Down letting you down?

[Youngblood]

Since we all know that down will migrate down the tubes when hung that way or with the baffles running the other way why not try a box construction? would keep the down where it needs to be?

I understand that more complex baffles would lend itself to higher price quilts.... but in order to get the down where it needs to be for maximum warmth maybe some experimentation with better baffles.
http://roberts.pl/Details1.html#H

That is one way to approach it. Another way is to recognize that you aren't always dealing with the nice rectangular baffled chambers that we sketch on paper.

When you look at the cross section of a single baffled chamber you get a nice, easy to calculate area, the width times the height. But we are not dealing with rigid materials, we are dealing with materials that have flexible walls. And where down migration is concerned, you need to look at what the expanded cross sectional area of that baffled chamber can be (with adjacent baffled chambers in place) when the down has the opportunity to migrate. When the amount of down used does not fill the space created by that expanded cross sectional area, it can more easily migrate. When the amount of down used does fill the space created by that expanded cross sectional area, then it doesn't shift so easily, if at all. That is what I call a zero-migration down-fill. It takes more down to limit down migration (and baffle spacing plays a huge role in how much more is needed) and it weighs and cost more, but it works better for many applications because in general it is warmer and the down has a very real chance of staying where you want it.

[Coffee]

That is one way to approach it. Another way is to recognize that you aren't always dealing with the nice rectangular baffled chambers that we sketch on paper.

When you look at the cross section of a single baffled chamber you get a nice, easy to calculate area, the width times the height. But we are not dealing with rigid materials, we are dealing with materials that have flexible walls. And where down migration is concerned, you need to look at what the expanded cross sectional area of that baffled chamber can be (with adjacent baffled chambers in place) when the down has the opportunity to migrate. When the amount of down used does not fill the space created by that expanded cross sectional area, it can more easily migrate. When the amount of down used does fill the space created by that expanded cross sectional area, then it doesn't shift so easily, if at all. That is what I call a zero-migration down-fill. It takes more down to limit down migration (and baffle spacing plays a huge role in how much more is needed) and it weighs and cost more, but it works better for many applications because in general it is warmer and the down has a very real chance of staying where you want it.

Would it be a safe assumption to make that the smaller the baffel spacing the less amount of down is needed to find the zero-migration? I realize there is a point where the baffels are too close together to be effective or easy to make and fill.

[Youngblood]

Would it be a safe assumption to make that the smaller the baffel spacing the less amount of down is needed to find the zero-migration? I realize there is a point where the baffels are too close together to be effective or easy to make and fill.

Coffee, when I work up a spreadsheet there is a point where the weight of the baffles themselves make adding more baffles, negatively offsets the weight savings on the down required for zero-migration. But sometimes I don't get to that point because of the issue you mentioned with them getting just too close together to make and fill.

When you look at down migration, it is the expanded volume 'condition' that allows that to happen. If you have a blanket flat on the floor that isn't all that much to worry about since gravity is going to basically keep it in place... where is it going to go? But when they are hanging or you taking them out of stuff sacks, etc, gravity or tossing them about can cause the down to migrate. In the extreme case, you can look at the cross section of a baffled air mat where the baffles determine the shape when it is pressurized. You would get something like this diagram for the shape and volume versus the ratio of the baffle width to height. Those shapes define the expanded (or maximum) area (or volume) that you are concerned with for down migration. You have to decide how concerned you are with it for your application. It may not be obvious in the diagrams but the area increases significantly as the ratio of the baffle width to height increases.

[tomsawyer222]

Would it be safe to say that using two lesser rated underquilts over top of each other would minimize the effect of the down moving? Instead of one quilt with twice the baffle size?

[Youngblood]

Would it be safe to say that using two lesser rated underquilts over top of each other would minimize the effect of the down moving? Instead of one quilt with twice the baffle size?

I'm not following your reasoning.

[gunn parker]

So would the migration be the same in an over quilt, if for example I lie on my side the down could migrate down from my shoulders?

And so would it be better to have narrower chambers running the length of the quilt rather than across it?

Cheers

[Just Jeff]

Dave - I think he's saying that the odds of down shifting in the same spots on both underquilts is very low, so you'd have less chance of a cold spot.

But the two additional layers of shell would weigh more than a significant overstuff of down on the thicker quilt, which would insulate better anyway.

[Coffee]

Good point Jeff. It might be more difficult to get 2 quilts adjusted right without a gap somewhere between them making a cold spot, or adjusting the outer one not to compress the inner.

Unless you are counting oz the overstuff only needs to be 2 or so oz for a quilt.

[Take-a-knee]

Good point Jeff. It might be more difficult to get 2 quilts adjusted right without a gap somewhere between them making a cold spot, or adjusting the outer one not to compress the inner.

Unless you are counting oz the overstuff only needs to be 2 or so oz for a quilt.

mikeinaz brought up a good point awhile back about doubling up quilts in the cold. He opined the JRB footsack was needlessly large, the upside of this is it works well as an outer layer over a quilt with a sewn/shaped footsack. Enough Mom 90 to make a quilt weighs about six ounces, so that adds a little weight, but if you can combine your summer quilt with your 3-season quilt and stay warm in most winter conditions you are likely to encounter, the weight penalty doesn't sound so bad. And, if you are like me, you are running out of places to store more stuff.

[Coffee]

mikeinaz brought up a good point awhile back about doubling up quilts in the cold. He opined the JRB footsack was needlessly large, the upside of this is it works well as an outer layer over a quilt with a sewn/shaped footsack. Enough Mom 90 to make a quilt weighs about six ounces, so that adds a little weight, but if you can combine your summer quilt with your 3-season quilt and stay warm in most winter conditions you are likely to encounter, the weight penalty doesn't sound so bad. And, if you are like me, you are running out of places to store more stuff.

Good point. I don't think doubling up ontop is a bad idea. I did it a lot last year with a quilt and sleeping bag. I was thinking more of as an underquilt it would be hard to get the fit right. I do not think the warmth of the 2 combined would be as warm a 1 quilt with the same thickness of the 2.

The Jacks put together a good article about combining top quilts. The gist of it was it is warmer but the weight of the top quilt compresses the one underneath.

On a side note it was 70 down here this weekend. Not worried about the cold anytime soon. Eyeing kayaks to get some trips in before the bugs come out now.