A thought about synthetic sleeping bags not compressing much...

**Macchina** · 06-26-2011, 22:12

The rage now-a-days seems to be down sleeping bags with no insulation on the bottom, you only use your pad as bottom insulation. This got me thinking... Down doesn't insulate when compressed because it almost totally relies on air space (Convection). However, synthetic sleeping bags don't compress nearly as much in their stuff sacks, meaning they also don't compress as much underneath you. Synthetic insulation also uses the loft to insulate, but on top of that it puts a barrier of highly insulative material between you and the cold (conduction) which doesn't rely on air space at all.

I have many nice synthetic sleeping bags, and a DIY fleece/nylon UQ. I was looking at getting a TQ but then realized I could just use my synthetic sleeping bags. My thought is if I use a 30 degree sleeping bag, I will have 30 degrees of insulation on top and a compressed insulation on the bottom. However, I wonder if that compressed insulation (which is synthetic so it's not completely compressed) + my DIY UQ will keep me warm on the bottom as efficiently as the top of my bag is?

Effectively I'm looking at using a heavy winter synthetic bag plus a light UQ to keep warm in cold temperatures. Anyone tried this? Results?

**oldgringo** · 06-26-2011, 22:24

I tested a synthetic Wiggy's bag, and it took me into the low 50s by itself. On another night, I added a hammock sock, and got to the mid 40s comfortably. I think you're onto something...certainly worth some experimentation. The only downside I see is the wrestling match that takes place every time you get into a bag in a hammock.

**JohnSawyer** · 06-26-2011, 22:24

My first hammocking attempt was in a 20 degree synthetic mummy bag... I froze my butt off in 40 degree weather.

Either way, you're compressing the insulation under you... I guess a really heavy synthetic bag would fare better, but why carry the weight?

Of course, OG's Wiggy's bag might have been much heavier than the one I carried...

**oldgringo** · 06-26-2011, 22:27

John, I would have been cold at 40, too...50, actually, without the sock.

**Macchina** · 06-26-2011, 22:36

Yup. I guess my point is that none of the conduction value of the synthetic bag is removed by laying on it, only the convection value. The light UQ will add some convection insulation back into the bag.

For this to work, you have to use a bag that insulates well, but also has a lot of "meat" to it. Meaning a synthetic "down alternative" bag will not work because it uses convection insulation. A bag that retains volume when squeezed between your fingers would theoretically work well...

**DemostiX** · 06-26-2011, 23:28

Good luck with this. I do mean it. I'd like others to better express what I think I've learned from a lifetime in beds and on pads on the floor: The blanket UNDER me doesn't have to be very thick to be effective.

A few things may be going on:

As you've suggested, there's more heat loss, and localized heat loss causing discomfort due to conduction than people are acknowledging, as with cold-butt-syndrome. CBS. Second, some insulation is much more effective than none. (Hello fixed-storm pane on windows with just 15mm =5/8 inch spacing?)

Third, there's the micro climate in the boundary between the cold hammock bottom and the skin, with whatever is going on in regulating the humidity and perspiration there. (See HF sticky on that by Youngblood) I suspect wicking of perspiration and a small amount of air circulation to parts of the fabric that are NOT in contact with the body are regulating thermal output of the body, too. Think of the headband that cools bec auseit provides greater surface area than just the portion under your sweating forehead.

I intend to try different fabric (wool, acrylic) and density blankets between me and the hammock bottom on cool nights. No cotton; it compresses and conducts. If I didn't need to keep zipped bug netting zipped in place, this would be easy with multiple clothespins clipping a draped layer. I do expect effectiveness and differences, maybe as great as the ones I feel wearing shirts of different fabric while cycling in hot weather (putting out 150-200 watts). It would surprise some that as long as I can keep moving and find the right pace, thin wool --- yes wool -- is very effective up to 80 degree F.

So maybe a real wool liner retains heat, is comfortable, breathes, doesn't stink, and wicks moisture. Not expensive, easily washed and dried, too. Too bulky? Maybe, which is why there have been claims for silk similar to the ones I'm making.

**Ramblinrev** · 06-27-2011, 07:16

This has been a topic of hot discussion in the past. The last I remember was probably a couple of years ago. If I remember correctly there was a consensus that everyone's experience is different and there was no clear "universal" result. The quants weighed in with the physics and everything. It was a fascinating discussion even tho it got a testy (pun intended) at times.

I'm no good at tracking down the archived posts from that long ago. But if someone can find them it might be a good read.

**BillyBob58** · 06-27-2011, 08:02

Originally Posted by Ramblinrev

This has been a topic of hot discussion in the past. The last I remember was probably a couple of years ago. If I remember correctly there was a consensus that everyone's experience is different and there was no clear "universal" result. The quants weighed in with the physics and everything. It was a fascinating discussion even tho it got a testy (pun intended) at times.

I'm no good at tracking down the archived posts from that long ago. But if someone can find them it might be a good read.

I think that might have been a thread I started. It did almost get a tad "testy" at times, as I think I remember there were a few folks who thought I was just a bit wacko to be putting such a theory forward. Or at least my theory was wacko!

Or both!

And I think my theory was ( if mem serves) something like: If you have a synthetic bag and IF you sleep in it, rather than use as a quilt, you might pick up a few degrees of BACK warmth in addition to the improved draft qualities of being sealed up inside a mummy bag. Therefore, if you use a synthetic bag in normal fashion, maybe you could get by with a thinner pad or lighter uQ than otherwise, or take the same pad/UQ to a somewhat colder temp.

I came by this idea after I resorted to getting inside my bag when I was just not warm enough using my SS plus bag as quilt, when pushing it past it's limits for me in the early days. Before I got better at boosting the SS AND better at using a bag as quilt. Usually I was not really aware of being cold on my back, but felt like it was more of a problem on top from drafts around my neck/shoulders. More than once, when giving up and getting in the bag, the problem was completely solved . But in addition to a quick increase in overall warmth, I would always also notice an immediate increase in back warmth. Often I would go from just "I don't think my back is actually cold" to "hey, my back is warm and toasty"!

I personally believe sleeping inside a synthetic bag def adds back warmth despite some compression of the insulation. Of course, the question is: how much does it add for a given bag? The thicker the bag, the more it adds, just like on top. BUT, it won't be near as much on bottom ( compressed a little or a lot) as on top. So if using a bag only, it will be hard to be warm enough on bottom without being too hot on top, or not being just right on top without still being too cold on bottom. Especially as bottom warmth seems to be the greater challenge.

And things will vary with the type of synthetic insulation. Some compress more than others.

But will sleeping inside a synthetic bag give more back warmth than a TQ or down bag, with any given pad or UQ? Or more (though often not enough) back warmth with no pad or UQ, than a TQ or down bag? Yep!

Based on my experience at least.

**burleyolebear** · 07-01-2011, 07:44

This is a great discussion. I have a nice synthetic bag that I am considering dissecting for the purposes of having a TQ/UQ system but I love the bag. It's a tough choice. I've slept in the bag in my BMBH with no pad and kept comfortably warm into the low 20s. The bag is rated at 15 degrees, so comfort in the low 20s is a pretty good test of a 15 degree bag in a hammock. If I use my reflectix pad in addition to the bag, could I go to the teens? I will have to wait til next winter to test, but better to try that that to ruin a nice piece of kit. Any and all results will be posted here for the benefit and humor of all.

**BillyBob58** · 07-01-2011, 20:49

Originally Posted by burleyolebear

This is a great discussion. I have a nice synthetic bag that I am considering dissecting for the purposes of having a TQ/UQ system but I love the bag. It's a tough choice. I've slept in the bag in my BMBH with no pad and kept comfortably warm into the low 20s. The bag is rated at 15 degrees, so comfort in the low 20s is a pretty good test of a 15 degree bag in a hammock. If I use my reflectix pad in addition to the bag, could I go to the teens? I will have to wait til next winter to test, but better to try that that to ruin a nice piece of kit. Any and all results will be posted here for the benefit and humor of all.

Wow, that is an impressive result for just a bag in a hammock!