How much does sleeping bag volume affect warmth?

Generally speaking, a wider bag means more air space inside that your body must heat, so all things being equal should feel a little less warm. But…”all things being equal”, they’re not. You have to factor in loft, baffle height, construction, length, many other factors.

Do you fluff up your bag?  Shake and beat at it.  That might make it a little warmer.  After a while the down can become matted.

Maybe you need to clean it?  If it becomes oily it can lose warmth.

It doesn’t seem like one would notice 3 or 5 F difference in a bag, that’s such a small difference.

When you get in your bag is it so narrow there’s no extra room inside?

If there’s extra room inside, you could wear a vest or jacket and get a lot more than 5 F increase in warmth.

+ 1 to the comments above.

Are you thinking you need a warmer bag for winter camping or are you getting cold in this bag during summer backpacking?

If the latter, you probably need more insulation under you. If the former, you want a bag big enough that you can add clothing layers like puffy jacket,

“If the latter, you probably need more insulation under you. If the former, you want a bag big enough that you can add clothing layers like puffy jacket,”

+1

What pad do you use? If you are looking for a 5 degree increase, that can be accomplished with more insulation underneath and a down vest/jacket if need be. Even a silk liner can add 5 degrees.

But back to the pad: sleeping pads tend to be overlooked and adding more ground insulation can make a big difference, even in mild weather. It doesn’t help that most air mattresses have unreliable R value ratings, or don’t have any at all.

(In the mid-Atlantic, I always bring one or two 1/8″ ccf pads to supplement my air mattress & I’ve always slept comfortably with a 40 degree quilt for 6-8 months out of the year. But I always bring a down vest/jacket just in case.)

Thanks for the responses. I use the Apache for summer backpacking in the CO mountains, where it’s occasionally in the 20s. The bag is narrow but so am I, so there’s plenty of room for clothing. I wear all my clothing except rain gear to bed, including fleece pants and down puffy with hood, plus a double-layer hat, gloves, etc. I run in place before bed so I can get into it warm, stay hydrated and well-fed, never sleep near streams, cinch the hood and draft collar, etc. I’m just a hopelessly cold person (and probably will never be able to go truly ultralight because of this… well, maybe once menopause hits!).

I’ve only used it maybe 25 nights so hopefully it doesn’t need to be washed yet (especially since I always wear clothes in it, and rarely sweat). I’ve tried moving all the down to the top of the bag, which does make a difference, but since I flop around during the night it’s not a super solution… just too hard to keep the down centered on top of me. I do try to take the bag out of its sack as soon as I get to camp so it can fluff up, but I haven’t tried hand-fluffing it. I guess I could also try wearing the rain gear to bed on top of everything else, I’m just concerned about the consequences if I do start to sweat.

I’m using a NeoAir XLite. I’ve considered switching to the XTherm, but there’s a small (3oz) weight penalty with that. Switching to a lower-temp sleeping bag (either WM Versalite or FF Lark UL) could be done with no weight penalty.

Do you think the XTherm (or adding a 1/8″ CCF pad) would provide more warmth than a sleeping bag 5 degrees warmer?

Switching to a lower-temp sleeping bag (either WM Versalite or FF Lark UL) could be done with no weight penalty

The manufacturer’s warmth rating is not as important as inches of loft (other variables being equal), IMHO. As for sleeping pad, yes, it makes a big difference and that XTherm will definitely get you and extra 5 degrees of warmth (or more).

probably will never be able to go truly ultralight because of this

You never know. If you have a 9 lb base weight with your current bag, you could add a full-sized 50F synthetic overquilt to your gear and get at least another 15 degrees of warmth for less than a pound. That still puts you under 10 lbs total weight. And add in an X-therm for 3 ounces and good measure and sleep warm and comfy.

“I’m using a NeoAir XLite”

On it’s own, I don’t believe it’s adequate whatsoever. There have been many folks over the years that have bitterly complained about the XLite’s apparent lack of insulation. Simply adding a 16 dollar 1/8″ Gossamer Gear Thinlight pad will make a measurable difference on your comfort (imho.)

While I haven’t used it, I understand the X-Therm is much “warmer” than the X-lite, according to many users out there.  Personally, I’m still a fan of sandwiching my current air mattress with ccf foam. No heavier, more versatility, and some good redundancy.

In the winter, I use a more robust system, but I still use ccf pads.

+1 to adding a thinlight on top of your xlite.  It’s an easy fix if it works.  Some people are just cold though.  If the thinlight doesn’t work, just get a warmer bag.

OK, sounds like a plan. The Thinlight is $22 with shipping, and I realized I can probably replace the back pad in my Mariposa with it, meaning minimal weight penalty. And as Ben says, if I am still cold I can then get a warmer bag (or try the XTherm).

Thanks everyone for helping me think this through.

The X Lite has an R3.2 rating.

The X Therm is at R 5.7.

You will gain much more by switching mat than by adding an ounce or two of down.

To give you a better idea , I would not use a 3.2 mat (by itself)  below 32 f or so.

X-Therm should fix your problem.  Otherwise I might send your bag back to WM for some overfill.

In addition to all the good advice above, if you’re cold, get a bag that’s 20 F warmer.  If you get one that’s 5 F warmer you’ll hardly notice it and you’ll still be cold.

The temperature rating of bags is not very good absolute – some people are colder, some warmer.  But the rating is pretty good to compare bags or if you get a bag that’s 20 F warmer, for example, it will be good for an actual 20 F colder temp (maybe 15 F just to be conservative).

This guy was a bit cold , so he put a nice down jacket on top.

but he was still a bit cold.

I wonder why ?

For people that doubt the difference a mat makes, here is a simple and free experiment you can do at home.

Put the mat on the floor and get inside the sleeping bag. Wait for an hour or so.

Now get on your bed and sleep inside the bag but on top of the bed.

Notice any difference in temps ?

The thing is

A larger bag has more surface area for heat loss, than a slimmer bag. At same loft, it will lose more heat, roughly in proportion to this increase in area. Its not really that you have to heat more air…air has low heat capacity, its minimal heat to do this, but that that air loses heat thru the additional area.   Also, stagnant areas get pretty darn cold with body heated air not making it to them.  Really cold.

My wife usually sleeps colder than I do.

But she is warm in a bag that is cold for me.

How can this be?  I gave her my Feathered Friends bag which is too small for me but just right for her.

When I sleep in that same bag I’m pushing against the sides and creating cold spots.

Size can matter.

I sometimes have trouble warming up the sleeping bag, especially if I get into it with a jacket on. I have to take the down jacket off in order to warm up the sleeping bag. If it is a cold night there is room for it in the sleeping bag next to me. I can put it on later in the night as temperatures drop.

I purchased a pair of possum down sleep socks from ZPacks. I don’t always carry them, but I find that if my feet are warm, I am warm.

The comments about the pads are right on.

Jo, I think you need to upgrade your sleeping pad.  I use the XTherm, also in CO and places north, and I swear I can feel the thing radiating heat back up to me.  In fact, this pad allows me to sometimes bring a lighter bag (25 or even 40 degrees, if forecast is for fair weather).  The better sleep you get will be well worth the extra few ounces of weight a better-insulated pad will cost you.  Hope you find a good solution.

Add a thin pad and, if you want to add a bit more flexibility to your sleeping system, get a good sleeping bag liner or an overbag.

The one below is expensive, but will add a few ounces to your system:

https://www.ultralightoutdoorgear.co.uk/equipment-c3/sleeping-bags-c21/sleeping-bags-c88/protium-125-down-liner-sleeping-bag-p9620

i live in the us and bought from here and never had a problem.

I got the next version up (with 200 grams of the best quality down available ) and is incredible, quality wise and light. It works as a summer bag too.

They have plastic toggles to connect to a Crux Torpedo bag, that you can cut to save weight.

i am 5 f 7 inches and the medium is tight, but they have a large option.

Similarly to what Erica mentioned…I have found that I am often warmer if, rather that wearing my down jacket, I use it like a mini-quilt.  I have found this to be true both inside a bag and under a quilt.  I have nothing to offer except a guess as to why this is, but suspect it is associated with all of the jacket being above and to the side of me, completely uncompressed, rather than having whatever part of the jacket is under me being compressed and thus, relatively useless as insulation.

completely uncompressed
In a nutshell, yes.

Cheers

Another vote for the Xtherm. They aren’t super comfy, but if you already like the X-Lite, the Xtherm is a big step up in regards to warmth.

Yes, sleeping bag volume does effect warmth. Too small will mean you will compress it when zipped. Soo, you end up colder. Too large means you have extra space to warm up and keep warm, you end up colder. But that is not the whole picture. There are many more variables, like pads, which looks like it has been pretty thoroughly discussed. Some one already said baffling, and style of cuts (differential cut as opposed to flat cut,) differential fills (more on top than on the bottom,) type of shell material (not just the lightest,) water/moisture resistance of the shell & down, and, the actual thermal conductance of the material used as fill. Oftentimes, this is offset with the desire to go light, but, in cool to cold weather, heavy may be your only option. The thermal differential also plays it’s part by varying between say a differential of 40F at 100% humidity and 40F at 10% humidity, a 30F temperature just ain’t always the same animal as 30F sometimes.

You cannot ignore loft and space in a bag, but it only accounts for a debatable 30% of a bags warmth in my estimation. A good down fill vs. a good synthetic fill plays a more important part, then pad and shell & material composition, size and design all play their parts.