Thermodynamics with down

"I'm trying to understand why it might be true that less clothing worn in a sleeping bag helps you stay warmer."

It does? I'm not sure that I believe that's true. When do you find that it happens?

I think the more clothing you put on inside your sleeping bag, the warmer you will be

If it's tight, adding more clothing will add progressively less warmth. If you want to get the most warmth for the weight, you want enough space inside your sleeping bag for insulation to fully loft.

Same thing – if you add pad in underquilt, it will make you warmer, but I'm no hammock user.

Theoretically, if it was really tight you might impede blood flow or something, but I don't think you'll ever experience that.

what kind of a sleeping pad?
When I used hammocks we didn't use an underquilt at all in winter.
We used to use a sleeping pad (closed-cell) like the large 4 foot wide ones from Gossamergear and we put it in the hammock and it came up around us blocking the wind and let the sleeping bag do it's job.

"I'm trying to understand why it might be true that less clothing worn in a sleeping bag helps you stay warmer"

That depends on how your body fills the bag. If the bag is to big for you, the air pockets will be moved around when you move around and you will feel the drafts. This is very typical for small women (and men) in "standard" sized bags. In that case extra clothes will control the air flow, and be in intimate contact with your body. (a better solution is an appropriately sized bag.) Similarly, people with cold feet may gain comfort by pushing a fleece or down vest into the foot box to take up excess volume and gain intimate contact with a conforming insulator.

You will get the best results if the bag hugs your (near-naked) body, and is fully lofted (not compressed at the hips, elbows, etc. It that case, adding clothing inside May compress the down and reduce its effectiveness. So in this case "less is better".

"… that increases in temperature and radiates that heat back to you …"

Nothing, including down, can accumulate more heat energy that it receives, plus it is constantly loosing heat to the outside. And while it will radiate heat, the Net result is that it will gain and you will loose. It is close to your body temperature, so the loss to the down is small, which makes that spot feel "less cold".

"I'm also trying to understand why I'm colder in my hammock…"

I don't hang, so adjust accordingly ….

I think the QU/pad issue comes down to "intimate contact" with you, versus air gaps between the pad and the UQ, and the pad and you, that promote convection. Anything you do that creates pockets of moving air works against staying warm.

Steve,

When I first got into backpacking, I recall reading in Backpacker about the "sleep naked" theory to sleep warm. I think it went by the wayside years ago, though.

I get warmer with more clothes. Many use a puffy jacket and pants to boost a bag's rating, too. This CAN be a problem when loft, anywhere in the bag, gets compromised.

That said, I firmly believe too many clothes, worn too early in the night(ie: before you need them) makes you sweat a bit, resulting in ultimately getting colder. That's been my experience anyway.

Todd

There are insulations and there are insulations. Down is far better than synthetics but it can be worse. When it is wet for example. You don't change the down, you simply remove the loft.

Some things ae far better insulators. But, they weigh a lot more. Cotton for example. For a half inch of thickness you will likely overheat at 20F. But it weighs a heck of a lot more than down. And, it doesn't perform well when wet. Wool pwrforms well when wet but it weighs a lot more than down or cotton. Synthetics really shine when wet because they don't loose as much loft. Ceramic insulations are light and very insulative, but not very flexible. So, there is a trade off between a high insulation value, flexiblity, water resistance, and weight.

In a bag, I know that adding cloths helps up to a point. Once you start compressing down, you loose as much insulation as you gain, sometimes more. Sometimes less clothing in a bag is warmer. As far as what is under you, this is pretty much conductive. The most insulative layer would do better than a less insulative pad directly under you. But you have to figure what loft is loss due to laying on it.

Well, its a 20 degree UQ being used with a Neo Air Trekker – not my setup. I advised the guy he was better off with just the UQ, but more than a few folks are saying that the pad should add to the overall insulation. One even calculated the effective r-value to be -3 degrees. I think that is bunk, but what do I know? (other than my experience)

OK, so why is less better? Compression of the down aside – why not wear a bunch of clothes? I, too, believe a form fitting, uncompressed bag is better. Of course, I hammock, so for me its about an UQ devoid of gaps, and a well draped, cinched and tucked top quilt.

Again, its my experience that less is more – figuring there is something more to the insulation equation. Could it be that breathability has an equilibrium enhancing effect, separately from what is going on with regard to heat loss?

"OK, so why is less better? Compression of the down aside – why not wear a bunch of clothes?"

Clothes can be an excellent supplement to a "light" sleeping bag, but not necessarily the most efficient use of the weight you are carrying. A well made sleeping bag will be a more "weight efficient" system for warmth than the small amount of down with the larger amount of fabric afforded by adding puffy pants and jacket to a lighter bag.

But your hiking style affects things. If you finish your day then get directly into your bag to eat and then go to sleep, an efficient bag is the way to go. If you finish your day, sit around to watch the sunset, have dinner on a rock, read a book, then you will probably want to take puffy pants and jacket for "camp warmth". In that case, going with a lighter bag and supplementing with the puffies you already have makes sense.

Better depends on the person and the situation.

For what its worth, I'm not asking about how to stay warm – I already know that. Been doing this for a long time, first with sleeping bags, now with under and top quilts – and with or without pads along the way. I'm either too enamored with the idea that down insulation needs to "heat up" to be more efficient – which I have to allow could be the case – or else there is an element missing in the equation of insulation that is not being widely accounted for.

Physics and thermodynamics don't step aside for backpackers. More clothes will always keep you warmer until/unless you compress the compressible ones to the point of detriment or the clothes are damp. The myth that you sleep warmer naked, I believe, stems from people's tendency to sleep in the same clothes they hiked in. Damp clothes make you cold, and your hiking clothes are almost always damp, the waistband of your undies if nothing else.

Yes, it's a myth.

I've even gotten in my bag with very damp clothes. As long as it's early in the evening (and not too cold yet), the air is dry, and you are using a super ventilated shelter like an a-frame flat tarp, you will dry out just fine.

Would be some gall to suggest that the laws of physics were somehow bent around backpacking, or anything else for that matter. No, just suggesting that our minds are imperfect in their understanding of it all. While I've probably failed more calculus and physics than most people take in their lifetime, I should still have enough going on to handle some technical details. I don't reject those details as much as I'm saying the story in incomplete. Just trying to align what imperfect minds are telling me about physics with my imperfect perception of my own experiences which tells me there is more physics going on than just "more is better".

Could be a myth, except it has worked for some, including me. And not like some shaky video.

Yes, sorry, your original explanation is bunk. Down doesn't need to heat up to work. I would argue the opposite point, once the down is warm there will be more convective heat transfer in the air between the down plumes due to buoyancy effects, but that's probably such a small effect that it is not even measurable. Adding more insulation (be it a pad or more clothing) will always make you warmer, absent other effects like evaporation or that fleece you just put on compressing your down sleeping bag. Keep in mind humans are pretty bad at evaluating heat fluxes, it might feel warmer to get in the bag naked because the bag fabric will heat up very quickly and no longer feel cold versus putting on a cotton hoody before you get in the bag that will take awhile to warm up but ultimately keep you warmer.

Trust me, getting in a bag semi-naked is colder. Would much rather get in with a lot of clothes on and wait to get warmed by my body heat being lost less. Unfortunately, that hasn't been happening. So, having read about "sleeping naked" – a misnomer as its more like one baselayer vs multiple layers of clothing – I tried it and it seemed to work.

I guess you all are figuring out by now that I'm not buying the physics, or better put, the set of natural elements as described. Again, not argueing against physics as much as saying something else seems to be at work when it comes to down insulation.

I appreciate all the explanations, but so far, its nothing I haven't heard before. Hasn't been enough to get me out of my comfortable bunk… or, eh, quilts.

p.s. One person previously argued "you wouldn't be warmer in your down jacket without any clothes on?", which is funny, because that is exactly what got me started with my line of thinking. Why am I actually warmer in my down jacket without clothes on? (and I am)

I think the rational can be compared with why mitts are always going to be warmer than gloves. By reducing the amount of clothing whilst sleeping you allow the body to naturally disperse heat inside the 'mitt' of the sleeping bag. Adding more layers isolates certain heat-generating regions from colder regions. i.e. that nice heat wave from your crotch, armpits, and core is suddenly trapped under your Marmot Zeus down sweater instead of being shared with your extremities.

That's exactly what I'm trying to get at. We'll see what others have to say, I've not seen much support yet. I'm bothered by the reluctance of others to account for what accumulated heat is doing inside the volume of the down. Maybe I'm just bothersome. I wish I were a technical person who could explain my viewpoint better, but the mitt example is a good one.

Well, Steve, it's the optimal solution but in the field it's a rather annoying 'best practice' to maintain. Imagine -10F and having to strip your layers in order to "maximize thermal efficiency." You'll be shivering in that cold bag until it warms up… and then struggle in the morning to get dressed in that cocoon. There are tricks – such as doing a bunch of exercises prior to entering a cold sleeping bag, and using hot water nalgenes to warm your body (and the loft) – but many lightweight alpinists (ueli steck comes to mind) will simply wear all their gear (boots included) and plunge into a SUL bag, having calculated their day clothing + the bag = acceptable level of warmth. It's kind of a ying/yang and entirely dependent on how comfortable you are with being uncomfortable at the beginning and end of the night :)

Ever tried wearing a liner inside a mitt? Hint, it's warmer…

A mitt works because there's less surface area through which the heat can "escape", a better comparison would be to wear a down suit with arms and legs compared to a sleeping bag.

For extream cold that probably works. Anything much less than 10F to 20F will, by definition, be dry. Water freezes at about 32F, give or take (a lot of variables, altitude, meineral content of the water, etc.) Even your sweat can freeze in a bag's fill, leaving it damp, and not very effective on the second or third night. But, for one or two nights, it is fine.

It is the inbetween temps that moisture is a big problem. Your bag is typically lofted with air. Stopping convetion in a bag, or your clothing, and still letting moisture escape always seems to be a problem. For instance, the old tent/bag design didn't work. There was simply too much air to be warmed. And, the heat loss from the larger surface area was too much. I think Stephensons tried that in the late 50's or early 60's.

Removing the lofting of the down will always mean it will not hold much heat. Not when a typical 900fill 6" bag is wet. It will compress to a 1" sodden mess. This is not even counting the greater conduction of the water towards heat loss.

Anyway, mostly, good points and interesting reading.

The reason gloves are warmer than mittens is less surface area

In mittens, the heat doesn't leak sideways out the fingers like a glove does

Yeah, and the air is warmed inside the mitt, which means more blood flow in our digits, which means air is further warmed in the mitt. If air is an insulator (the best?), then there is air in between the skin and surface area of the mitt. No air in a glove except for in the insulation which is going to have more conductive properties than just air. While I get the heat loss argument of why we feel cold, it sure seems like it ignores an awful lot, and therefore, is oversimplistic.

I know that we release a lot of moisture and that some of this will be leftover in our bag or quilt. I haven't found it to be a lot – maybe not as much as could be expected – and wondering if there isn't a drying effect that down has which 1) helps to eliminate the moisture instead of accumulating it, and 2) has a drying effect at skin level which has a beneficial effect with regard to heat loss?