Why draft tubes?

[R]

Hi all,

I’m working on a down sleeping bag.  I’m contemplating zippers.  I understand the purpose of draft tubes, but I question that they’re the only solution.

What happens if you just mount each side of the zipper an inch back from the edge?  It seems like that would produce two lofted flaps that press against each other to seal drafts from passing through the zipper teeth.  The advantage would be simplicity and weight – you wouldn’t have to sew and stuff a separate draft tube.

Another variation would be mounting one side of the zipper 3 inches back from the edge, and then mount the other side on, say, a 2″ flap that’s attached 1″ back from that edge.  This would again press the lofted edges together, this time moving the zipper away from the junction.

If it makes any difference, I’m using a differential cut.  The circumference of the inner liner is smaller than that of the outer shell.  I’m using baffles w/ a finished height of 1.8″, aiming for ~30F.

[Franco Darioli]

I don’t know but it might be easier to snag the fabric that way.

[Erica R]

FWIW I purchased a default Z-Packs bag which comes without a draft tube. The theory was you don’t need a draft tube as you can keep the zipper under you. Didn’t work for me. A year later it cost me about $50 to send it back and have them install one.

So…. I wouldn’t mess around, just put a draft tube in.

[Sam C]

I built a sleeping bag with draft tubes but I don’t think they are absolutely necessary.  I am a warm sleeper and prefer drafts;  I like to poke my feet out if not an entire leg or arm.  However, Ms. Geek can’t stand any amount of draft, so I suppose it depends on what type of sleeper you are.

[Ivo Vanmontfort]

You can try with velcro (like lafayette from valandré)

But I don’t like velcro . I Used two zippers for my winterbag. It’s only usefull with a central zip (see pictures)

https://ivovanmontfort.blogspot.be/2014/02/winter-dons-slaapzak.html

[Craig B]

Hi, Rene,

Yeah, I’ve thought about this too.  It MIGHT work out OK offsetting the zipper from the edge of the fabric, but a couple of potential problems:  I agree with @franco that it might be easier to snag the fabric here.  You might protect the zip path with another strip of something like 0.5 Oz cuben fiber.  The other problem depends on how you plan to attach it.  Sewn through both layers, or just on the inner or outer layer?  If it’s just on the inner or outer layer, you may have a strength and durability problem since only one layer of ultralight fabric will be taking all the tension of the zipper as you move around in the bag.  If you sew through both layers, you no longer have loft on the zipper line and you’ll loose heat there, kind of defeating the purpose of the draft tube in the first place.  Sorry, no good answers, but maybe some food for thought :-)

Craig

[Ryan Smith]

First of all, Ivo. Geez. Nice job on the sleeping bag.

Rene- I think it was Feathered Friends that did something like your first option a couple years ago on a bag. They mounted the zipper 3-4 inches back from the edge on one side. When zipped up that 3-4 inches became the draft tube. I assume it worked fine, but I never saw one in person.

 

[R]

Thanks for the replies.

Ryan, FF still uses their ‘crossover zipper’ on their Flicker quilts. https://youtu.be/s4BbfKRSkKU?t=1m13s  Thanks for the tip.

Craig, you made me realize that if I sew the zipper to the outer shell, I’ll lose some of the loft assurance afforded by a differential cut.  An elbow or shoulder protruding anywhere close to the zipper will pull the inner liner as if there was no differential cut and compress the down at the point of pressure.

Ivo, I have considered a double zip, or using velcro instead of a zip w/ a flap like FF – hadn’t considered it as used by Valandre.  Velcro edges against bare skin is really uncomfortable.

If I was just closing the bag, I’d put the zipper on the bottom, a la Zpacks, and not worry about drafts.  But I’m looking for a multi-use option where the bag transforms in to a garment that’s hopefully a little more elegant than the serape you get if you just put a head slit in the middle.  A few years ago I made a synthetic summer quilt that zips in two and the small part turns in to a vest.  I want to make a top quilt with zero insulation on the bottom, which complicates things a bit if you try to wrap a portion of it in to a vest.  I’m also entertaining the pants conversion.  So potentially several zippers.

A related question: at what temp or loft does a differential cut begin to make sense?

Seems you’d benefit as soon as you stop using sewn through baffles, but maybe there’s a range where you don’t really need it yet?  Again, I’m aiming for 1.8″ / 30F.

[Sam Farrington]

Quilts have become popular, especially on BPL, but I have stuck with the cocoon approach.

Some years ago, MontBell dropped thick draft tubes for some of its bags, and substituted insulated but thin flaps not much wider than the zippers.  Will Rietveld’s review of one of these bags contained a photo and description of the flaps at:  https://backpackinglight.com/mb_down_hugger/                              (Later the spiral became a super spiral, and Will did another review.)

Immediately purchased the #3, the 30 degree F model at around 21 oz.  The flaps made it much easier to zip and unzip the bags.  Do they release much body heat?  For a long time I had always stuck to synthetic bags rated at 30 degrees F due to early experiences with down bags that just became useless if moisture got to them.  This can happen despite maximum care, such as when a tent fabric fails, and leaks.  But went back to down to get the weight savings, as the best synthetics I could find with the same temp ratings weighed more than twice as much.  And the DWR treatments on the MontBell bags repelled moisture well.

The synthetic bags left me chilly at 30 degress F, and below, and sometimes everything in the pack with any insulative value, even the raingear, had to worn inside the bag to keep warm.  With the spiral down bag from MontBell, this never happened – it was much warmer, even with just the flaps instead of puffy draft tubes. And wearing a puffy synthetic pullover and pants purchased from BPL, I could be comfortable at much lower temps and enjoyed a much wider temp range for sleeping.  The spiral construction and other features also added to the performance of the bag.

So resolved that the next bag project would use the MontBell flap construction, probably with a strip of 3M Thinsulate in the flap.  But am still using the Monbell spiral almost ten years later.  (Someone commented that it must be pretty rank by now, but it isn’t because I backpack mostly in the high Rockies or mountains in northern New England, where the nights are cool even in the summer, and with the 3/4 zip closure, the bags can be opened up and kept well ventilated while sleeping.

So unless you are making a zero degree F bag for winter or arctic zone use, I think the flaps are better.

 

[Jerry Adams]

I made a quilt without draft tube along zipper

The zipper is at the top, baffles perpendicular, like Ivo’s pictures of sleeping bag

One thing I noticed was that the down in baffles tends to shift away from the ends of the baffles.  So right at the top of my quilt along the zipper the loft was less and I got cold.  Also, the #3 zipper snags the fabric.

So, I sewed on a cylinder filled with down and sewed it to the zipper tape.  I put twice as much down in rather than the normal 20%.  It’s now much warmer although the zipper still snags.  I had put some heavier fabric, 200D, right at the edge which helped reduce snags.  (I had this in mind all along, just tried it out without draft tube first.)

[R]

Thanks for the responses.

Really seems like MontBell let go of insulating the zipper and just wanted to keep it from snagging.  Located where theirs is, that strikes me as reasonable priorities.

I’ve talked myself in to going for a pants conversion so one zipper will be top, center, but none will be involved in getting in/out of the bag.  Snagging is never fun, but it’s much easier to deal with when you’re not trapped inside it.  The main opening will be along the back – in that area there won’t be any insulation, and just a few snaps to hold it together under me.

I think I’m going for a zipper/velcro combo.  A #3 zipper will be attached to the inner liner to transmit the tension of me rolling around inside the bag, and a 1/4″ strip of velcro will be attached near the outer shell which really just has to handle the tension created by down loft.  I’ll probably put the velcro on a flap that closes over the top which will load it in sheer rather than peel.

I will be sure to post photos of my fancy-pants when they’re done :)

[Sam Farrington]

Rene,  I think the Montbell construction does more than just reduce snagging.  For years, I felt cold air coming into bags at the zippers.  With the Montbell flaps, never.

 

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