Over the Top:
Using a summer bag in the middle of winter
Overbags may seem quite unusual, but the idea’s really no different from what we all do with clothing: when it gets colder, we add layers. We don’t strip everything off and replace it all with a single monstrous coat!
Well, this is the same but with sleeping bags: when it’s too cold for your lightweight bag, you can put a warm layer over the top.
We’re often asked about how to size overbags, which is perfectly reasonable: if the outer bag was too tight then the inner one would get crushed and wouldn’t hold any warmth. So just like with clothing - where a medium coat fits over a medium jumper - the outer layer is sized to go over the inner one. A standard overbag goes over a standard sleeping bag, a wide goes over a wide, etc.
For maximum thermal efficiency, overbags are shaped as well as sized: Minim Overbags fit Minim bags, Hispar Overbags fit Hispars. The Greenlandic Overbag fits the Greenlandic and Icelandic bags, and the M.Degree Overbag fits the M.Degree bags.
How warm are they?
The Minim Overbag boosts the performance of a Minim K Ultra, Minimus or Minim 400 by a whopping 23°C; the Hispar Overbag will add 24°C to a Hispar 400, 500 or 600; the Greenlandic Overbag boosts performance by 23°C, and the M.Degree Overbag adds 20°C warmth to any of the M.Degree bags.
As an example: a Minimus has a TOT of 5°C, but with a Minim Overbag you can create a system that’s warm down to -18°C. The Overbag can then be used on its own too, giving you an intermediate option for cool conditions. It’s rated at -5°C when used like that, but bear in mind that it’s a roomy cut so although it will work below zero, it won’t be quite as warm as a more snug-fitting bag.
For conditions where moisture is likely to cause problems, our Zeta synthetic bags are made with Primaloft insulation, which maintains performance better when damp. The Zeta Overbag allows the layered principle to be used with these specialist bags, but also works well as a synthetic alternative to the Minim Overbag for those who routinely venture out in the wet.
Thinsulate overbags are a different animal really, but we’ll touch on them briefly. In extreme cold, freezing point can occur within a bag’s insulation, so any moisture can freeze inside the down: to be avoided! A synthetic Overbag adds about 10°C but, crucially, moves the freezing point outside the down to protect the main bag.
The Snowline Bivvy is an insulated bivvy/come sleeping bag cover, that acts like an Overbag. Whilst protecting against damp, the insulation (Primaloft) adds 10°C warmth and also prevents condensation rubbing against & wetting the sleeping bag.
What you’ve told us…
When Chris Vardy went to Iceland for the 2017 World Scout Moot, he took his Minim 300 and a Hispar Overbag (an unusual choice, but it seemed to work), and had this to say about it:
“The overbag was spot on. The first week was warm so I used my Minim at night but the second week I used the overbag on its own when the temp dropped somewhat. A couple of nights the temp crashed and I used the two together.”
That’s pretty much textbook – exactly how we intended the idea to work – but it’s good to hear that it’s living up to expectations.
Slightly less textbook was what Stephanie Peluchon told us about dog sledding west of Kiruna in Sweden, 200km north of the Arctic Circle. “There was some condensation in the tent but the bag never got wet. I got some snow on it but again, no problem.” Once more, it’s great to hear that it’s behaving as we intended. What we honestly hadn’t foreseen though was that “The material was very resistant as I had a dog lying on top of me while I was lying in the overbag! No scratches on the material.”
We designed them for sleeping under canvas - or maybe under the stars - not necessarily under a dog. Still, it’s nice to know there’s an extra element of versatility to the kit!
So, if you have one of our bags and you want to go colder…or if you’re going somewhere super-cold but want to be able to camp in less extreme temperatures too, it’s well worth considering the benefits of overbags. And if you do, let us know how you get on.