Dec 20, 2013 at 12:46 pm #1311251
@hamericaLocale: Northern Virginia
This may be a stupid question but does the order matter in this equation for layering quilts: [x – ((70 – y)/2) = z]
I mean does it matter if the lower temp rated quilt (20°) is on the inside of a higher rated quilt (50°) given compression is not an issue?Dec 20, 2013 at 1:31 pm #2056346
Mike VBPL Member
The formula I have always used and found pretty reliable is:
x -(70 – y)/2 = z
x = higher rated/lower degree bag/quilt
y = lower rated/higher degree bag/quilt
z = combined rating
Which bag is inside of the other is irrelevant though, it will still give the same amount of insulation. The only consideration should be which bag you want the condensation from your perspiration gathering in (outer bag). I would advocate using a synthetic as the outer since that manages moisture better than down.Dec 20, 2013 at 1:49 pm #2056350
Tad EnglundBPL Member
@bestbuilderLocale: Pacific Northwest
Keith, yes it does matter if you use the equation when making the decision because the x value give you a different answer depending on which rated quilt you use for x (20* or 50*).
Now that I have stated the obvious (and didn't answer your real question)- the real issue is loft.
You will have the same loft regardless of which is on quilt is on top. The total inches (2+3 or 3+2 is the same). If one was synthetic I'd use that on top. If both are down, I’d put the 50* on top, better to saturated the 50 with condensation than the 20* IMO.
Looks like the question was answered while my girls were trying/fighting to borrow the car during there break- good to have them home.Dec 21, 2013 at 9:53 am #2056522
What this equation says, is that one of the layers will be compressed to 1/2 of its normal loft, and be less effective due to that.
This would obviously be the bottom layer.
This is also a bit non-scientific, probably overly conservative.
How much will a UL 14 oz 40F quilt compress a 30 degree bag? Not much at all. Possibly not any at all. Definitely not by 1/2.
However, for a heavier synthetic quilt on top of a UL down bag, it might do some.
The net result , is the heat transfer is by conduction , and it is linear. With 70F being "comfortable" with no insulation for people, the layers are additive in insulating ability. With the caution that dont expect perfection, you lose something.
Add it up, take away 10 or 20F if you like, or use the formula. Its just guesswork.
Just take something away, its not going to be perfect. The colder the condition, the more you may want to take away to be conservative.Dec 21, 2013 at 10:08 am #2056530
but 92 F is a better number to use than 70 F
that is the skin temperature when you're sleeping, around the torso, when you're a bit cold
that is the X axis intercept on Richard's plot, the Mammut document plot, and others
assuming the bottom layer gets compressed 1/2 is arbitrary. It can be more or less. And like Richard has shown, if you compress down, you lose only some of the warmth.
another factor is when you have more loft, the surface area of the outer surface is bigger. If the loft is increased 1 inch, the circumference is increased 2 * pi * 1 inch. If the outer circumference is 48 inches, that would be a 1/8th increase, so the outer layer would add 7/8th of the warmth it would have otherwise.
I think it's too complicated to have any formula that is very useful. You have to try it. Maybe derate the outer layer by 1/2:
combined limit = inner limit – (outer limit – 92) / 2Dec 23, 2013 at 5:34 am #2057028
By no means take this as any kind of expert advice.
I would think the actual real life temperature of a quilt would have to do with heat retained rather than outside temperature rating.
I.E. If the designed comfort zone was 70*, a quilt designed for 50* weather should retain 20* of body heat to reach 70*.
In 0* weather a 50* quilt plus a 20* quilt would therefore reach the comfort zone.
(70-50)+(70-20)=70*Dec 23, 2013 at 7:14 am #2057043
But you shouldn't use 70 F comfort zone
What temperature would be comfortable if you didn't wear anything? 70 F is comfortable room temperature with a thin layer of clothes if you're up and doing things, but if you lay down to take a nap, you'll be cold without putting a light blanket on.
92 F or +- a few F is a better numberDec 23, 2013 at 11:17 am #2057098
What I don't know is what temperature the quilt manufacturer is basing your comfort zone at.
The formula [x – ((70 – y)/2) = z] tells me that the rating of the first quilt is lowered by half the difference between the design zone of the second quilt, where as the formula,
A-[(M-x)+(M-y)]= desired rating
M = manufacturers comfort zone
A =average desired temperature
X= rating quilt 1
y= rating quilt 2
ex. if A = M = 70
seems more accurate.Dec 23, 2013 at 12:50 pm #2057123
I am trying to wrap my head around this particular rule of thumb.
Looking at Richard Nisley's example in an earlier thread: http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=83567&skip_to_post=716303#716303
It doesn't seem that the "[x – ((70 – y)/2) = z]" formula applies to his layering of a 25 and 40 degree quilt. Richard gets a rating of -4 yet the formula above gets a rating of +10. That's a BIG difference to me (although my math brain is turned off right now.)
Furthermore, it seems even the EN13537 standards from which manufacturers ratings are based upon have come under heavy scrutiny over the bast few years, so who really knows what to rely on these days:
Since people all fall asleep at different points of comfort, and "comfort" is clearly a range based on temperature, RH (and even noise & smell), it seems that accurately gauging sleeping bag performance has become as elusive a goal as measuring the overall performance of a piece of footwear, and applying it to all people.
(p.s. I guess that "keeping you from dying" temperature number is perhaps useful info to keep in the back of one's mind. No use trudging out in 0 degree weather with a 50d bag.)Dec 23, 2013 at 1:22 pm #2057129
Richard adds the clo of the two bags together (subtracting the 0.6 clo boundary layer of the 2nd bag)
This implies that the formula is x + (92 – y) (and subtract 10 or whatever to account for lost boundary layer of 2nd bag)
What's good about EN13537 is it allows you to compare two bags.
Your comfort limit will be a little different. If you found you were comfortable 10 degrees above the comfort limit on one bag, you could figure the same for other bags, although this would be approximate.
You really have to try a bag on different trips to find what the real comfort limit is, and even that's not accurate because maybe the next trip you'll be more tired or eat less.Dec 23, 2013 at 4:13 pm #2057170
x + y – 70 or 25*+40*-70*=-5*Dec 23, 2013 at 5:14 pm #2057185
@hamericaLocale: Northern Virginia
Thanks guys, good info here. I realize the formula is just a baseline but it should get me close.Dec 23, 2013 at 5:20 pm #2057188
That certainly proves to me that my math brain was turned off.
Thanks for making that fit, but what if I have two zero degree bags? That formula won't apply.
MDec 23, 2013 at 6:48 pm #2057210
"What's good about EN13537 is it allows you to compare two bags. "
After reading the document on the "issues of EN13537", it appears to me there is still much work to do, especially in regards to the potential variablility between the testing labs. To me, this might make it more difficult to compare two bags than it seems in the surface (or even the same bag from one year to the next.)
From the report:
"This example indicates that even if the inter-lab variability in manikin results is within the ideal ±5% range, the affect on the resulting temperature predictions is unacceptable to manufacturers. It has become difficult for companies to manage the changes in temperature ratings on product labels and in technical literature from year to year that are due to sample variance and laboratory test variance – and not necessarily due to a minor change in bag design or materials. The constant changes in labeling and re-testing increase costs for manufacturers."
Your advice to try different bags is spot on.
MattDec 23, 2013 at 7:11 pm #2057218
What I hate about EN13537 is it's so expensive. A manufacturer will have to test each size of each bag. It's prohibitive for small manufacturers.
The problem is that, pre EN13537, different manufacturers are more or less conservative so it's hard to select which bag from which manufacturer. Some manufacturers arbitrarily rate their bags colder because they cynically know it will help sales.
It would be better if they just had ratings such as 2.5 oz Apex is 45F, down could have linear relationship between loft and temp rating only you'de have to factor in that if you over-stuff, the clo/inch goes up. And you'de have to factor in mummy bag vs rectangular. The calculation would be a little complicated and there would be some error, but like the article says, EN13537 has errors too.
Then, different manufacturers could be compared easily.
And if you knew you slept cold, for example, you could just select a bag that's rated 10 degrees (or whatever) colder than the minimum temperature you want.Dec 23, 2013 at 8:32 pm #2057239
x + y – 70 or 25*+40*-70*=-5* is just coincidence
Richard refers to EN13537 lower limit to clo conversion and vice versa.
EN13537 lower comfort limit (Men) is:
10 C for 0.62 m2K/W
-15 C for 1.3 m2K/W.
And it’s linear, so, formula is:
Lower comfort limit (men) = 32.8 C – 36.8 * RSI
where RSI is the insulation value in m2K/W
Convert to clo and degree F:
Lower comfort limit (men) = 91 F – 36.8 (W/m2) * 1.8 (F/C) *0.155 (m2K/W/clo) * Rclo
= 91 F – 10.27 * Rclo
Where Rclo is the insulation value in clo
And inverse is:
Rclo = (91 F – lower comfort limit ) / 10.27
Richard said 25 F rated bag is 6.39 clo. I calculate 6.42 clo from formula above so there’s a small difference in round-off somewhere but it verifies the above formula.
Richard said 4 oz Prodigy is 0.82 clo/oz = 3.28 clo total (Richard said ~3.44 which is close enough). This is just the spec’d value for Apex.
Richard added these and subtracted 0.6 clo, which is the boundary layer. 6.39 + 3.44 – 0.6 = 9.23 clo. Plug into the formula above – lower comfort limit = -4 F – close to Richard’s calculation of -5 F.
Richard said that 40 F Prodigy has 4 oz Apex of 0.82clo/oz = 3.28 clo, but if you plug that into the formula, it would be EN13537 rated to 57 F. or 55 F if you use 3.44 clo. The 40 F rating is exhagerated compared to EN13537. Maybe that assumes you wear some clothing inside to get it down to 40 F.
So, the formula would be 25 F (for the MB bag) – (91 – 55) = -11 F. Except he subtracted 0.6 clo for the boundary layer which is 7 degrees, so the formula is 25 – (91-57) + 7 = -4.
The value 70 has no basis except anecdotally.Dec 24, 2013 at 8:50 am #2057306
25 – (91-55) + 7 = 25 + 55 – 91 + 7 = 25 + 55 – 84 = -4
X + Y – 84 = Z (Epic)
Two 0* quilts would be good for -84*. Two 50* quilts would equal a 20*.
70* was anecdotal based on ambient temperatures.
I won't disagree with your figures, I see where you get them. The standards do seem a bit wishful though.
I'll let Mikey try it.Dec 24, 2013 at 9:38 am #2057323
I agree the theory will exhagerate lower limit
Because of the factors mentioned – outer layer compresses inner layer, as the loft increases the area of the outer surface increases so you have more surface area conducting away heat, your head sticks out the end and needs to be insulated too,…
Since that depends on each case, you can't really make a good formula, so any of these ad hoc formulas are as good as any other. Maybe lowering limit by 10 or 20 degrees is a good "formula" but you really have to try it.Dec 24, 2013 at 10:11 am #2057326
I guess that'll be good for ole Santa – who (according to NORAD) is on his way to Dubai right now.
Happy holidays everyone!
Thanks for the enlightenment, Jerry & Scree.
p.s. for Christmas, I just wish bag manufacturers would post temp ranges AND permeability measurements. That info would be very helpful for me, in my pursuit for pure comfort in the wilderness…. (sigh)Jan 2, 2014 at 11:46 am #2059668
Piper S.BPL Member
@sbhikesLocale: Santa Barbara (Name: Diane)
Ha ha these formulas are crazy!
My thinking is you put the bigger (by dimension) quilt on the outside. You want it to wrap around the other quilt and if you use straps, to be able to thread the inner quilt's straps through the outer quilt's loops, or if you don't use straps, you want the outer quilt to drape completely over the other quilt to close all the gaps. It's those gaps that make you cold more than anything else.
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