Why did this work so well?

[larryschwartz]

Let me start this off by saying that i am looking for actual experience here, not guessing as to the reason what I describe below worked so well.

I spent a week camping and hunting in the Appalachians last month and it got down into the 30's most nights and maybe into the 20's some nights. I used a TNF 20* Cats Meow bag with a SOL Escape Bivy inside of it and slept OK but did wake up a few times each night from the cold. The last night (the coldest night per the forecast) a friend loaned me a canvas bedroll/bivy cover that I laid over my bag and slept like a baby.

So, I am wondering why that canvas cover made so much of a difference...

~ Did the thickness of the canvas cover trap more of what little heat escaped from my Escape bivy and sleeping bag?

~ Did the weight of the canvas laying on top of the sleeping bag reduce the amount of air inside of the bag and therefore the air warmed up faster and stayed warm longer?

~ Does having a bag/cover of some sort enclosing the sleeping bag make that much of a difference in warmth retention? Does it have to be heavy or would a light bivy, of tyvek maybe, work as well?

~ What was it about that canvas cover that made the difference?

Thanks to all.

Larry

 

[larryschwartz]

BTW, this is the SOL Escape Bivy. It has a mummy style drawstring hood, has a reflective inside, and the fabric is breathable. It will always be in my ovenight kit from now on and will replace a space blanket as my emergency sleeping gear. A great way to extend the effectiveness of a sleeping bag, especially if you weigh 200# or less.

http://www.adventuremedicalkits.com/product.php?product=263

 

[mdog]

Last hunt I took a 30 degree MontBell bag and my wife had a 15 degreeMontBell. The night time temps hit 20 as a low and she got cold. I used a really old bivy-goretex top and coated nylon bottom- and slept really warm...even with a condensation issue at the foot of the bag. I've heard that a bivy,used outside/over your sleeping bag, will lower your bag rating another 5-10 degrees.

 

[Darren Best]

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.

 

[larryschwartz]

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

Elkhunter, so you are saying that it wasn't the weight of the canvas bedroll covering the sleeping bag that for whatever reason made for a warmer night but the additional air pocket outside of my sleeping bag that allowed the moisture to evaporate while also trapping the escaping heat in another air pocket...yes?

So, just having a lightweight cover over the sleeping bag would have done the same thing...yes?

Larry

 

[HellsCanyon]

I know one thing, I've used canvas "manti" tarps from my outfitter days as a "bivy" and have slept in some really cold weather and woken up just fine. Even woken up with snow on top of it and stayed warm. They're usually cut from old wornout canvas wall-tents. Heavy as hell but they keep the moisture out for the most part and really upped the temperature retention.

Mike

 

[larryschwartz]

I know one thing, I've used canvas "manti" tarps from my outfitter days as a "bivy" and have slept in some really cold weather and woken up just fine. Even woken up with snow on top of it and stayed warm. They're usually cut from old wornout canvas wall-tents. Heavy as hell but they keep the moisture out for the most part and really upped the temperature retention.

Mike

Mike, exactly!!! I am just trying to find out WHY it really upped the temperature retention. Was it the thickness of the fabric, how it pushed down on the sleeping bag to keep less air in it that needed to be kept warm? I don't know but want to find out, which is why I started this thread. I just don't want to have to carry an additional 5#s just to get that nice warm feeling again.

 

[HellsCanyon]

I suspect it's a big part of the material used. We all know that properly treated wall tents will resist snow and rain for a long time before leaking through. They also do a great job at holding in the heat from a wood stove! I think the canvas acts as a breathable high insulating outer layer. Cuts the wind great as well. There's a reason it's been used for shelters forever.

Mike

 

[Darren Best]

The canvas creates a micro climate when used as a bivy sack that keeps the air next to your sleeping bag just warm enough that it's still above the dew point and your sweat is still vapor until it passes through the canvas, thus keeping your sleeping bag dry.

If you look at early arctic explorers and mountain climbers they wore canvas anoraks and pants for this reason, it actually worked really good. The garments had to be baggy in order to work though. But this only worked if the canvas stayed dry, hence the reason it was used in sub freezing applications.

The canvas when used for a wall tent works a little differently, the fibers swell when wet and make it water proof to some extent. Older better quality canvas that had a tighter weave was better at this, the new stuff is so porous it needs chemical treatment to be waterproof, but it doesn't work in the same way. There is a canvas made in England that has really tight weave that is used for rain coats that is very water proof, I can't remember the name of it offhand.

 

[fillthefreezer]

The canvas when used for a wall tent works a little differently, the fibers swell when wet and make it water proof to some extent. Older better quality canvas that had a tighter weave was better at this, the new stuff is so porous it needs chemical treatment to be waterproof, but it doesn't work in the same way. There is a canvas made in England that has really tight weave that is used for rain coats that is very water proof, I can't remember the name of it offhand.

i believe youre trying to think of sail canvas.
wintertrekker still builds some tents using the closest thing to the old style finely woven canvas ive heard...

 

[Darren Best]

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.

 

[larryschwartz]

OK, so we have established the idea of the top layer creating a micro-climate that adds another layer of insulation which could have increased the warmth on my last night with the canvas bedroll laying over top of my sleeping bag.

Does anyone have any experience that would point to its weight pushing out the "excess" air inside of my sleeping bag and therefore making it easier to heat less air being the reason I slept warmer?

Larry

 

[Nick Muche]

You may have just been more tired that last night.

 

[larryschwartz]

Nope, that wasn't it.

 

[Buckman]

Larry just a thought I had. Did your bag have frost on it the nights you didn't use the canvas? If it did and didn't when you used the canvas, maybe the frost didn't let the bag breath. this may make your sweat make you cold. Just a thought, prolly wrong.

 

[UPnorth]

OK, so we have established the idea of the top layer creating a micro-climate that adds another layer of insulation which could have increased the warmth on my last night with the canvas bedroll laying over top of my sleeping bag.

Does anyone have any experience that would point to its weight pushing out the "excess" air inside of my sleeping bag and therefore making it easier to heat less air being the reason I slept warmer?

Larry

It sounds to me like you aren't getting the key takeaway from Elkhunter's posts, which, at least in my opinion, is moisture management. (not "micro-climates") You absolutely want the "extra air" trapped in your sleeping bag...that is how the insulation is designed to work. If we restrict the discussion to conductive heat transfer (i.e. no air circulation), the heat transfer rate of your sleep system will depend on the material(s) it's made of, the thickness of the material(s) between you and the ambient air, and surface area. Air is a poor conductor of thermal energy - the insulation in your bag is designed to trap a think layer of stagnant air to impede heat loss. Reducing the thickness of this layer and its ability to trap air (i.e. reducing loft) is not a good thing if you want to stay warm. Down loses lots of loft when it gets wet, which is why down and water tend not to mix well. Also, this is (one of the reasons) why the quilt guys decided to throw away the bottom of their sleeping bags. Your body weight eliminates most of the loft of the insulation and severely reduces its ability to insulate.

It sounds to me that your "breathable" bivy sack is not breathable enough. It trapped enough of the moisture given off by your body to degrade the performance of the insulation in your sleeping bag and you got cold. In contrast, the canvas let moisture pass through, your insulation didn't degrade, and you stayed warm.

My two cents...it's worth what you payed for it.

On a side note, if you have a good scale and lots of free time, it would be interesting to compare the weight your sleeping bag when it is completely dried out and then in the morning after you've slept in your SOL bivy.

 

[Darren Best]

UPnorth is right, pockets of warm dry air are your friend.

Cold air, sweat, condensation and flattened insulation are not.

Loft creates these pockets.

As soon as the moisture laden air (from your sweat) touches cold air below the dew point, it condenses and wets what ever it's touching. This is why people see condensation on the outside of their sleeping bag or the inside of their bivy sack on a cold morning.

Weigh your sleeping bag after a cold trip, it will be heavier.

Moisture management is the key to staying warm.

 

[HellsCanyon]

Another thing to think of. Is your bivy exposed to any possible drafts? I know I got substantially more cold with my setup when I pitched my shelter for ventilation. Even a <3-4mph draft was substantial in reducing my warmth. A top layer of canvas would've prevented that allowing my main insulating layer (quilt or bag) to work to it's maximum potential...

Just

 

[123 4/8 P&Y]

Is that SOL escape bivy designed to go inside the sleeping bag? Or outside of it?

 

[larryschwartz]

Is that SOL escape bivy designed to go inside the sleeping bag? Or outside of it?

When I talked to them they didn't know which one would be best, I had asked about whether I needed to have the reflective layer next to me or if it would work OK as a bivy with my sleeping bag inside of it. My experience showed that as an outer layer it did make things warmer, but when I had it inside as a sleeping bag liner it worked even better. This was probably because it kept more of the heat my body was kicking off next to me.