GSM - is the weight in grams of 1 metre squared of insulating material. So in general the higher this value the warmer your bag will be.
However, not all synthetic fills are the same. So a high quality fill weighing 200gsm could be as warm a low quality fill weighing 300gsm.
There are other factors that come into play. Is the bag rectangular or mummy shaped? Mummy shaped bags are more efficient for the same gsm but some people feel restricted in them.
A better indication of warmth would be to look at the TOG rating or the comfort rating. You may also want to look at the warmth to weight ratio of the bag, especially if you are going to be carrying it.
yes in general a lot of large retailers like argos are a bit fluffy on clarifying exactly what they mean by a summer/spring/autumn/winter bag and whether the value they give is a comfort rating or extreme rating. The two things are very very different.
If in doubt contact the manufacturer and ask how they tested their bag, best to know before you spend your money and have a cold night.
Rather than rely totally on your bag for warmth you will find far greater flexibility for our uncertain climate by also using fleeced covers and clothes when cold and unzipping or just using a fleeced cover when warmer. The fleeced covers I use come from the carp fishing scene and are around £25-£30 delivered. These are designed to go round a bedchair but can be used equally well without.
Also bear in mind that some bags have different thickness on front to the back and therefore assume the bag if not you will remain with the thicker front to the top.
If you cant decide what sleeping bag to take, although I have only ever take one, then instaed of fleece covers or liners, take an el cheapo sleeping as a spare, you can use the two sleeping bags together, unzip one and use it as a blanket etc.
Never quite understood why fleeces seem so popular as a standby, a cheap sleeping bag (from Lidl etc) would be cheaper than a fleece liner, is about the same packed size as a fleece liner, is lighter and most importantly is a great deal warmer than a fleece liner could ever be.
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Topic: Advice on Sleeping Bags
