I was planning on using 1.1 uncoated downproof ripstop from Ed Speer's site, along with his 900 fill down.
Can I just make the pieces, stuff them with down, spread the down evenly, and sew a pattern similar to the Montbell's, in order to keep the down in place, and then stitch it together like normal? Or is that ignorant? I've never worked with down before, so I don't know what to do really. But I've heard the horror stories, so any advice would be much appreciated!
I've never done down pants, but I've done a couple of jackets and a sleeping bag. I think you'd sew the quilt lines before stuffing with down (that's how you do a jacket.) Then baste the edges to keep the down in, and sew as usual. If you tried the diamond-shaped quilting pattern, I think you'd have to sew through the down. With the small amount of down required, either way ought to work.
I imagine a rain pants pattern would work OK, as long as you made it large enough to accompany the extra thickness. A slight differential cut, that is, having the shell slightly larger than the liner, might make them fit a little better, but may not be worth the trouble.
Down isn't all that hard to work with. Just work in a draft-free room, and work slowly. Figure out by trial and error how much down you need for one compartment, and use a digital postal scale that reads in grams to weigh the down for each compartment.
Good advice so far, especially from MNS on adding extra fabric for the puff-factor. I would recommend you start with the Thru-Hiker wind pants pattern and modify that to fit. I think the name of the pattern is Liberty Ridge wind pants. I would not recommend sewing through down; it wastes down and can make for a sloppy looking seam. You would be much better off quilting ahead and then stuffing each compartment. Of course, this precludes the diamond quilting but I think that diamond quilting is more for fashion than function. If you were to make outer pants and a liner pair leaving the outside seams unsewn, stitch the two together horizontally for quilting (there is no fly on the Liberty Ridge pattern) and then stuff with down, baste and then do a finished seam it would probably work. There would be a lot of fussing and measuring though. Before I tried the above, I would look hard for a pattern. As an alternative, see whether you can locate an old pair cheap and then deconstruct them for a pattern. Have fun and good luck!
Dang man, this is becoming more difficult than I thought it would be. I'm not sure exactly how to modify the pattern to account for the down...or maybe more to the point, I don't know how to predict how much puffiness there will be.
I have not made down pants, but I have made insulated pants using Polarguard 3D insulation. In dealing with the loft of the insulation, you have to add a suprising amount of fabric to the outside layer of fabric. If you do the math, adding 1" of loft means adding 6.28" of circumferencef or each leg. I did not allow enough at first and had to add a gusset in the thighs and at the hips to make things work out properly. As to dealing with the quilt lines, my only experience is from way back when a friend of mine made a down jacket from a Frostline kit. The instructions had him fill each panel (front, back or sleeve) with down, and then pin the quilt lines, carefully shifting the down around to get it even, then sew the lines. But pinning an ultralight fabric isn't the greatest idea if you want it to staty downproof. I think I'd quilt it almost all the way, leaving a few inches of each quilt line unstitched, fill each channel, sew the edge closed, then use the fewo inches of open space to fine-tune the balance. The tricky bit would seem to be getting the amount of down you want into each panel. The sources of down I am aware of sell in packets of 3 ounces, and you might want more or less than that in any given panel - of course with pants you can just decide to have 3 oz. in each leg and then it's easier - that would probably be about right for a nice warm pair of pants anyway. I decided not to do down because I didn't want to deal with the added hassle. Sewing with synthetic insulation in batts is much easier. My pants are about 9 1/2 oz and very warm.
Loc: On top of the North Downs, UK
If you haven't done much sewing before, or have never handled bulky stuff, this may be a good way to go. The other advantage is that it is so much easier to launder!
If you don't already have one, invest in a walking or even-feed foot for your sewing machine. This will help enormously in feeding the bulk and layers through without creep, stretch, wodging up, and other bulk nasties. <img src="/forums/images/graemlins/ooo.gif" alt="" />