Why did this work so well?

[larryschwartz]

UPnorth, I do understand the points about the vapor barrier and I wasn't trying to disregard them. I am sure it does play a role in keeping warm, but I also know that often in life, if not always in life, there is more than one part to the answer for any question. I had a few theories about what helped keep me warm and I just wanted to explore all of them.

As for the bivy not breathing, I think that it did. The inside of the bivy and the inside of my sleeping bag did not have any moisture on them that I noticed.

HellsCanyon/Mike, I think that the draft may have been a part of it. I would sleep fine and then wake up cold during the night, so since my SL-5 was not pitched tight to the ground (it is designed to have a little distance for ventilation) I might have woken up after a breeze came through the tent. The canvas cover would have shielded me from the breeze and the resulting loss of insulated warmth.

Thanks for all the input guys, let's keep the discussion going. As Aron noted in his gear thread, let's keep this fact/experience based only please.

Larry

 

[UPnorth]

Larry,

I apologize as misread your initial post...I thought you had your sleeping bag IN your bivy. I stand by my comments about loft and insulation, but agree that if the bivy was inside your sleeping bag, my theory of degradation of the insulation due to moisture is not likely to be correct. In addition, having the bivy right next to your skin would tend to make it obvious if it was trapping too much moisture.

If your bag was exposed to moving air, that changes the discussion entirely.

Good luck getting your system dialed in!

 

[larryschwartz]

Thanks UPnorth. My next test will be with a tyvek bivvy I am making outside of the whole thing.

 

[MOcluck]

I bought a cheap bivy made by ems to protect my bag its supposed to be waterprof does this actually hurt my temp rating? Would I be better off laying on top of it?

 

[Hunt'nFish]

It had a specific brand name type, was very common and popular in England, started with a V if I recall.

I have seen the Snowtrekker tents, they use a light weight tightly woven canvas that they used to sell separately that was popular with home crafters and it turned out to be more popular than their tents so they stopped selling the canvas.

Empire canvas has anoraks and they explain a little bit how they work.

Larry, not just any fabric will work, you need to keep enough warm air in that it stays above the dew point and yet let water vapor pass through it easily. Most synthetics have too much difficulty passing water vapor, even breathable types.

A different approach is vapor barrier clothing and sleeping bag liner. You trap the sweat vapor next to your body and this signals your body to slow down heat generation, this of course only works if you are not exerting yourself a great deal, otherwise you have to vent off the excess heat. By trapping the vapor inside a waterproof layer, this protects your insulation layers and they stay lofty and warm, in fact you need less insulation because your body isn't constantly cooling itself.

I know it seems backwards, but our ability to sweat and keep cool in hot weather works against us when it gets cold. By wicking the sweat away from our skin, it cools us, this makes our body think we are cold and keeps generating more heat to "warm up", thus the runaway sweating that many experience when it's cold out. This is fine as long as we are moving and working, but can be dangerous when we stop. It's also why we feel cold, especially our feet when we have been out in the cold for a few hours, our insulation has become damp and can no longer keep us as warm as when it is dry.

Here is some reading on it to get you started. The third article is the best.

http://sectionhiker.com/vapor_barrier_clothing/

http://sectionhiker.com/vapor-barrier-socks/

http://andrewskurka.com/2011/vapor-barrier-liners-theory-application/

The name of the game of staying warm when it's below freezing is moisture control, no matter how you do it, it's what you need to pay attention to in order to stay warm. This is why wood stoves are so popular, it allows you to dry your gear out each day to stay ahead of the game. Without a stove though, you must come up with other methods.

Interesting thread. Like Mike I too have used manty tarps as bedroll covers and even woke up w/ 6" of fresh snow once out on a ridge.
Although what I want to share is my use of Gore-Tex coated 1000D Cordura as a bedroll cover. Some of you may recall my DIY Cowboy Bedroll cover project from last year, or maybe I didn't share it here. Well I put it to use on a 14day solo elk hunt recently in cold ass condition in a foot of snow. I've used this same bedroll it before w/o the new cover, but this season's elk hunt was the first real test in freezing temps. I can report to you that I saw a HUGE increase in sleep comfort over the same bedroll (two coleman sleep bags one inside another w/ coleman air pad underneath) w/o the cover. I normally have to sleep in my fleece base layer in order to stay warm, but not the case w/ the cover on. I sleep so warm & comfy that I accidently slept through the night a couple times & let the stove go out. After that, I started drinking a fair bit more liquid (Seven & 7) before bed just to wake me up to pee & re-bank stove. I also note that even after many, many nights in the bedroll, the sleeping bags didn't appear to be getting over-loaded w/ moisture. (which was a fear of mine.) I was really impressed.

So I guess what I'm saying is, cotton canvas doesn't appear to be the key here.
I'm on board w/ the micro-climate theory.
Hunt'nFish

 

[Hunt'nFish]

Thanks UPnorth. My next test will be with a tyvek bivvy I am making outside of the whole thing.

Larry, I played around w/ Tyvek for my cowboy bedroll cover.......... doesn't breath.
And, it's really loud/noisey when you roll over or shift position.
I do use it as a ground sheet though, works great for that.
Hunt'nFish

 

[larryschwartz]

Larry, I played around w/ Tyvek for my cowboy bedroll cover.......... doesn't breath.
And, it's really loud/noisey when you roll over or shift position.
I do use it as a ground sheet though, works great for that.
Hunt'nFish

Hunt'nFish,

Thanks for the warning, but you ruined a great excuse to get outdoors for the night! I have found that just crumpling it into a ball will eliminate a great deal of the noise. No need to put it in the washer or dryer to beat it up.

Larry

 

[Hunt'nFish]

Good tip Larry, I didn't do that. I wonder if you do "fatigue it" would it not breath better?
Might still be some hope for a tyvek bivy yet. See, I just gave you your excuse back.


At one time I toyed w/ the idea of sewing a pyramid tent out of the stuff. The noise factor dashed that idea.
So maybe I should experiment w/ fatigued material for a tent.
Hunt'nFish

 

[fillthefreezer]

Good tip Larry, I didn't do that. I wonder if you do "fatigue it" would it not breath better?
Might still be some hope for a tyvek bivy yet. See, I just gave you your excuse back.


At one time I toyed w/ the idea of sewing a pyramid tent out of the stuff. The noise factor dashed that idea.
So maybe I should experiment w/ fatigued material for a tent.
Hunt'nFish

i have a 9x9 square of tyvek i use under my sl5 sometimes. when i first got it, it sounded like i was bending metal roofing handling it. put it through the wash. its 2 season old now and hardly makes a sound...



larry: going off the VBL theory, what if you made a tyvek liner bivy?

 

[larryschwartz]

larry: going off the VBL theory, what if you made a tyvek liner bivy?

FTF,

I don't know if I would like sleeping on the tyvek, if you are talking about using it as a liner INSIDE of the sleeping bag. Like I said, I am really impressed by the SOL Escape Bivvy as a sleeping bag liner to boost the warmth of the sleeping bag, especially with the reflective inner layer. It serves as a vapor barrier too.

Larry

 

[Nick Muche]

What did you use for a pad? Not sure if it was mentioned already....

 

[larryschwartz]

BTW, I have found the most effective way to improve the warmth of a sleeping bag is to make sure that it has a storm collar, or whatever you call a roll of insulation that snugs up around your neck when the bag is closed. This prevents the air that you have warmed up from getting pushed out by the bellows action of the sleeping bag when you move around.

 

[larryschwartz]

What did you use for a pad? Not sure if it was mentioned already....

Nick,

Of late I have been using a Thermorest RidgeRest Solar closed cell pad with the reflective coating on it to reflect heat back. It is .75" thick, I prefer more cushioning for my old bones but it does the job. Sometimes I will use two or add an inflatable if I want more comfort. For the trip I mentioned in the OP I just used the Solar.

Larry

 

[HellsCanyon]

BTW, I have found the most effective way to improve the warmth of a sleeping bag is to make sure that it has a storm collar, or whatever you call a roll of insulation that snugs up around your neck when the bag is closed. This prevents the air that you have warmed up from getting pushed out by the bellows action of the sleeping bag when you move around.

Yep the "draft collar" really helps on my Draft Dodger quilt. I also wear a neck gaitor when sleeping, seems like it really helps to keep me from getting chilled.

Mike

 

[Where's Bruce?]

The canvas cover creates a warm micro climate just like a anorak in arctic conditions. The canvas allows water vapor to escape, but retains warmth.

We sweat when we sleep, this sweat also pulls heat away from our bodies, this is called evaporative cooling. The sweat vapor moves into our sleeping bag which then encounters the cool air near the outer edges and condenses, which in turn wets the insulation and also makes us feel cold in addition to the evaporative cooling from sweating. After a certain point, our bodies can no longer keep up with the heat loss and we feel chilled.

By creating a warmer layer of air outside of the sleeping bag, the canvas keeps the sweat vapor from condensing in our sleeping bag.

This cooling and condensation isn't a problem until temps fall below the dew point.

He's right.

 

[Rizzy]

I have noticed that adding a layer over the top of my sleeping bag does more for me than using a liner or a layer inside the bag. I packed in 2 sleeping bags on a hunt this year, my 0 slick and my off brand 20 down bag. By putting the 20 down inside my 0 slick, I ended up sweating a little but still stayed warm all night with no problems. It only ended up getting down to the low 20s that night, so this was overkill until about 330 AM. On another occasion in similar conditions I draped the extra down sleeping bag over my slick bag and was even warmer but didn't sweat. I'm an even bigger fan of Climashield at this point, it seems to have great breathability

I notice that my knees, toes, chest, and top of my legs feel cool to the touch when I'm cold in my bag. These are also the body parts that are usually in constant contact with the bag and closest to the surface of the bag. I think it's convection that sucks the heat out of your body and transfers it to the surface of the bag, due simply to the temperature gradient between your body and the surface of your bag. It's worse where your body is in contact with the bag because there is no air pocket to slow down the convection. Adding another layer provides that air pocket.

This is an interesting discussion and I think it may rekindle the Down vs. Synthetic dilemma. I have always felt that a down bag seems to hold heat better than a synthetic bag. I'm beginning to speculate that this is due to down being less breathable than synthetic. Water has a high heat capacity......

 

[HellsCanyon]

Larry,
While not 100% related I thought my experience this last weekend was relevant for this thread.

First off where I was camping has a hard freeze every night. Lot of frost and humidity there for some reason... Nighttime temps were between 5-8* F.

First night from the ground up I had a ground sheet, thermarest expedition pad, borah gear M90 bivy with BA Q-core SL pad inside + Marmot 20* "tall" down bag + 20* Down quilt inside the bag. I had no problem staying warm in these temps and I slept in my clothes and a had with mummy hood cinched down tight. I had zero condensation inside the bag/quilt and didn't sweat.

The first morning I woke up and got out of my bivy. I noticed I had a pretty solid layer of frozen condensation BETWEEN my bivy and sleep bag. Obviously my bag/quilt was doing a great job of keeping the heat inside as the condensation was not thawed at all. And while the bivy this summer proved to breath extremely well, I think the fact that it was 6* inside the shelter had that water vapor freezing before it could breath through the M90 fabric of the bivy.

With this in mind the second night I got rid of the bivy and just slept on top of the two pads with my quilt doubled inside my sleeping bag. Zero condensation or frost on the bag when I woke up the next morning.

Its interesting because whenever I've use a canvas manti tarp over a bag I've never had an issue with frost between the bags outer layer and the canvas but the material of the bivy allowed the vapor to freeze before it escaped through the material...

Mike

 

[Mike7]

My experience is similar to others here in cold weather...but I have found that the condensation inside of my Montbell sleeping bag cover/bivy only occurs if I have to sleep with my head inside of my sleeping bag hood. This condensation/ice mostly occurs in my anterior chest/neck area inside of the bivy near where I breathe. This past week I have slept in similarly cold temps (15 degrees) with my head outside of my sleeping bag/bivy while wearing a Katabatic hood and had no condensation whatsoever inside of the bivy (despite a lot of ice inside on the tipi walls).

I am usually in a pyramid pitched up off of the ground with good ventillation and much less condensation compared to a tipi, and seem to have increased warmth in these drafty conditions when using my breathable sleeping bag cover over my sleeping bag.

I wonder if a Tyvek cover wouldn't work just fine even in cold humid weather, as long as it is cinched down around the neck along with the sleeping bag or quilt?

 

[Darren Best]

You do not want a waterproof barrier on the out side of your insulation, you will soak your bag and begin to feel cold.

There are two approaches here, one, stop the vapor from reaching your insulation by using a vapor barrier liner or wear vapor barrier clothing. Campmor sells vapor barrier liners.

Two, let it all breathe the vapor out completely. This is the approach that most people take.

If you stop the vapor from leaving your insulation, then it absorbs it and makes you cold, hence the Tyvek covering your sleeping bag will not work.

The reason the canvas seemed to work so well is because it passes vapor very easily in comparison to a synthetic bivy sack while holding in just enough warmth to keep the bag above dew point.

The variable here is humidity, not temperature.

It still comes down to moisture control, not temperature control.

Here is an example, last summer the troop took a week long trip to a Boy Scout camp literally right on the Pacific Ocean in Oregon. My tent was but a scant 200 yards off of the beach. My usual summer bag is a very old Slumberjack canvas bag that is very roomy but not really warm for cold temps. However I knew that the coast would be wet so I brought my Kifaru Slick bag. So the challenge here is keeping the bag dry enough that I would stay warm at night and I was right. The air was literally saturated with moisture 24/7. There is a constant mist/fog that hangs over everything and the air itself is literally soggy. Had I brought my usual summer bag I would of been cold at night because I would of been unable to dry the bag during the day. Out of the week that we were there I think I only saw sunshine one and half days. This is where a wood stove comes in so nicely, you can dry your gear out with the stove and keep going.

Why do down bags work so well in high altitude environments, because the humidity is so low in the extreme cold temperatures.

Moisture control gents, learn it, love it, live it.

 

[MarshLaw303]

Im getting to the party late so I speed read the replies so please forgive me if i'm repeating what has been suggested.

I couldn't find anywhere where you stated what shelter you were in. Since you were inside a vapor proof bivy i don't think moisture in the bag was a problem since it should have been trapped in the bivy with you where it belongs. If you're shelter wasn't blocking wind well that explains everything to me. The canvas cut the wind which was stealing heat out of your bag. Some fabrics are too breathable and wind can easily rob you of stored heat. Adding a layer over top that cuts the wind can greatly increase the heat that is retained. This was stated in some of the posts i read but I don't think the concept was explained, just that adding a cover made it warmer, this is why. If your shelter was blocking the wind then I'm obviously barking up the wrong tree here.

-Tim