Furthest below your bag's rating?

[DougPaul]

DougPaul, can you explain more about this "frost point" and how it's affected by the different types of bags. My winter bag is a huge old down bag, weighs 5+ pound, from Eddie Bauer. Tons of down, about 25 years old.

The frost point is actually a surface...

Let's assume the outside temp is 0F. You are at 98.6F and evaporating moisture off your skin. As the moisture diffuses outward through your bag, the temp drops until it reaches freezing. Beyond this point (actually surface), the moisture can freeze in the down. Frozen down loses its loft and after several nights, the bag will have significantly less loft than it started with. (The insulation of the bag is primarily related to its loft (thickness).)

Polyester bag fills are significantly less affected by moisture and frost.

A traditional method of dealling with the moisture in down is to dry (air and sun) the bag whenever possible. Another method is the vapor barrier liner (Just like a vapor barrier on a house, it must be on the warm side of the insulation to work.) The VBL blocks the moisture at its source preventing it from accumulating in your insulation and now your body heat actually helps to keep the bag dry. The VBL also reduces the heat lost to moisture evaporation from the skin.

The same logic applies to VBLs in all forms: clothing, socks, etc--they reduce evaporative cooling and keep your insulation dry.

Doug

 

[BorealChickadee]

Doug- Thanks for explaining it so well. That's a huge help and yes in my ignorance I thought it was a temperature point, not a surface.

I love down, sleeping bags, jackets etc so this is good information. When we go out in the winter cross country skiing or snowshoeing sometimes we get sweaty. I am more aware of it and take off my coat but my son sometimes lets the inside of his coat get wet from the moisture given off when he sweats. I was considering having him wear a windbrealer inside of his down so this wouldn't happen. Makes sense, it would be a vapor barrier.

 

[DougPaul]

DougI love down, sleeping bags, jackets etc so this is good information. When we go out in the winter cross country skiing or snowshoeing sometimes we get sweaty. I am more aware of it and take off my coat but my son sometimes lets the inside of his coat get wet from the moisture given off when he sweats. I was considering having him wear a windbrealer inside of his down so this wouldn't happen. Makes sense, it would be a vapor barrier.

You don't have to sweat to emit moisture from your skin. The body always tries to keep its skin moist (to keep the skin soft and flexible) with the net effect that it always keeps the humidity right next to the skin rather high (~97% humidity at ~98.6F, IIRC). So moisture is _always_ diffusing through your insulation unless you are using a VBL. Obviously worse if you are sweating, but it never stops.

Using a VBL requres more concious control of thermal balance since sweating can no longer cool you.

Don't know how old your son is, but adjusting one's clothing to stay just slightly cool requires dicipline. Modern moisture tolerant clothing (moisture wicking polyester underwear, fleece, and nylon shells) is much better than many older materials at dealing with the moisture.

I have seen people insisting on keeping an expedition down parka on while sweat is dripping from their foreheads... Our party ended up having to rescue such a group in one case...

Doug

 

[Neil]

Using a vapour barrier also adds some R factor due to the dead air space between the barrier and the inside of the bag. Another advantage to having 2 bags is that depending on the combination of bags and liners you use you can cover a wide range of temps more economically. As mentioned, the bulk and weight are cons.

Re: sleeping pads. In my previous post we only had skimpy closed cell jobs but we put every bit of spare clothing we had underneath us to help keep us warm. It remained dry.
Sleeping on snow must be a lot warmer than a lean-to floor.

Sleeping in a quinzee is toasty. (measured on one trip: neg 40 outdoors, plus 30 indoors) Lots of work but worth it for multiple nights. It can get humid in there so poking lots of holes is a good idea. Claustraphobic people (I've got a funny story...)are better off with another solution. We would dig right down to the ground because we read somewhere about geothermal heat being able to boil one cup of water in 5 mins. per square meter. DougPaul, what do think about that?

About the heat given off by digestion, the easiest is simply to eat a big meal of anything just before turning in and not worry about protein vs. carbs. vs. fat. We did the meat thing because it worked real well for us (nothing like a humungous steak freshly grilled after a long day outdoors in winter!) and that gave us lots of fat, too. (DouPaul, now I'm curious as to which bonds release the most delta G when broken: amino, fat- I think fat is simply stored away with little breakdown- or carb. I'll be back quick with an answer: my winter camping pardner teaches biochem at U of Mtl)

Is summer ending? When winter subjects start cropping up it makes me think so.

 

[DougPaul]

Using a vapour barrier also adds some R factor due to the dead air space between the barrier and the inside of the bag.

Probably only a small amount compared to the bag--free convection in the gap.

Re: sleeping pads. In my previous post we only had skimpy closed cell jobs but we put every bit of spare clothing we had underneath us to help keep us warm. It remained dry. Sleeping on snow must be a lot warmer than a lean-to floor.

Snow is a good insulator as long as you don't melt it--lots of airspaces. But after a few days sleeping in the same spot, it turns into hardpack/ice (loses the airspaces) which does not insulate nearly as well.

Sleeping in a quinzee is toasty. (measured on one trip: neg 40 outdoors, plus 30 indoors) Lots of work but worth it for multiple nights. It can get humid in there so poking lots of holes is a good idea. Claustraphobic people (I've got a funny story...)are better off with another solution. We would dig right down to the ground because we read somewhere about geothermal heat being able to boil one cup of water in 5 mins. per square meter. DougPaul, what do think about that?

Depending on conditions, the ground can be frozen or not. Ground also conducts heat moderately well. I'd guess that sleeping on snow is warmer than frozen ground, don't know about hardpack snow vs frozen ground.

Sleeping in a shelter or hammock can be the coldest (or coolest in summer) because you have to deal with the ambient temp both above and below you. My earlier quoted instance of being cool in a -20F bag at a temp of -5F was because of the cold air under a marginal (for the conditions) pad.

About the heat given off by digestion, the easiest is simply to eat a big meal of anything just before turning in and not worry about protein vs. carbs. vs. fat. We did the meat thing because it worked real well for us (nothing like a humungous steak freshly grilled after a long day outdoors in winter!) and that gave us lots of fat, too. (DouPaul, now I'm curious as to which bonds release the most delta G when broken: amino, fat- I think fat is simply stored away with little breakdown- or carb. I'll be back quick with an answer: my winter camping pardner teaches biochem at U of Mtl)

I'm not a nutritionist. I learned about the approx digestion times in winter school many moons ago... IIRC, fat is first broken down into glucose then reconverted to fat for storage. (Fat is also poorly digested at high altitudes (>20K ft) because it requires more oxygen to digest than does carbs.)

Is summer ending? When winter subjects start cropping up it makes me think so.

Why wait? I spend 9 months of the year waiting for the snow to return...

Doug

 

[Ken999]

Thanks Doug...I kinda figured as much but have kept an eye open for someone to comment on such a combo. All up I'd have a real flexible setup that could handle most winter conditions for less than 4.5 lbs...cool.

 

[truepatriot09]

Hope I'm not being redundant, I breezed through the thread pretty quickly, but when winter camping we've found that filling a Nalgene with near boiling water and putting it at the foot of your bag helps turn on the furnace at bedtime as well as giving you an unfrozen bottle in the morning...

 

[jfb]

I kinda figured as much but have kept an eye open for someone to comment on such a combo.

I've also used a combination bag for winter camping. In my case, it was two down bags that were designed to nest together. The inner bag was sewn through construction with 3 in. loft (2 1/2 lbs., rated to 40 F) and the outer bag had slant baffles with 6 in. loft (3 1/2 lbs., rated to 15 F). Together they were rated to -20 F. They were barrel-shaped with full zippers that could be zipped together and sleep two. Unfortunately neither bag had a hood, but I added one (back when EMS sold them) to the outer bag and it worked fairly well.

In all the years I owned the bags they never seemed to be quite as warm as the ratings, but at temps. about 15 degrees above their ratings they were comfortable. As far as the vapor barrier liners go, I've never tried one so can't comment on how effective they are.

 

[Capoken]

me too

I too, find myself a little chilly whenever I am at a bags comfort rating. If its zero degrees and I'm in my zero bag, I'm a little cool. Same thing with my 32 degree bag at 32 degrees. Some people just "sleep cold" and I guess we fit in that category. I guess it has to do with metabalism, %body fat, and a lot of other factors that are tough to quantify.

 

[jfb]

I guess it has to do with metabalism, %body fat, and a lot of other factors that are tough to quantify.

Some of those other factors have to do with the construction of the bags. A good winter bag will have differential cut, mummy shape, good footbox, draft tube behind the zipper, draft collar with drawstring, a baffle that runs the length of the bag opposite the zipper to reduce down shifting and generous hood with drawstring.

 

[Warren]

I've taken my 15° bag down to a measured 4° at bed time. At some point during the night I need to put a jacket on- I assume it was colder then.

Coldest I've been was was first year out, usiing an old Campmor synthetic bag at the old Denning lean to in late October. I was damp, it was cold and the bag was on it's last (20°?) legs

 

[SAR-EMT40]

I have taken my -10 bag down to a measured -25 more than once. I did wear most of what I brought though. I was comfortable enough. I tend to sleep warm though.

Keith