I have a Marmot Helium with a 1/2 zipper that I would like to convert to a full zipper. I definitely don't have the skills to do this, so does anyone know where I can get this done? I am also thinking of having some additional down inserted since the bag would already be cut open.
I would imagine that converting it into a quilt would be easier, but at the intended temp I think the draft would be troublesome.

Adding extra down to a bag isn't necessarily a good idea. If the bag is properly designed and made in the first place, the compartments will be sized so as to allow the down to achieve its full loft. Or, to put it another way, the compartments have just enough down in them so they are completely filled (a little extra is usually added to allow for degradation of the down over the lifetime).

Adding extra down won't increase the loft, as there should be no more room for it to expand into. In fact, there's an argument that overstuffing replaces dead air space with more conductive down, so you may even degrade the performance.

The only benefit you may achieve from adding down is when it's added to the base of the bag, to increase the thickness of the weight-compressed down. But you're probably better off using a more insualtive mat.

Hopefully, the down bag and quilt makers will be along later for other points of view.

Edited by captain_paranoia on 03/11/2010 12:04:41 MST.

I may have completely misunderstood, but I just read a thread where Richard Nisley said that the optimum performance of down was at a 2.5x compression. I don't have the actual thread, but here's a quote from another thread, basically dealing with the same thing. However, these are mathematical equations that aren't true in all circumstances for all people.


"So, assuming I have understood this right, in the linear region of the curve (where UL backpackers operate) if you decrease the thickness of your typical UL sleeping bag by a factor of 2, you will not affect the warmth of the bag. Why? Because the density will be doubled, and so the clo/inch will double, but the thickness will be halved... so 2*0.5=1 and the warmth of the bag will stay the same.

I think is a particularly interesting fact. Surely it means that you needn't worry so much about needing lots of room inside your sleeping bag or hard shell to allow a down jacket to loft fully. A partially compressed jacket should be just as warm, so long as you are not completely crushing it (eg. lying on it!)."

This is my understanding too, based on several threads/posts from Mr. Nisley and others. There was even a comment a while back regarding why "fully lofted" has become the marketing standard due to the numbers game manufacturers have to play with each other. ("yeah, well we get 7" of loft!!).

Anyway, I took this "optimum density" thing and ran with it recently on a down sweater for my girlfriend. Each compartment was stuffed to approximately 2x density. It has proven, at least anecdotally, to be quite warm for it's thickness.

Edited by nmiller08 on 03/12/2010 15:29:53 MST.

The insulation value is about the same in the range from fully lofted to 2.5x density increase. Down isn't warmer at 2.5x density.

In the case above though, adding down--to an extent--would increase warmth since there is more of it, and the slight compression of adding more down would not be a detractor of the insulating properties.

Or did I just botch the whole concept?

Sounds right to me Travis, it seems the difference is the two possible scenarios...here we're talking about filling a fixed space with more and more down (warmer and warmer, but diminishing returns beyond 2.5x even though warmth continues to increase) and the alternate idea of shrinking a space with a fixed amount of down in it (retains it's warmth up to 2.5 compression, then actually loses insulation value).

> In the case above though, adding down--to an extent--would increase warmth since there is more of it

I'm not sure that's what Richard has said (and I'd appreciate a more detailed explanation from Richard, or a pointer to his investigations).

My understanding is that it's not the down that insulates, it's the air trapped within the down that insulates (the 'dead air space'). All the down does is expand into the space provided between the shell layers, and prevent air circulating (thus preventing convection losses).

Since down is made from a solid, it must conduct heat better than air, so, if you replace the air with down, the insulation will be reduced. On the other hand, I can see that increasing the down filling slightly may further reduce convection within the down cells, and thus improve performance.

At the limit case, where we ram in so much down that the bag is a solid mass, we no longer have dead air; we have a fairly conductive, solid lump of keratin.

I note that Richard said:

"The insulation value is about the same in the range from fully lofted to 2.5x density increase. Down isn't warmer at 2.5x density"

If I understand the second sentence properly, Richard is saying that increasing the amount of down over the fill density will not make the bag warmer, which would tally with my thinking.

Edited by captain_paranoia on 03/12/2010 07:54:54 MST.

I found the thread dealing with this.

http://www.backpackinglight.com/cgibin/backpackinglight/forums/thread_display.html?forum_thread_id=12505&startat=20

Here's a graph from Richard that I think is relavant.



Quote Richard: "You can make a down bag or clothing warmer by increasing its density even if the loft is less."

Edited by T.L. on 03/12/2010 08:10:33 MST.

Travis and Others,

Thank you for attempting to help Kevin understand by pointing him to some of the prior relevant discussions on this topic.

Kevin,

Heat will transfer through three mechanisms simultaneously (conduction, convection, and radiation). The convection is effectively stopped when the volume of the baffles matches the fill power of the down. By continuing to overstuff down beyond its fully lofted amount, the air pocket sizes will continue to be progressively reduced (results in less conduction heat transfer) and the increasing filament intersections with the radiation (results in less radiation heat transfer).

Thanks for checking up on us amateurs, Richard! Your posts are most informative.

"The convection is effectively stopped when the volume of the baffles matches the fill power of the down. By continuing to overstuff down beyond its fully lofted amount, the air pocket sizes will continue to be progressively reduced (results in less conduction heat transfer) and the increasing filament intersections with the radiation (results in less radiation heat transfer)."

This puzzles me too. As the conduction is reduced as the air pockets get smaller, isn't it simultaneously increased as the pathways across the down become more numerous?

Mike,

Yes it is but, to a lesser degree that the other heat-transfer-mechanisms positively offset its negative influence. This positive offset region averages approximately from fully lofted density to 2.5x fully lofted density.

Edited by richard295 on 03/12/2010 11:06:16 MST.

Travis & Richard,

Thanks for the pointers to the other information, I'll have a read. (hmmm... page not found: what was the thread title, please?)

I'm familiar with the three heat loss methods, but my understanding was that convection is the most significant (and, in turn, it's driven by conduction and radiation).

I'm prepared to accept that radiation may be reduced by increasing the density of the fill, providing more material to reflect IR back inside the bag, but I'm still not convinced by the conduction issue; replacing a gas with a solid seems unlikely to reduce the conductivity.

My suspicion is that the increased density is also further reducing the convection losses within the down, and that convection is indeed the dominant loss mechanism.

There still seems to be a contradiction between your comment "The insulation value is about the same in the range from fully lofted to 2.5x density increase. Down isn't warmer at 2.5x density." and the graph posted; it appears from the graph that the insulation value isn't the same as density increases; an increase in insulation by a factor of about 1.29 (5.8/4.5) [checks: clo is a linear-scaled measure] for a down density increase of 1.75 (28/16), which doesn't seem to be a cost-effective solution.

[edit: not having the article to hand, I don't know whether the above graph includes the 1 to 2.5 fill density range; all I know is the fill weight.]

Edited by captain_paranoia on 03/12/2010 11:19:59 MST.

> By continuing to overstuff down beyond its fully lofted amount, the air pocket sizes will continue to be progressively reduced (results in less conduction heat transfer)

Now, if you'd said 'results in less convection heat transfer, I'd have been tempted to agree with you; reducing the convection cell sizes, and therefore the speed at which heat can convect through the down.

Thanks Richard.
This has got me thinking. I have an Arc Specialist that i am happy with for most of the year. I was thinking of buying an Arc Alpinist with overfill for colder temps.
Would i be correct in thinking then, that a Specialist with the same fill as an Alpinist would have similar insulation value then? Is the taller baffles of the Arc Alpinist not of any benefit, as long as the amount of down is the same?

Woops, sorry for the broken link. Try searching for

"Fill weight and temp rating: Western Mountaineering vs. Montbell UL SS"

Maybe this link works.

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=12505

"There still seems to be a contradiction between your comment "The insulation value is about the same in the range from fully lofted to 2.5x density increase. Down isn't warmer at 2.5x density." and the graph posted; it appears from the graph that the insulation value isn't the same as density increases; an increase in insulation by a factor of about 1.29 (5.8/4.5) [checks: clo is a linear-scaled measure] for a down density increase of 1.75 (28/16), which doesn't seem to be a cost-effective solution."



Without knowing the setup and testing that resulted in the graph, it is impossible to tell, but it appears it was done with a fixed volume by increasing the down amount. Hence the x-axis is down weight, not density, and hence the increase in insulation value. So I don't think Richard's comment is contradictory so much as the graph doesn't depict the situation he is describing. i.e. a fixed amount of down does not get warmer if compressed, but it won't lose insulation value unless you compress it beyond 2.5 times.

So as relates to the original poster, because his current bag is essentially a fixed volume, he would obtain greater insulation value by adding down if the density is kept below 2.5x.

Edited by nmiller08 on 03/12/2010 15:31:49 MST.

Kevin,

You said, “… my understanding was that convection is the most significant (and, in turn, it's driven by conduction and radiation).” It has been at least 33 years since your first premise was disproved both by calculation and experimentation. See:

Pelanne, C. M., "Heat Flow Principles in Thermal Insulations", J. of Thermal Insulation, Vol. 1, July, 1977, p. 49.

Tye, R. P., Editor, Heat Transmission Measurements in Thermal Insulation, ASTM Special Technical Publication 544, Chapter on Definitions and Thermal Modeling, January, 1980.

Both early studies showed theoretically and confirmed experimentally that natural convection within ANY fibrous insulation material is unimportant as compared to conduction and radiation if Ra is less than 40 for the material. Countless others have reaffirmed these earlier studies. The Ra number for fully lofted down averages ~ .18 and improves to ~.012 in the 2.5X density increase range. Natural convection loss has never been a significant loss factor even in the very first solid fiber synthetic polyester insulation bats. They had a Ra value of 6.79 which was very poor compared to down but good enough to prevent natural convection from being a significant factor.

Your second premise is also incorrect; “conduction and radiation drive natural convection." Neither are variables in the Ra related calculations. The topic is far too complicated to explain in this forum. If you want to understand the variables that factor into the Ra natural convection calculation, see http://en.wikipedia.org/wiki/Natural_convection

Your two erroneous premises did not lead you to a valid conclusion regarding the relationship between down density and thermal conductivity. I hope that you understand that I mean no disrespect, but I have grown weary from this topic discussion.

Edited by richard295 on 03/13/2010 13:27:29 MST.

Richard,

Thanks for the references and links. I'll go and do some reading.

Sorry that this thread has wearied you. I like to discuss things, and learn by thinking around problems, and trying to understand the issues involved for myself. I'm also quite happy to be told I'm wrong, provided that the correction is adequately explained. Unfortunately, the discussion in this thread didn't seem to provide a clear explanation. If someone had provided suitable links earlier, you might not have been so wearied.

Anyway, to bring the discussion back to the OP's thread, can anyone help with recommendations for where Dave might get his quilt modified? If he was in the UK, there are a couple of places I'd recommend, but I don't know about the US.