05 December 2008 12:22
Does a sleeping bag inside another sleeping bag significantly boost the performance? Or, is it better to get one thick one? (Don't really want to do the second option as I would probably never use a 5 season sleeping bag again!)
Sleeping bags keep you warm as they are able to trap small pockets of air. The more air trapped inside a sleeping bag, the more insulation it will provide and the warmer you will be. So in theory, yes, placing a sleeping bag inside a second sleeping bag should be warmer. The Ultralight range of sleeping bags by Khyam for example uses this principle to create a warmer bag.
However, for this to work the sleeping bags need to fit inside one another without squashing all the small air pockets - otherwise the benefit will be reduced. The perfect set-up is an inner bag which is a little smaller than the outer bag, so that it fits neatly inside to provide the maximum number of air pockets and therefore maximum insulation. For these two bags to fit inside one another perfectly, one would have to be tight when used on its own and the other would have to be baggy when used on its own. So they might work well together but no so well on their own?
As a general principle it may be better to have just one bag. Two bags are only better in very specific conditions, such as trekking from low altitudes in hot countries to high altitudes - say Nepal.
Finally, buying two bags will be more expensive and heavier and have a greater pack size. So the short answer is, I think it is better to have separate bags overall.