Sleeping Bag Liners and Quilts: Convective Heat Loss, Draft Control, and Practical Benefits

[Ryan Jordan]

Companion forum thread to: Sleeping Bag Liners and Quilts: Convective Heat Loss, Draft Control, and Practical Benefits

Sleeping bag liners are often sold as warmth boosters, but their real value in quilt-based sleep systems is managing convective heat loss. Drawing on field use and ongoing experiments, this article explains how liners affect drafts, when they help, and when their weight is better spent elsewhere for cold sleepers.

[Terran]

I like the ZenBivy system. Rock Front makes a similar one. I can seal one side while leaving the other open for easier access. I theorize that the sheet adds some R value to the pad. I never liked the restrictiveness of a mummy, hence I’ve avoided using a liner. The AD60 does look inviting. More so if it was open on one side.

I’ve taken to wearing warm clothes to bed while counting less on the quilt. Less of a cold snap when I get up. Not the most weight efficient, but dual purpose.

[Gerry B.]

I have developed a very versatile system that works great for me from the low 70’s(F) down into the low 50’s(F) that weighs a total of 12.5 ounces, a Cocoon Silk Mummy Liner (4.5 oz) and JacksRBetter Alpha Direct 90 Quilt Liner/Summer Blanket (8 oz).  It has worked for ground camping, hammock camping, and during a pilgrimage walk I did last year.  Here is how I use it in rank of warmest to coolest conditions:

1.  Silk Liner alone

2.  Alpha Direct 90 Liner alone

3.  Alpha Direct 90 Liner OVER Silk Liner

4.  Silk Liner OVER Alpha Direct 90 Liner

[Matt Dirksen]

Thanks for this, Ryan!

In the building science world, we’ve always used the rule of thumb that one square inch of air gap in a building’s envelope is equivalent to eight square feet of no insulation. So the impact of air exchange in a sleeping system is very important to consider. Additionally, body movement during the night should not be overlooked, since this not only affects the convective air between the user and their quilt/mattress systems but the air-only mattresses as well. (As we know, ASTM F3340 does not take body movement into account, which would be awesome if it could somehow but understandable that it doesn’t.)

For years, I’ve sworn by the use of a silk liner with my quilt setup but with a slight but important modification to what you’ve described.

 

I have my liner drape over top of my quilt, and have it wrapped in between myself and my sleeping pad, NOT under the pad. I cut very small slits on the sides of the liner so my quilt straps would still securely connect under the mattress.  While this connection would cause the liner to be tucked under the sides a little due to the strap tension, I noticed it actually created a better air seal as a result.

Admittedly, I did this to mitigate two other issues:

1)  I find the “plastic” surface of the mattress to feel terribly uncomfortable, especially on more humid nights.

2) The silk layer over top of the quilt was superb at catching and absorbing rogue condensation that would eventually drop from whatever tent or tarp surface that was overhead, a very normal occurrence in mid Atlantic climate throughout the year.
And the liner was always pretty easy to dry out.

Perhaps you could explore this system in your testing?

 

Cheers!

[Terran]

Matt and Gerry. 👍👍 for the idea.

The A.D. quilt liner at JrB fit the bill. I do have a old silk sheet from Rab that is as meant to slip over a sleeping pad. Basically a sheet doubled over, with the sides seen together. Unfortunately what I see on their website now is cotton. Turning it inside out, I found that my 20* EE quilt fit comfortably inside. That might just become a thing.

 

[David D]

Sea to Summit frame their Reactor liners as not providing much if any draft control.  Reminds me of alpha-D needing a wind shirt:

“When sleeping outdoors, the main factor to be aware of is moving air. The Reactor liners are primarily designed for use inside a sleeping bag, so they allow enough air to pass through their fabric to guarantee a comfortable night’s sleep in that configuration. Breezes and drafts in an outdoor situation will also pass through the fabric and rob you of the warmth you have generated.”

It added  up to 5C warmth inside a 10c bag in a backyard test in a hammock but was constraining and didn’t really offset its weight in my use case.

The Reactor’s too heavy for Gerry’s good idea but the MLD might work and be only 2 or 3 oz.  Oh man, trying not to buy more gear….

[Terran]

Oh man, trying not to buy more gear….

MLD discount…

ADVENTURE15

JrB

[David D]

Thanks for the JrB link but Apex is a much better insulator for the weight than alphaD

A 40F Apex/7D quilt might be my first MYOG project when I retire.  They go for $310Canadian up here which seems a bit excessive for what it is

[Terran]

I think a 40* Apex quilt has been on your list. I did order the MLD liner. Thank you for that. If my 20* quilt fits inside, it might be good down close to 30*. Using my sheet over the 10* ZB worked well. Fit like a spacious mummy bag. Better than the ZB alone, which otherwise still has a slight gap by the upper leg.

[Monte Masterson]

I find sleeping bag liners to be totally not worth it. First of all, it’s already hard enough to secure yourself under a quilt getting the straps properly around the pad and all. Then with a liner you also have to shimmy like a worm getting in and out of a tube. Makes urination breaks even more of a hassle. Secondly liners provide very little warmth for their weight. Add that extra weight in down or synthetic insulation and you get a much more warmth per gram. I used to laugh at the estimates S2S touted with their Reactor and Reactor Extreme liners. I see they finally stopped giving warmth estimates which is a good thing. If you can’t seal off the cool spots with a quilt then go with a sleeping bag

For just in case overnights on day hikes I pack a thicker and warmer 7 oz Thermasilk silk liner with a 4.2 oz SOL emergency (mylar) bivy.

[Terran]

Would the same be true with something like the HMG 40* with sewn through baffles?

[Geoff Caplan]

I’m with Monte here.

For around the same weight as an Alpha liner, you can carry a down filler bag like the Cumulus Magic 100 mummy. It’s a basic sewn through bag with 100g of decent down.

This will cut draughts, give some insulation, and reduce convection in dead space.

Tests by PHD Designs of similar bags report increases of 5-10 degrees centigrade, which is pretty impressive for 240g of weight.

[David D]

The reactor was a waste of money for me, it’s never been used once because of the weight, modest warmth and extreme fiddle factor.

The mld with Gerry’s idea looks worth the look. Still a fiddly garden hose to slip in and out of but less than a third the weight of a reactor and this could be much more effective at blocking drafts than any interior liner because it forces the quilt to block the drafts and doesn’t rely on the porous liner material to do so.  All depends on whether a quilt fits inside it

I’ve tried going strapless but I twist and turn way too much.  Far too many broken bones and torn ligaments over the years to sleep in one position for too long

[David D]

Oops, sorry, I meant Matt’s idea!

[Terran]

It depends if you need extra insulation or just a draft stop. As with the alpha, going back to Gerry’s idea. Using it inside a liner as a summer blanket or a just in case overnighter. As a quilt liner, it should be nice for car camping.

The MLD has a shoulder girth of 64″. I’m thinking one could sit up, slide it down, then shimmy out. If not, modifications might be in order.

Draft control would be the main benefit of a liner or a bivy. If a 10* quilt needs further insulation, chances are that I’ll have insulated clothing to use.

 

[Monte Masterson]

@Geoff

The Cumulus Magic 100 Zip is 240 g but the regular non- zip weighs only 214 g. I’d take the 25 g weight penalty of the Zip for the much easier entry and exit. 7D shell is only a mere 114 g. https://cumulus.equipment/en/ga/p/down-sleeping-bag-magic-100-zip

But if I was in fact going to use a liner, the question would become which provides more warmth per weight, a down fill bag with 7D shell or Alpha Direct liner with no shell? Sure, down beats synthetic, but when you add in the shell weight much of down’s advantage is negated. Of course Alpha Direct on its own without a shell on the outside is not very good when exposed directly to the cold, however inside of a quilt/bag it performs well. I just don’t know where you’d find an AD liner though. JRB offers an AD quilt and Timmermade an overbag/quilt, but neither would completely seal you off like a liner or summer bag. Probably would have to make one yourself.

[Scott S]

I experimented with AD on some quilt designs and concluded it is not worth its weight. Down is so much better than AD that even with the penalty of some 7D to contain it the down will be much warmer per weight.  There are admittedly things that down won’t work for, but for those APEX will still be better than AD.

I also experimented with some liner-type sheets for draft blocking and decided it was easier to instead just add that to the quilt itself by making the quilt wider.  A 58″ wide quilt can be snapped to the sides of the pad to provide complete draft blocking but still give plenty of room for movement.  I put Kam Snaps on the lower sides of the pad with squares of tenacious tape to hold them in place, as well as on the sides of the quilt.

[Jerry Adams]

Yeah, well said Scott, I have same experience except just using regular fleece rather than AD.  And the quilt is sewn rather than kam snaps, but same result.

One problem is what Geoff mentioned, there’s now an internal air space at your sides where the quilt goes down to the edges of the pad.  I’ve measured the temperature of that air space and it was about half way between ambient and skin temperatures, when I was also wearing insulation.  Which would say that wearing insulation maybe prevents this problem.

I should repeat that just wearing a base layer.

[Jerry Adams]

Rock Front has another solution, different than zen bivy.  They have zippers to connect the sides of the quilt to the sheet covering the pad.  If it gets too warm, you can unzip as needed.

(Bill told me about Rock Front)

[Terran]

The zippered up RF quilt looks like it would create that sort of clam shell effect leaving air space. For me using a large ZB, it sort of creates the same affect while an XL doesn’t. An XL will fold in and surround me. ZenBivy doesn’t directly attach at the edge. The sheet does while overlapping the quilt. A minor, yet a big difference.

Edit: effect/affect. Still gets me.

[Scott S]

Yes the goal is to have the quilt draping down on the pad and to not have so much tension that there are air pockets on the sides.  If the quilt is not wide enough there will be air pockets on the sides.  Smaller bodies need less width on the quilt; I am an average sized male and 58″ works OK for me.  Even wider would be a bit better as the quilt is sometimes off-center and in that case there is a bunch of slack on one side but it is tight on the other side and creating an air pocket.

[Terran]

An advantage over straps is one side of the quilt can be fastened with the second side left loose. Easier for ingress. Laying on my side, my back goes against the fastened side while I bunch the loose side around me. Any draft comes from where the wings end  and the footbox begins. A short sleeve with adjustable compression?

[Jerry Adams]

“Yes the goal is to have the quilt draping down on the pad and to not have so much tension that there are air pockets on the sides.”

I have been working on that for years and have not found a good way to make that happen.

Maybe when I’m laying on my back, the quilt will drape down and have no air space.

But when I roll from side to side, there’s an air gap.

[Scott S]

I have my tent set up outside now for testing, and I popped in to see exactly what the gaps were so I wasn’t just going from memory.  When on my back the quilt would settle down to a 0-1″ gap at the mattress juncture.  When on my side there was something like a 4″ gap.  I’m overall less concerned about the side sleeping gap though, the side sleeping position is warmer even with the gap due to the well-known advantages of the fetal position keeping you warmest.  That has been confirmed by testing with my current setup.

[Monte Masterson]

I know a lot of people love quilts for cold conditions and I get it, but count me as one of those who prefers a sleeping bag instead. In temps above 40* F quilts are great, however from 30’s downward I’ll take my Western Mountaineering bags thank you. Even though it’s going to be about 5 to 7 ounces heavier than a same fill weight quilt, the superb WM draft collar, zipper tube and well-fitting hood is going to make it warmer. And it doesn’t involve a lot of mental and physical gymnastics with straps, liners, etc. All you have to do is just zip the bag open, easily slide in and then zip it up. Simple, quick and there’s almost zero chance of drafts or leaks if the bag fits properly. As many options as WM has that should be doable for any body type. Toss and turn all you want, no worries.

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