30°F / -1°C sleeping bag vs a 50°F / 10°C sleeping bag whilst wearing layers

Question

I'll be doing a multi day hiking and backpacking trip in a few weeks with a company that recommends that participants have a 30°F (-1°C) sleeping bag. I'm wondering if a 50°F (10°C) sleeping bag could work provided I wear some layers.

Answer 1

Summary

• Layering clothes to "boost" the sleeping bag about 11 °C is unpractical: too heavy and too bulky, and likely won't fit into the sleeping bag.

• A 2nd summer sleeping bag to layer/combine with your sleeping bag may be an option, though.
  That's also heavier and bulkier than going with one thicker sleeping bag, but can be much cheaper. Particularly, if you can borrow the additional sleeping bag. Make sure it is large enough.

Long version

Sleeping bag temperature ratings (according to ISO EN 23537) already assume that you wear a base layer of thermo underwear. AFAIK, it will also assume that you close the hood except for a little breathing hole. Then, at

• comfort temperature: a standard female can sleep comfortably in relaxed position.

• lower limit temperature: a standard male can (just still) sleep, but already needs to take a curled up position to avoid waking up because of feeling cold. I don't know whether this also assumes breathing into the sleeping bag hood - which IMHO in practice creates nothing but misery due to moisture.
  Side note: if you cannot curl up inside, or your sleeping bag has less insulation at the back (quite common), don't expect to sleep at that temperature.

  Unless you are substantially more muscular than the standard man (or know that you react to outdoor activity plus good feeding in the evening by being very warm during the night), it is not a good idea to consider lower limit rating other than telling you that there's bit of margin for temperatures getting lower than expected without endangering the rest of the tour.

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Back of the envelope calculations

for the additional layer(s):

• Skin temperature during sleep ≈ 33 °C (Kräuchi et al: Circadian and homeostatic regulation of core body temperature and alertness in humans: what is the role of melatonin? In: Circadian Clocks and Entrainment. Volume 7 (pp.131-146, 1998)
  (We have lower core body temp, but higher skin temp when sleeping compared to being awake. Or maybe rather: we need higher skip temp in order to sleep.)

• FWIW, the warmest thermo underwear I have (270 g/m2 fleece), for me makes a difference of maybe 4 °C at rest. I think it's similar to the effect of a thick fleece pullover + pants.

• The 10 °C sleeping bag thus covers a temperature difference of ≈19 °C.

• Another 11 °C are needed, plus a bit of margin because you'll loose more heat through breathing at -1 °C than at 10 °C.

  • Extrapolating from my X-warm thermo underwear, you'd need to layer up like Michelin Man, and that also corresponds to my experience of how much stuff I wear when sitting in camp in the evening at temperatures where I'd barely wear long sleeves when active during the day (e.g. sitting comfortably at slightly below 0 °C: in downhill ski suit).
    In general, sufficient additional clothing layers are much heavier than additional sleeping bag insulation.

  • If you go for layers, I'd recommend to test your plan. I currently have night temp around 0°C, so perfect test conditions. If you don't, you may be able to approximate it by sleeping in a room with, say, 15 °C in your layers but without the slightest covering. (I added a safety margin of 3 °C for breathing cold air and uncertainty about the thermo underwear).

If that suggestion sounds crazy and miserable: that's the level of misery to expect when planning to sleep in a +10 °C sleeping bag at -1 °C...

• very rough guesstimate from the specified differences of a series of sleeping bags vs. temperature rating suggests that maybe 6 cm additional loft between you and the outdoors is needed to go from +10 °C to -1 °C.
• However, you can also layer up sleeping bags (obviously, the outer one needs to be larger so the inner one doesn't get compressed). A 2nd summer bag would do here.
  When I went winter camping long ago with university sports, an additional large summer sleeping bag was lent out to those who didn't have a suitable sleeping bag. We stayed in quincies, so temperature was approx. 0 °C, and that combo worked well.

Answer 2

I would go with the bag they recommend. It's much easier to cool down than to warm up.

You state a "multi day hiking and backpacking trip". I have been caught out with an inadequate bag, and for a single night it was manageable, but I was able to change my plans.

What will you do after the first possibly miserable and sleepless night? All you will have is what you carry.

Answer 3

It may not be enough, but sometimes you can add a sleeping bag liner or a bivy to enhance the insulation and humidity and wind resistance of your sleeping bag.

Liners can be made of cotton, silk or polyester fibers, and can come in handy for sleeping in hostels or huts.

Answer 4

If you are either going alone and can easily go back, or have enough time to collect e.g. leaves to bolster insulation, you can give the lighter bag a try. Then it's your decision.

If you will be traveling with a group, please don't start with disregarding explicit safety advice given by the organizer yet. Being not properly rested or unable to function (e.g. in aptly handling a belay device) puts a lot of unnecessary strain on the group, might be an impediment in reaching an intermediate target, or even endanger others. So this might be a bad idea.

If you have good reason to make do with the lighter bag (e.g. weight considerations, or maybe the cost of buying a new sleeping bag), you should ask the organizer. Be open with them. In any case, please don't silently disregard their advice. They should have a very clear idea of what temperature to expect in average and outlier years, and what safety margin is acceptable.