How could a lighter winter sleeping bag also be warmer?
Western Mountaineering makes both the Bison GWS (rated -40) and the Cypress GWS (rated -30) sleeping bags. But the Bison has a lower fill weight (42 oz) and is cheaper ($1095) vs the Cypress (50 oz fill weight for $1130).
How could the supposedly warmer Bison have less down in it and cost less? This makes no sense to me.
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Your looking at the wrong value. Fill Weight is not a measure of warmth. It's a measure of the weight of the down. Weight does not equal warmth. The difference in fill weight here is likely (as strong bad says) the cut. The cut of the bison is more generous (more space inside) and it has thicker insulation (11" vs 10"), so it's fill weight is slightly higher (the total weight of the down used is greater).
So because the cut is more generous it has a greater surface area, greater surface area means more heat lost. Also there is more air inside the bag that will need heating, so it takes more energy (heat from your body) to heat this air. They've compensated (to some extent at least) by increasing the thickness of the down but it's still isn't as warm as the bag with a smaller surface area (Mummy).
They real meausre of the effectivness of the down is the fill power, these both have a fill power of 850+. So the qulaity of the down is equal.
It's a bit odd that your paying more for essentially less down, but the manufacturing of the mummy style bag is likely more complex and the company has also decided that you should pay extra for the extra warmth rating.
I've also just noticed the the more expensive one has a more complicated internal structure. Acording to the links on the web site the more expensive bag is:
V-Block Baffle w/two (offset) draft tubes & full down collar...etc.
Where as the cheaper one is:
Side Block Baffle w/draft tube and full collar
So the more expensive one is designed to prevent gaps in the loft on the sides.
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On reason why the Bison costs less is simply because it has less down. At 8oz for 35$, that's not a big difference though.
The bison is warmer because it is more snug. The semi-rect shape of the Cypress loses a lot of heat that way, because it exposes a larger outside area.
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This is what the Western Mountaineering said:
The Bison is a considerably narrower bag so even though it has less down/less insulation than the Cypress it has more loft. The design of the Bison is also quite different. Both bags take roughly similar amount of production time to build (a little over 8 hours per bag in terms of manufacturing time/labor) but the Cypress uses quite a bit more fabric and down which bumps its production costs (and subsequently its retail price) up above that of the Bison. I can see how the rating and fill weight specifications might appear to stand in contrast to the retail price difference between the two bags. Hopefully this additional information helps clarify the reason why our Cypress is more expensive than our Bison.
The Cypress is just a much wider bag. Its for people who are large framed and have a tough time fitting (or sleeping comfortably) in a Bison. We designed it after receiving requests from big guys who are not mountain climbers but hunters in places like Alaska and northern Canada, Montana, etc. We have sold these bags to people who are the animal doctors along the Iditarod who are stationed at checkpoints. They are not climbers, but just people with a bigger build. Not everyone is a fit climber who uses our products so we created this design for those people to use at cold temperatures.
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I think it is easiest to see the difference here: Specification Chart
The heavier one is a semi rectangular bag while the lighter one is a mummy bag. Both bags have the same amount of loft, which is usually the key to warmth, but in this case the difference in cut matters.
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