A sleeping bag’s role is to trap a person’s warmth and prevent it from escaping, keeping them toasty. However, the warmth of the bag is determined not only by the bag itself. The environment in which it will be used, as well as the attributes of the person using it, dramatically affect how warm it will feel.
- Environment: when using a sleeping bag, the outside temperature, shelter type, presence of a breeze and what you are sleeping on will all affect your warmth.
- Individual makeup: women feel the cold more than men – it’s a fact. Other factors that will affect your warmth are your metabolism, what you are wearing and when you last ate and drank.
And then we come to your sleeping bag…
The warmest bag for its weight will be a mummy-shaped bag with heaps of excellent down, great technical features and a fantastic design. But if you’re generally going to use the bag in spring around Sydney, you will swelter! So before you buy the warmest bag on the market, think about when and where it will be used and the minimum temperature you’re likely to encounter.
Many manufacturers give their bag a temperature rating, specifying the temperature range at which a bag will keep a person warm. However, this rating should be used as a guide only, as it does not take into account the environmental and individual factors outlined above. Often there is little information about how the temperature range was determined and – unless independent testing is done – such ratings can be inconsistent and of limited use.
As there isn’t a consistent, universally adopted method of rating sleeping bag warmth, comparing sleeping bags can be complicated. Many brands have adopted ISO 23537-1, the international standard for sleeping bag performance, as it is a reliable, scientific rating method that gives accurate results and certainty for customers.
However, not all manufacturers test to this standard, so care should be taken when comparing warmth claims across brands. (This rating is also done when the bag is clean and new.)