LOFT ABILITY OF PARTICULAR SLEEPING BAGS FILL VOLUME is a measure of how much the TOTAL of all the down in a particular sleeping bag will expand. The greater the FILL VOLUME, the warmer the sleeping bag. The ability to loft of a sleeping bag depends on the weight of down that it contains and on the fill power or quality of the down being used. A comparison between sleeping bags can be done using the equation : (weight of down in grams) divided by 30, multiplied by the fill power For example, compare a sleeping bag filled with 900 gms of Chinese down, with one filled with 700 gms of Polish, diamond grade goose down. fill power weight of down (gms) TOTAL FILL VOLUME Using Chinese Down 600 900 18000 Using Polish Diamond Grade Down 870 700 20300 Here the lighter sleeping bag is actually warmer than the heavier one since it has a higher overall Fill Volume. When comparing the temperature or "season" ratings quoted by different manufacturers it is worthwhile doing the above type of calculation and comparing Fill Volumes. The so-called "minimum comfort temperature" that is quoted by most sleeping bag manufacturers is supposed to be a measure of the suitability of a particular sleeping bag for use in cold conditions. However, it is a very subjective measure (different people can tolerate different levels of cold), and furthermore, there is no "standard" to which every manufacturer adheres. Sometimes these ratings are overstated to make particular sleeping bags appear more attractive, hence such information should be treated only as a rough guide. The above FILL VOLUME equation is a measure of a sleeping bag's ABILITY TO LOFT and is, therefore, a useful tool for cross-checking such information when comparing several different sleeping bags. However, it should be borne in mind that good design also has an effect and, for example, a sleeping bag which utilises a Trapezoidal baffle construction will have a comfort temperature rating of 2 to 3 °C lower (better) than a sleeping bag containing exactly the same quality and weight of down, but constructed using the Boxwall method. Other constructional design features also have an effect. The better the overall design the lower the risk of "cold spots" arising when a sleeping bag is being used. Technical Note - temperature ratings and overall weight 1. The effectiveness of insulation materials is affected by moisture. You will feel colder in your sleeping bag in high humidity conditions. Moisture will also add to the weight of a sleeping bag. 2. How warm you feel is also dependent on where you are sleeping. The size of the body of air surrounding the sleeping bag and the amount of air movement/ventilation both affect heat retention. The comfort temperatures that we quote for Cumulus sleeping bags assume that they are being used in average humidity conditions inside typically-sized 1 to 3 person tents (with a relatively small body of still air surrounding the sleeping bag). Use them in a drafty, unheated mountain hut in damp conditions and you will feel significantly colder. That said, independent tests, carried out on all of our models in compliance with British Standard BS4745 and ISO 5087-1, gave temperature performance figures that are as good as or better than we claim.