleeping bag. It is machine washable too. It costs around £35.
Sea to Summit claim that there are temperature gains to be had to increase the range of your sleeping bag for what is a lightweight liner. Summarised thus: Reactor (+8C/15F, 243g), Reactor Plus (+11C/20F, 263g) and Reactor Extreme (+14C/25F, 399g). The former made with 80g/m2 Thermolite, the latter two are made with a heavier 110g/m Thermolite either in some areas of the liner (Plus) or the entirety of the liner (Extreme). All of the temperature gains are a maximum and many people may well experience, say Sea to Summit, less. Mine, I should say, weighs 259g in the stuffsack (rather than 243g) - always important when assessing the overall efficacy of a product to ascertain if a manufacturer is prone to hyperbole. The liner measures 210cm x 90cm and is too roomy for me. It could've been smaller, easily - it has seams rather than one piece so Mrs M may well butcher this for me to a smaller size. Packed in its silnylon stuffsack, it measures 9cm by 15cm (which is larger than the 7.62cm by 12.5cm as advertised - remember: hyperbole).
If those claims are correct, for £35, my Summerlite would be rated to -8C (I consider more like -4 or -5C, to be on the safe side, if the claim is believed at all) for an additional 259g (a total of 834g, although some judicious ditching of labels and stuff sacks saves around 20g and I imagine if Mrs M gets the sewing kit out, another few grams would be shaved). Additionally, I'd have to wash my bag less which is a good thing with a down bag. Looking at the WM Ultralite (-7C, 820g) and PHD Hispar 400 (-9C, 770g). The Hispar is, if PHD are accurate, the more effective regime but at £408, it's a Scots Bonnet of a purchase. Also, on a longer trek, with varying temperatures, the Reactor may well prove more versatile than two bags or a bag that is too warm - if it simply enhances the range of a single bag then it is a worthwhile consideration.
I am sceptical about the Reactor. Reviews of its performance are wildly divergent (have a look at BPL here and here and Outdoors Magic here). This is to be expected as sleeping is a strikingly personal thing (a fact demonstrated by someone in 2005 and then again in 2010 still willing to write at length about the assertion it does not work). A 0C rated bag might well be warm enough at 1-3C for one person, but not for another. If you get into a bag cold, you'll likely stay cold. If you've exercised or eaten before getting into a bag, then you'll be warmer. If your mat is poorly chosen, either because of its thermal insulating properties or it's too short and a part of your body is in contact with the ground, then few bags will assist and you'll still be conducting your heat into the ground which is a far bigger, and colder, surface area than you are. You'll be cold.
So, most people would suggest that +8C/15F is unlikely. To be fair to Sea to Summit, even in an email to me about the fabric and the tests they've conducted, they do say that. They are even considering spending money of having it EN13537 rated but as yet they have not. Their tests are of the field-report type but they did not elaborate on what they were. Clearly, the fabric will have some insulating properties but I doubt very much if it is as effective as suggested. Having sat in it in a cool room with nothing but boxers on (how very scientific) it feels warmer than not having it. I would conclude it is likely to add something to your bag but, as I said at the start - there is no adequate way to test this and people are very different. We'll see, in -5C to -8C conditions, where it should, if their claim is accurate, keep me comfortable in the Summerlite, how it performs. I'll always have the Yukon if it does not...