Down filler bags – full body, or half-body pied d’éléphant?

[Geoff Caplan]

Hi

I need to extend the range of my current down mummy bag without breaking my budget – with minimal added weight and pack volume.

High-end British bag make PHD has done a lot of testing with layering systems for sleep.

They have found that a 240g down filler bag will add around 10C of warmth. This is partially though the added insulation, but mostly because it fills the air pockets in the bag and reduces heat loss by convection. That’s a lot of bang for your buck – the two bags combined would be no heavier or bulkier than a dedicated cold-weather bag.

Here’s a PHD video explaining the concept:

https://www.youtube.com/watch?v=DU6faR5kKFA

Annoyingly, PHD are coy about the amount of down they are using, even when you ask them directly. An odd way to treat customers. But by the rule of thumb that the shell is 50% of the weight, it must be around 120g.

The price for their own filler bag is an eye-watering £491 – not sure how they can justify this. But the affordable Polish supplier Cumulus, who use the highest quality down, have a couple of suitable offerings.

My cold spots are my hips and my thighs, never my trunk. So one option is a half bag that would reach to my hips. This concentrates the down where it’s needed most.

But it’s the trunk that generates most of our body heat, so I’m wondering if it would be smarter to get the full-body bag, on the theory that it will retain more heat in the bag overall and it will filter down to my nether regions.

Any advice about which route to take?

Both bags weigh around 200g with 100g of 900FP down, on the EU scale. You literally can’t get better than that, so I should be getting almost as much benefit as the PHD offering at well under 1/3 the price.

Full body

Half body

[Jerry Adams]

Maybe wear insulated pants and or jacket/vest

Adding insulation to your torso may make your legs warmer.  If your torso is warmer, then more blood will go to your legs and keep them warm.  Or, if your torso is cold, then more blood will be redirected to your torso.

I used to have cold feet, so I wore insulated booties.  But then I added torso insulation and don’t need the insulated booties anymore

[Jerry Adams]

There has to be enough room in your mummy bag for the added insulation whatever you use

If your mummy bag, the added insulation will be compressed, not allowing it to completely loft to get all its insulation value

[Dan]

Jerry’s last post took the words out of my mouth. I’m worried that you will lose loft by trying to jam so much stuff inside your bag. Maybe not the most efficient way to do it.

[Geoff Caplan]

Hi guys – thanks for the responses.

Down trousers are definitely an option, and more versatile around camp. But in terms of warming up my sleep, a bag will be significantly more efficient, I think, weight for weight and volume for volume. Plus good down trousers are spendy and budget is tight right now…

As for compressing the down, that’s not really the point of a filler bag, as I understand it. The main idea is that you fill all the dead space inside the outer bag, reducing convection.  PHD claim that this offers greater benefits than the insulation value alone – for example the same amount of down in an overbag. So any additional insulation is a bonus.

PHD outfit many serious expeditions, and have done a lot of pretty scientific testing over the years on modular sleep and clothing systems. Their advice can be trusted, I think. Plus the concept is strongly endorsed by the awesome Andy Kirkpatrick, who knows more about lightweight gear than most, to put it mildly:

https://www.andy-kirkpatrick.com/blog/view/phd-k-bag

UPDATE: PHD have just got back to me and recommended the full-length bag, so that’s probably the way I’ll go. But I’m still open to counter arguments from anyone with experience of this filler-bag concept.

[Dustin V]

There was a recent thread discussing insulated pants where budget options are discussed.  https://backpackinglight.com/forums/topic/insulated-pants-recomendations/#comments

[Jerry Adams]

I have noticed that when I’m sleeping, my quilt is against me on my top half.  And the bottom touches my pad.  But there’s a strip about 3 inches wide on each side where the quilt is away from me.  There’s an air gap.  Inside that air gap I’m sure there is convection happening.

I’ve tried to design the quilt to reduce that air gap with little success.  It sounds like PHD has a way to do this.

But, the air gap is so narrow there won’t be much convection.

I think I’ll try to measure how big this gap is.  And measure the temperature of the air in the gap…

[Geoff Caplan]

Given that PHD find that a mere 100g of down in a filler bag adds another 10C/18F of warmth, I suspect that convection is a bigger issue than more of us imagine. Filling the dead space is the only thing that can account for this spectacular performance. And this is in a sealed up mummy bag – not a leaky quilt.

Personally, I’m over the quilt thing – I find I’m much warmer in a conventional bag of equivalent weight. Luckily, I enjoy the experience of sleeping in a mummy.

[Scott L]

I have tried wearing a down vest upside down over my upper legs. Seemed to work well.

[Geoff Caplan]

Way back in my student days, my rope-mate and I zoomed up to the Highlands for a quick weekend of winter climbing only to realise that I’d forgotten my sleeping bag. I’ve always been something of an idiot…

Luckily we both had serious down jackets, which we’d picked up as surplus from Graham Tiso, who was fitting out an Everest expedition.

So I padded up with every piece of clothing I had, wrapped top and bottom in the jackets, and had a decent nights sleep.

Which was pretty amazing – because that night was a record frost, and we were half way up a mountain. It must have been approaching -30C, and we were only sleeping on closed cell Karrimats. When you’re young and your metabolism is at its peak, it’s surprising what you can survive…

Footnote – the Karrimats were prototypes that had been supplied to the Everest expedition for testing. They were white and covered with manufacturing flaws. But they were a breakthrough compared to the beach lilos and piles of heather we’d been using till then. I still have it, and it still works fine. That stuff is indestructible.

[Haakon R]

In my experience a “filler” bag is very comfortable and works very well short term, but it’s a challenge when spending several nights outside.
The added insulation inside your down bag, moves the condensation point further inward into your down bag, so it will effectively retain more moisture. This moisture can be a pain to keep under control.

If you on the other hand add a lightweight, synthetic quilt on top of your down bag, this will add insulation without hardly any compression of the down while at the same time moving the condensation point outwards, and most times all the way into the synthetic layer, which is much easier to dry out.
And it also adds protection from the frozen condensation inside the tent/shelter that to some extent unavoidably will fall on top of your sleep system, either as ice melting on the surface as the temperature rise, or as liquid falling off the roof as the frozen condensation warms up.

Whatever you choose, don’t under estimate the importance of a good sleeping mat. No combination of bags or quilts can compensate for inadequate ground insulation.

On the topic of filler bags, I didn’t even know you could buy something specifically made for that purpose. My own approach was to use a very light weight, synthetic, 1-season mummy bag inside my 3-season down bag. This way I could achieve an almost optimal sleep solution thru all four seasons while getting away with only owning two bags. Before this I used to own a three season bag and a fully specced winter bag. Both synthetic and both incredibly bulky and heavy. The 3-season bag was too warm on many occasions, and the (by far) most expensive bag only got a few weeks of use during the winter, which didn’t feel like a very good return of investment.
All this was before I learned about quilts, which has now replaced the synthetic 1-season bag.

[Terran Terran]

The 800fp Naturehike down pants at around $60 were recommended to me on a different thread. I’ve been very satisfied with them and had a few envious comments while camping last weekend. Surprise, when I told them the cost and fill power. Slightly warmer than my old EE Torrids. Wearing the pants makes it easier to adjust temps by sticking a leg out. Easier to get up in the morning.
Trying to balance whole body insulation is a bit tricky. Thermal imaging might help.

[Geoff Caplan]

Haakon – you make some very fair points.

My two use-cases are:

1) Local camping here in the English Lake District – these are mostly just weekenders where the damp problem is a non-issue.

2) Longer projects where I may face the occasional hard-frost on a high pass, but would have regular opportunities to air my bag. I would keep it in my bounce box for use on colder stretches.

For hard-core winter projects, the synthetic overquilt is much the better solution, as you say. But my own approach when I did such things was to use a vapour barrier – I was influenced by the Stephenson philosophy, as outlined in their eccentric nudist gear catalogues!

Like you, the main attraction is that this is a frugal way to cover a very wide range of temperatures. And to easily adapt to changing conditions on long walks.

As for a specialised filler bags – I think PHD are the only option. But I don’t know how they justify their pricing of £491/$620. It’s filled with a mere 120g of their rare 1000FP down, with simple sewn-through quilting. Labour must be minimal, and the super-expensive down will save all of 12g compared to the excellent 900FP down offered by Cumulus at literally a fraction the price. People buy from PHD when they want the best of the best and are prepared to pay, but this is is simply taking the ****.

The Cumulus I am eyeing is simply a minimalist bag, not specifically designed as a filler. It will also be useful for hut-hopping in the Western Alps, where you have to carry your own liner to protect their blankets, or for festival camping on warm summer nights…

[Geoff Caplan]

Terran – thanks for the heads up on the NatureHike pants. They do seem like a bargain – and well worth having for general use if nothing else.

They don’t weigh all that much more than a pair of Capilene mid-weight long-johns, and will offer far more warmth.

And they are a similar weight to the filler bag. I suspect they would offer less warmth-for-weight to the sleep system, but be much more useful around camp and huts.

In the Alps, I tend to walk in shoulder season when everything is less crowded, and sometimes use freezing winter rooms. These pants sure would be nice to have on those trips.

I think I’ll snag a pair!

ALERT: As I write, there’s a Black Friday deal on these pants on Amazon.co.uk. Can’t see everything on the US site because I’ve registered a UK address, but I presume it’s the same there.

[Terran Terran]

I couldn’t pass them up either. I bought a pair of Ghost Whispers on sale at the same time. Still close to twice the price of the NHs. I do like them. I do tend to wear the GWs more often. A little more tapered with leg zips. The NHs are just slightly lighter and slightly warmer. Cold mornings, I might wear them both. If you can slip a loose pair of wind pants over them without over compressing the down, it makes up for the sewn thru baffles and traps a little more air making them a bit warmer.

I used a closed cell pad last weekend. I was surprised how well it worked.

[Geoff Caplan]

Order is in – £44! Anything branded I can find is over 3x the price. I review gear and all the NatureHike stuff I’ve been given has been decent value. If the worst comes to the worst, I’ll use them about the house to keep my heating bills down!

Yes – before the closed cell pad sleeping on cold ground was an issue. For basecamp we used beach lilos – they were heavy and uninsulated so not much use to man or beast, to be honest. Away from the road we’d simply pile up heather under the groundsheet, which worked surprisingly well. Then in the ’60s Karrimor developed the closed cell mat and everything changed – camping on snow was much more enjoyable. As I said above, I managed to snag a pre-production prototype from the local outfitter. And then Thermarest released the first inflatable foam mattresses – but they were so heavy and bulky we usually stuck with the Karrimats, until the advent of the insulated inflatable.

[Jon Solomon]

I’m late to the discussion, Geoff, but I would suggest that if the goal is to reduce convection inside a bag, then a thin, narrow light down quilt in a highly breathable fabric would be even better than the Cumulus bags you’ve been looking at.  The open nature of the quilt means that it has a greater margin of flexibility to fill in all the irregular space between your body and the sleeping bag.

I actually use an EE 50F quilt in 7D inside my layered wool blankets at home for precisely that reason. The quilt effectively fills in the spaces left by the Pyrenean wool blankets. The high CFM of the 7D fabric is another key element of the system. I personally wouldn’t want the very low CFM pertex fabric used by Cumulus on the inside of my sleep system.

[Jon Solomon]

erratum: the fabric that Cumulus uses on the Magic 100 is Toray Airtastic 7D. It also has a CFM around 1 0r 2, i.e., virtually windproof.

Here’s a thread that mentions the CFM (under 2 CFM): https://www.reddit.com/r/Ultralight/comments/k3thei/low_cfm_fabric_toray_airtastic_in_sleeping_system/

For some (most?) people, the MVTR alone will be high enough. I tend to sweat in my sleep after working hard all day and usually prefer a higher air permeability (CFM) in addition to a higher moisture transfer rate (MVTR). YMMV.

[Geoff Caplan]

Jon

Your input is always insightful – permeability is something I’d never really considered when buying a bag, though it’s obviously a factor now you point it out.

I suspect I’m not a sweaty sleeper, because I’ve used all kinds of bags over the years and never noticed an issue. I wake up dry, and my bag doesn’t seem to accumulate too much moisture on longer projects – even in torrential Scottish conditions.

I always wear the lightest base-layer I can find, just to keep oils and dirt off the lining. So I’m never in direct contact with the fabric. Perhaps that helps regulate the flow of moisture?

As for using a quilt as a filler, I take your point. But I seem to be quite an active sleeper, and I’ve got a nasty feeling it would end up tangled around my body and leave areas exposed. How do you find it works for you?

Turns out that this may be an academic issue – earlier in the thread I was pointed at some very affordable down overtrousers and initial experiments are promising. Same weight as the filler bag. Digging deeper into the PHD site, they claim that an insulated jacket with down trousers and socks will  also add 10C to a sleep system. Heavier than the filler bag, but I carry the jacket anyway, which evens things out, and the trousers are more useful in huts and around camp.

For example, here are their suggestions for high summer camps in the Western Alps:
https://www.phdesigns.co.uk/sleep-systems-for-alpine-summer-mountaineering

[Jerry Adams]

I use the same warmth sleeping bag for all temperatures.  I just need a warmer jacket, both inside the sleeping bag and when I get out of the sleeping bag

[Terran Terran]

I’ve graduated towards wearing insulation. It makes it easier to get up. Brynge baselayer with insulated pants and an Alpha top. . Then I may wear a vest or a light jacket, but I’m usually pretty warm by then.

[Geoff Caplan]

@Terran Terran – great minds think alike! My basic setup is a Brynje top capped by a Montane base-layer and an Alpha fleece!

My default sleep kit is the ultralight base layer, but when the frost is pushing the limit of my bag I’ll sleep with the same setup you’re suggesting.

By the way, I may have misunderstood you, but the Alpha directly on top of the Brynje mesh isn’t optimal for warmth – it’s too permeable to air. I use a light, wicking base layer to cap the mesh and trap the air against my body. It has a 1/3 zip to help dump heat when I’m active. The Alpha goes on top of that. With a wind shirt and very light rain shell that doubles as a jacket, this handled 90% of conditions here in the UK while I’m on the move. I also carry a belay jacket for stops and extreme conditions.

[Jon Solomon]

I read with interest, Geoff, PHD’s sleep system suggestions. I haven’t bothered to check their website in ages because of the outrageous prices. The sleep system suggestions are fun to look at but ultimately not very helpful. It’s not even worth it to start going into all the things that are missing.

Alpha works really well in an overbag with a nylon outer. But that’s not part of the “filler” concept that this thread started with (but from which it now seems to be straying).

I still find the “filler” idea intriguing and would love to see some more experiments with this.

This thread reminds me: A long time ago, about 25 years ago when I was living in Seattle, Moonstone had a going out of business fire sale. One of the things they made was an 800FP (high for the day!) down top in nylon taffeta, not ripstop, that would clip in over their normal bags to boost the rating by a claimed 25F.

I picked up one of those tops for something like $30. Since I didn’t have a Moonstone bag, there was no way to use it as intended. Instead, I used it a couple of times inside of a bag and was impressed, but it was just a tad too narrow for the job.

Suddenly, I’d be curious to try out a very simple narrow sewn through down liner with a closed foot box that could be used as an air space filler. Of course it would be without straps or a draft collar or anything really, but would just be a minimalist internal top quilt. The weight probably wouldn’t be all that different from the Cumulus Magic 100, maybe 30 grams less at best, but it would pack up smaller and be easier to use, especially for active sleepers. It sounds to me like something that would offer a lot more warmth than a pair of down trousers at basically the same weight.

[Terran Terran]

I find wearing Alpha over Brynje adds more insulation when I’m still and vents when i move. Under a quilt, it’s very comfortable. I think the way the air moves around the different weaves helps keep the heat in while keeping dry. It’s that warm, cool feeling. Invigorating.  Like a cold plunge after a hot sauna, but simultaneously. Perhaps for us who are less active. I was wearing them with the Alpha as the base layer, but it was pilling with the lint getting caught in the net. They meshed together pretty nicely though.
The reason for an inner bag is mostly for economics. I think the lightest option is a lower rated bag or overstuffing. Clothing is the heaviest, but it’s multiple use, more versatile and the greatest value in that you can get everyday use from it. Sometimes I leave the quilt off until the wee hours, when it gets cold.

[Jon Solomon]

The lightest option may well be a lower rated bag but is it the warmest/most thermally efficient per weight?

Geoff’s initial post suggests that at least one manufacturer thinks a minimalist “filler” can provide more warmth for the weight by eliminating internal air space. So it’s hypothetically not just a question of adding more or less down to get to a certain rating.

By design, a single layer bag cannot eliminate internal air space because you need to leave extra wiggle room to get in and out of the bag across a wide range of body shapes. The only exceptions I can think of are the Down Hugger series from Montbell, which comes at a significant weight penalty, and the custom false bottom bags (Wren and Serpentes) from Timmermade that allow you potentially to dial in the internal girth measurements of the bag as close fitting as you want (with a sacrifice in comfort perhaps) and include snaps at the top to reduce girth.

The advantage of an internal filler bag is that you would eliminate air space while leaving some room for movement. Nice for active sleepers!

[Author]

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