View Full Version : Down loft to warmth ratio
vaporjourney
03-13-2007, 09:54
Is there a general rule of thumb out there which helps determine the amount of down loft needed to stay warm at various temps? I know that this is wildly variable and dependent on things such as metabolism. It'd be nice to try to decide if 2 inches of loft is enough compared to 4 inches of loft. Or another way: is 5 oz of down preferred over 4 oz of down while sitting in camp at 20 degrees. I've seen ratings on the R-value of sleeping pads, so assumed there may be something similar related to down clothing.
Frolicking Dinosaurs
03-13-2007, 10:11
Many use the Jardine formula: 100 - (40 * inches of loft) = Fahrenheit temp rating.
Down comes in different fill powers (amount of loft it provides per oz) so there is no magic number in ozs. It would take twice as many ozs of 400 fill power down to give the same warmth as 800 fill power.
QHShowoman
03-13-2007, 10:18
Well, what you are forgetting to take into consideration is the fill power of down. The higher the quality of down = the greater the fill power = the warmer it will keep you with less down needed. For example, fewer ounces of 800 fill down will keep you as warm as more ounces of 650 fill down.
Down with a higher fill power is also more compressible, which is why high quality down bags are usually lighter and more compressible than their counterparts of lesser quality. Although some may disagree, I don't think loft height by itself is enough to determine the warmth of a sleeping bag. There are also other factors to consider, such as the type of baffles sewn into the bag and the size of the bag relative to the amount of fill used. The fewer the baffles, the more likely it is that your fill will slide around and the more likely you are to develop cold spots. Obviously, the longer or bigger the bag, the more down fill you will need.
vaporjourney
03-13-2007, 10:32
Perhaps I should add that this is for a jacket, and not a down bag. I'm sure the same rules apply, but just wanted to clarify. All of the down jackets I have been looking at are of 800-fill down, which is nice. The only difference is ~2 oz of down filled in them.
That Jardine formula is pretty crazy: I just input 4" of loft into the equation, and it says the jacket would be good down to -60* F?
That Jardine formula is pretty crazy: I just input 4" of loft into the equation, and it says the jacket would be good down to -60* F?
I had the exact same reaction. 4" of down is good to maybe, maybe 20*, at best. Not -60*.
I don't think you measure/multiply 4" of loft total but 2" on either side.
hammock engineer
03-13-2007, 12:19
If you are making it I would add a 10% or so overstuff to each baffle. It helps to keep the down from shifting.
Programbo
03-18-2007, 00:17
That Jardine formula is pretty crazy: I just input 4" of loft into the equation, and it says the jacket would be good down to -60* F?
There are so many variables that come into play with temp/loft ratings..As others have said it depends on the quality of the down 500/650/800..The amount of the down..The baffling in the jacket/parka/sleeping bag...4 inches of loft with a 800 fill down in enough quanity could very well be good for -60*F...That`s about what all your 8000 meter parkas are....If it`s saying 800 fill and -40/-60 and it ain`t costing you like $700 throw it back! :p
Sly is correct, loft in sleeping bags is measured from the ground up but you get half as much insulation since you are inside the bag with 1/2 below & 1/2 above you. Therefore, devide the bag loft by 2 for the calculations.
Check Western Mountaineering & RJB FAQ for the loft = temp ratings.
http://www.jacksrbetter.com/index_files/FAQ.htm
Most Bags are tested new never compressed. First compression lose 15 percent. 2nd compression 10 percent of remaining, after that it is 3 percent of the remaing each time. On the plus side the weather is getting warmer so the difference isn't noticable till fall.:)
Skidsteer
03-21-2007, 07:12
Most Bags are tested new never compressed. First compression lose 15 percent. 2nd compression 10 percent of remaining, after that it is 3 percent of the remaing each time. On the plus side the weather is getting warmer so the difference isn't noticable till fall.:)
Great.
Pay $400.00 for a good bag and it's worthless after 27 days. ;)
peter_pan
03-21-2007, 07:17
Most Bags are tested new never compressed. First compression lose 15 percent. 2nd compression 10 percent of remaining, after that it is 3 percent of the remaing each time. On the plus side the weather is getting warmer so the difference isn't noticable till fall.:)
grysmn, et al,
I'd like to see an authoritative source on this.... This seems extremely high and doesn't pass common sense for the continuing 3 percent statement....Doing the math after a 7 nite hike the bag would lose37.5 % or be down to 62.5 % at 17 nights it would be at 50%.... this is not consistent at all with actual experience with down.... It may hold for synthetics , but I doubt that too.....
Pam
Most Bags are tested new never compressed. First compression lose 15 percent. 2nd compression 10 percent of remaining, after that it is 3 percent of the remaing each time. On the plus side the weather is getting warmer so the difference isn't noticable till fall.:)
Sorry, this is hogwash. I've got a 35 year old down bag and two "newer" down bags, a bit under 20 years old -- all still in very good condition. All three have seen heavy use over the years.
Most Bags are tested new never compressed. First compression lose 15 percent. 2nd compression 10 percent of remaining, after that it is 3 percent of the remaing each time. On the plus side the weather is getting warmer so the difference isn't noticable till fall.:)
I ran out the math. After 1 week, a sleeping bag has lost 1/3 of it's insulating value. After 1 month, it has lost half of what's remianing, and thus has 1/3 of it's original insulating value. After 100 days, it has 4% of the insulating value, and at the end of a 5 month thru-hike, it has about 1% of the value.
What's the source of your formula?
Sorry, this is hogwash. I've got a 35 year old down bag and two "newer" down bags, a bit under 20 years old -- all still in very good condition. All three have seen heavy use over the years.
I concur, too. Other than an increase in odor with increased usage, perhaps just a slight, barely noticeable, reduction in warmth, but no more. With a good, very careful washing the odor is gone and that little loss of lost warmth (loft?) returns.
Highway,
You bring up a good point. What does everyone do about the odor? Freebreeze? How often do you all wash your bag / quilt?
Egads
Footslogger
03-21-2007, 18:00
Highway,
You bring up a good point. What does everyone do about the odor? Freebreeze? How often do you all wash your bag / quilt?
Egads
=============================
YUP ...Fabreeze on the trail and a good washing with "Downwash" when I get home.
'Slogger
Most Bags are tested new never compressed. First compression lose 15 percent. 2nd compression 10 percent of remaining, after that it is 3 percent of the remaing each time. On the plus side the weather is getting warmer so the difference isn't noticable till fall.:)
That's total BS, even with a cheap Campmor bag...
You bring up a good point. What does everyone do about the odor? Freebreeze? How often do you all wash your bag / quilt?
When it needs it. For summer thru-hiking, maybe every three or four weeks or so. Less often if you sleep with a liner. YMMV and all that.