Is the loft loss(warmth loss) of a down sleeping bag caused by the weight of the shell fabric or th bivy sack fabric negligible?

I don't think so

I tried to measure the difference between down sample on the bottom and synthetic sample on the top, and vice versa. Heat source similar to human on the bottom. Measure temperature difference across the two insulation samples. The temperature difference was the same whether the down was on the bottom or the top.

Of course, this was in controlled situation. one foot square samples, could be different on a sleeping bag. The synthetic sample was 4 ounces per square yard which is heavier than most bivy covers. 700 g/sqmeter seems heavy

i've used various heavy fabric weight bivys (three layer goretex and event)and haven’t had a problem with high loft sleeping bags not attaining full loft. if you can, loft your bag before you put it in the bivy because while it won't prevent full loft, it can slow the lofting. what is important for full loft is that the volume/shape of the bivy is sufficient for you and your sleeping bag.

Thanks for the replies.
I found that in standard methods, the added pressure is about 1500g/sqmeter. Comparing to 1500g/sqmeter, maybe any heavy bivy sack fabric is not a big deal. Maybe wind pressure is more significant.
Besides, I’m wondering that if there is not any added pressure, how much more would high loft down loft? Has anyone done fill power test with different pressure added?

"what is important for full loft is that the volume/shape of the bivy is sufficient for you and your sleeping bag."

+1 to that

This can especially become an issue when someone brings a sleeping pad into a bivy, which generally makes sense to do so, but not if it will prevent your bag to fully loft. I have as direct experience with this, and it compromised my system… Once.

"what is important for full loft is that the volume/shape of the bivy is sufficient for you and your sleeping bag."

"This can especially become an issue when someone brings a sleeping pad into a bivy, which generally makes sense to do so, but not if it will prevent your bag to fully loft. I have as direct experience with this, and it compromised my system… Once."

I agree.
I've tried several times with my klymit inertia x-lite in my bag, and found my bag a bit tight on some area making it unable to full loft.

if you lose some loft, all is not lost

for example, if you squish the down to 50% of the loft, you will not lose half your warmth, but you'll only lose 25% of your loft

to put it another way, the clo/inch increases when you compress it. The density of down fibers in the volume is greater, so internal air flow is reduced

I've measured this, and Richard has some data that I think is consistent with this, but it's a bit confusing

Basically, it is negligable. The interesting bit is why.

The insulation power of down lies in the surface film or boundary layer of air around every tiny tip or spicule of the down. That boundary layer does not move: it is 'attached' to the down itself. The bulk of the air in the bag is actually far more free to move around, and if there is enough space it will move, taking heat from the inside of a bag to the outside. That has several consequences.

First, a prime determinant for insulation is the quality of the down. The finer the down and the fewer feathers in it, the greater the number of tips present and the better the insulation. The interlocking of the down plumules also matters.

Second, another obvious determinant is the amount of down: the more down there is in the shell, the more insulation there is.

Third, the actual thickness of the loft is not such a good indicator of the insulation power of the down. We use it more as a guide to the amount of down inside the shell, as it is the amount of down which controls the insulation.

Fourth, if you have too much loft, then the bulk air between the bits of down starts to matter: that bulk air can transport heat (air) past the down plumules, making their insulation ineffective.

Compressing the down a limited amount does not change the boundary layer, and does not reduce the insulation power. This is why many people, more concerned with warmth than fanatically shaving the last few grams off, go for a slight 'overfill'. Yes, overfill can work, both by increasing the amount of boundary layer and by reducing the chances of bulk air flow.

Despite all this, there are a few other factors which influence the warmth of a down item – or the user.

First, if the shell fabric lets lots of air blow through, you are going to be cold. Cold air infiltrating deep into the down layer (or going right through it) may not alter the boundary layer, but it will alter the heat flow through the whole thing. This means a fine weave and calendaring of the fabric can be vital.

Second, if you crush the down layer by sitting on it or by stretching it tightly over a hip or elbow, then you can disrupt the boundary layer to the point where it ceases to work. Conduction starts to play a part. This is why a quilt is just as effective as a sleeping bag. Do remember: you sleep under a quilt at home, not in a bag.

Finally, something so many novices fail totally to understand. You might have a super down layer over the top of you, but if what you are sleeping on is cold, you will be cold. Your mat is just as important as your quilt. Those thin 1/8" layers of foam barely smooth out the bumps under you; they certainly will not keep you warm! (1/8" of foam over 3" of soft dry pine needle duff – another matter.)

So why do we go for UL shell fabric? To reduce the weight of the shell. No other reason.

Cheers

Great reply, Roger. Thanks.

"So why do we go for UL shell fabric? To reduce the weight of the shell. No other reason."

That brings up the obvious question, is there a significant difference in air permeability between the various shells available? Does getting the lightest possible shell fabric reduce the warmth of the bag compared to one of more typical thickness? What about nylon or whatever versus Goretex versus Epic?

On a related note: I bought a really puffy puffy on sale and love it. But my Rab Demand smock is a medium. There is 'some' compression when I wear the puffy under the smock. But the smock keeps out the wind and adds a layer.

Is there a rule of thumb here vis a vis trade offs? (And yes, I'll buy a large shell next time).

edit: I understand that some people wear their down jackets over their shells; this always struck me as counter-intuitive, but who knows?

There was an SOTM review on BPL a while back that measured the loft and efficiency loss of down under a shell where there was some compression …

The verdict i believe was that the extra layer and wind resistance it provided outweighted any loss in loft

As to poofays over shells …

Thats a climbing concept for minimizing "faff" on belays

When yr hanging off the side of a rock face, you arent going to take off yr shell, put on yr poofay every time … Youll lose the built up heat (wind is often blowing) and risk dropping more gear

Thats the "belay" jacket concept …

;)

> Does getting the lightest possible shell fabric reduce the warmth of the bag
> compared to one of more typical thickness?
Not per se, but you need to check the air permiability.

> What about nylon or whatever versus Goretex versus Epic?
Most Goretex uses a nylon fabric.

However … the problem with Goretex as a shell material is that it blocks the transmission of water vapour far too much (read my lips), and in cold weather you get ice forming on the INSIDE of the shell. Very stupid idea.

EPIC: I have not seen UL EPIC fabric to compete with the UL shels we have today.

Bottom line: a good DWR over a tightly-woven light nylon fabric. Not cheap though: not Walmart stuff.

Cheers

@Roger,
Thanks for the elaborate reply. I haven’t study much about down before and thought 800FP down compressed to loft just 600FP could be just as warm as 600FP.
Beside I have several further question:
1. Now I understand that the pressure given by the shell fabric of a down bag is negligible. But how much pressure should be seem as noticeable? Or How much loft loss would reduce the insulation power?
2. It seems obvious that at the same weight (not over filled) 800FP down is usually warmer than 600FP down because the former has finer down and more loft. But could I say that at the same loft (not overfilled) 600FP down is usually warmer than 800FP down because the former has more down? or it’s hard to say since the later has finer down?

Cheers

> could 800FP down compressed to loft to the equivalent of the same mass of 600FP be just as warm as 600FP. [paraphrased for clarity]
The 800 FP down would likely be warmer than the 600 FP down, as I think it would have more fibre tips to hold more captive air at the boundary layers.

> But how much pressure should be seem as noticeable? Or How much loft loss would
> reduce the insulation power?
Short answer: I don't know.
A lot will depend on the air permiability of the shell fabric, as the compressed down may allow more air flow – but it may not if the down is very good. If the down is really interlocked, air flow may still be limited enough.
I suspect that compressing the loft to 50% should have some noticeable effect; compressing it to 25% should be serious. That is, reducing a 40 mm loft to 10 mm is going to make you a bit cold.

> could I say that, for the same height of loft, 600 FP down is usually warmer than 800FP down because the 600 FP has more down? [paraphrased for clarity]
That is possible. You would have about 33% more down in the same volume, and it is likely that the extra down would prove effective, even if only 600 FP. We don't do it that way only because the weight of down would go up by 33% as well, and we sometimes place excessive focus on weight, ignoring warmth. What the cost effect would be I don't know: probably slightly dearer.

> overfill
There is no Standards or legal definition of overfill as such. It is a term used by some manufacturers to mean that they can put more (or less) down into a shell – but if you want more than their standard amount you have to pay extra for it. We all know what it signifies of course.

No simple answers here.

Cheers

I think that if you overstuff by 30%, it will be 15% warmer. Overstuffing is not just wasted down, but it's not as efficient as providing enough room for it to fully loft.

And, it will make up for loss of loft due to humidity, down shifting within a baffle, or inaccuracies measuring the correct amount of down for each baffle.

"you have to pay extra for it. We all know what it signifies of course"

You get what you pay for?