Found this information for what it is worth
Bag / Mat Combination
Which sleeping bag and mat combination is for me?
When and where are you going? How high up will you be? How far do you have carry everything? How knackered will you be when you get there? Are you likely to get very wet?
Temperature Ratings
In the UK you can pretty much expect to encounter close to freezing conditions at any time of the year (ask anyone who has hiked on Dartmoor, frost in early summer is not uncommon), but rather than getting the bag that is going to suit this one occassion, buy one that will be ideal for you the majority of the time, and wear a few items of clothing in rare colder cases or get a fleece sleeping bag liner to add a season when necessary. In summer at low level it may be possible to use bags rated for only one or two seasons but as altitude is gained, warmer gear should be considered.
In terms of seasons the following is what is generally meant in terms of average temperatures.
Season 1 (Summer) 2 (Spring-Summer) 3 (Spring-Autumn) 4 (Winter) 5 (Expedition)
Rating (°C) +5 0 -5 -10 -15
Main Use Warm travel Camping May-Oct Allround Year-round UK High altitude and polar
Although the bags are grouped into seasons we feel that this is somewhat misleading, particularly in the UK, as the conditions experienced throughout the year vary greatly from area to area throughout the country. For instance a three season bag would generally be acceptable here in South Wales for year round use. However, the same could not be said in the Cairngorms.
Manufacturer's Temperature Ratings
Each manufacturer tends to use separate human field testers and also separate laboratory testing agents. This alone tends to make comparisons subjective, especially as each tester is somewhat biased towards keeping their job.
There has been a recognised series of tests that all bag manufacturers can subject their bags too. This combination of ISO 11079, TOG testing and field testing has been around for some time. Recently another stand alone test has been created - EN13537. These tests differ significantly and whilst some manufacturers are giving information produced by the new tests on their goods many are still going with their own ratings. We feel that the results produced by EN13537 are generally no better than the preceeding tests and not much better than manufacturer generated values as the test has a number of questionable areas. Here are a couple of the main problems with EN13537:
The base mat that the test is conducted upon has a comfort temperature of around -7 degrees C and therefore influences the results of different temperature bags in different ways (summer bags performance is improved and winter bag performance reduced)
As can be seen the test doesn't work for winter bags which is the most important area to be confident about as failure here is more likely to cause death or injury
There is no use of reference materials in the tests to check whether the results are accurate (a standard requirement of good scientific analysis)
Be aware that the extreme temperature rating given means frost bite likely but death unlikely, so it really isn't something you want to do!
As a result of these problems we have been forced to conduct our own field tests. Although somewhat crude, we attempted to use standardized conditions and ensure the person in question had completed a similar amount of exercise and had eaten a similar amount before going to bed. In some cases the tester was woken in the night to change from one bag to another. The tests were all done in a small backpacking tent with two people in it. Clothing worn was just a T-shirt and underpants, if extra thermals or fleeces needed to be worn to stay warm then this was deemed to be the extreme temperature rating not a comfort rating. The temperature given was measured inside the tent and was generaly found to be about 5°C higher than that outside.
I believe this is a fair comparison and would equate to Winter conditions in Tasmania and other Alpine areas of Australia.
corvus
) further I could be wrong but I believe the EN testing of bags is done on a -7 rated mat.
. I have used the silk liners before and they can be handy for keeping your bag clean.