Sleeping bag ratings?

[GaryM]

I understand this is very subjective but how are they determined? What assumptions are made concerning the dress of the occupant?

[4eyedbuzzard]

Do a google search of EN 13537 and also the ISO standard for sleeping bags. It's a complex subject with plenty of articles out there. Test mannequins are typically dressed in socks, long base layer, and knit hat. Test mannequin is fully in the zipped bag, on an insulated sleep pad (sleep pad R value is important as well), and the test is done indoors in a temp controlled room. In the real world, wind/breeze, higher humidity, dirt, etc., all work to reduce insulation effectiveness. A tent probably adds 5° to your comfort in cold weather.
TIP: I would always go with the so called "womens" or "comfort" rating when selecting a bag. I don't want to have to assume a fetal position and struggle to keep warm based on the "lower" limit - you'll understand this statement better after some research. And while the standard is a good intention, some manufacturers don't choose to use or test to them. Western Mountaineering makes perhaps the best sleeping bags out there, and they are reported by most people to have kept them warm at the advertised rating, yet they don't choose to cite the EN or ISO standard ratings on their .com / US website [EDIT: It seems that their resellers in Europe do use this info - perhaps a legal requirement in the EU?]. From what I can tell, WM, Feathered Friends, Enlightened Equipment, and Zpacks - all companies that are known for high quality gear and sleeping bags, don't test or choose not to cite those numbers - but if you ask hikers, they are the ones always mentioned when looking for top quality gear. The bigger companies that make good bags - Marmot, Montbell, North Face, Big Agnes, etc. all seem to report the EN ratings. But there's also a lot more to what makes a good sleeping bag than just the temperature rating. There's shell fabric, quality of fill, baffling, draft tubes, zippers, quality of sewing/construction ...

[soumodeler]

This should be helpful:

https://thermarestblog.com/en-iso-sleeping-bag-ratings/

[HooKooDooKu]

The sleeping bag company marks it at what ever temperature they want. About the only limitation might be legal liability should someone die because they believed the temperature rating.

However, there is one rating system that has very well defined legal parameters - EN 13537.
Of course their definitions of "comfortable" will be different than yours depending upon how warm/cold you sleep. But at the very least, you'll have an accurate comparison across bags that have been tested against EN 13537.

If a sleeping bag doesn't say it has been tested against EN 13537 (or one of it's other names), you can't be sure what you're getting. If it's a high quality name brand, you'd expect that if they didn't explicitly test against EN 13537, they would have their own rating system that is in the same ball-park... just to maintain a reputation. Lesser known and lower quality companies are much less likely to be close to the EN 13537 rating.

Here's a few articles about the rating system:
https://en.wikipedia.org/wiki/EN_13537
https://thermarestblog.com/en-iso-sleeping-bag-ratings/

But otherwise, the basic answer to your question is that these tests assume the user has a sleeping pad and wearing a base layer (i.e. long underwear).
I would EXPECT (no guarantee) that high quality name brands would have a rating system

[OwenM]

The sleeping bag company marks it at what ever temperature they want...However, there is one rating system that has very well defined legal parameters - EN 13537.
Of course their definitions of "comfortable" will be different than yours depending upon how warm/cold you sleep. But at the very least, you'll have an accurate comparison across bags that have been tested against EN 13537.
And even with the EN tested bags, they're still going to round up or down, since the test can get down to tenths of a degree, but bags are usually rated in 10F increments-though sometimes 5.
My first down bags were a "40F" that EN tested at 42.6, and a "15F" that tested at 10F.
It took time and use in a variety of temperature ranges to see where I stood relative to those ratings, and that's something each person has to find out for themselves.

EE has a good writeup on their site, and something anyone shopping for a bag or quilt for the first time should read:
https://support.enlightenedequipment.com/hc/en-us/articles/360021022052-About-our-temperature-ratings

But look at the variation, even there. Their 20F tested just 11F warmer than their 40F, which is only 6oz lighter...

[MuddyWaters]

I understand this is very subjective but how are they determined? What assumptions are made concerning the dress of the occupant?
Many are simply made up or guessed at
Copying " loft" of other bags that were tested

[Franco]

There are several other variable that are difficult or impossible to enter into those figures.
Humidity is one. At low temps high humidity will make us feel colder than it is, the converse is true at high temps.
If you get into the bag cold , you will need a much warmer bag to work (for the first hour or so at least).
If you don't have a good amount of calories to burn, your body will have a hard time warming you up.
How tired you are and your mood will also come into play, so ...it depends.

[Christoph]

Everyone sleeps different. This is a generalization and may NOT be tailored to you. I'm a hot sleeper, my 32* mummy military issue bag worked more than enough for me. Might not for some though. Best guess is to try it out in the back yard, freezing conditions, in shorts and a T-shirt like I did. I felt pretty comfy form there and went with that.

[Venchka]

The EN testing assumes a mid-20s fit human being.
Western Mountaineering bags sold in Europe are EN tested. My Alpinlite is rated 20 F in the USA and 16 F Lower Limit in the UK. I have slept in the bag at 15 F in the EN prescribed clothing and a R-5.7 pad. In a tent.
EN testing becomes unreliable below 20 F. I have never seen EN ratings for 0 F and below bags.
Bottom Line: The User is the only person who can know what bag rating they need for a given temperature range.
Wayne

[moldy]

It's simple. The rating is determined by what the overnight temperature was when your hiking companions found your frozen dead corpse in the morning.

[blw2]

when doing research a couple years back, I had an "epiphany" of sorts when looking at some information that was published about adjusting ratings for stacking multiple bags/quilts.... all subjective, but it seems to me a pretty good measure based on my limited experience....

based on what temperature you'd set your thermostat at home...or more accurately how you react when the AC is down for power failure or whatever....
Imagine sleeping in standard summer weight underwear (or commando I suppose).. no long johns, no warm PJ's....at what temperature do you feel comfortable kicking off all the covers?
If that number is 70 degrees F, then I feel like the bag ratings would be pretty good for you. so, a 70 degrees F baseline.

For me, in a 70 degree room, I need a blanket. When the room gets up around 80 degrees F, I'll start thinking about kicking off the covers.....Of course humidity, air movement, lots of variables might change it a bit...but ballpark 80 degrees F for me....so a +10 degree delta.
This has always played out for me... a 20 degree bag is sorta ok for me to down around 30....a 40 degree quilt is good for me to down around 50...etc...

...and then adjustments can be made from there based on clothes worn for sleeping....