On August 21 I finished building a two person quilt, and on Aug 22-31 we took it on its maiden voyage - ten days in SEKI NP. Others have done such a great job of describing/showing construction details that I won't repeat that information. But I'll give a rundown of my quilt, and show some details that are a bit unique.
I made my first double-top-bag in the mid-80's by taking a commercial rectangular bag, cutting out the zipper, and sewing a nylon bottom to it. I replaced that bag in the mid-90's with a similar design. About 10 years ago, I bought a custom Nunatak Backcountry Blanket and added a nylon bottom from the hips to the feet and velcro attachments to the pad couplers. I also added a pleated neck collar (more on this later). The Nunatak bag is old and worn and not too lofty anymore, so it was time to replace it. Since we've been using a double quilt for almost 30 years, I had a pretty good sense for the dimensions and features that work for us.
Description of my Love Bird Quilt:
M50 fabric, nanoseeum baffles, and down were all purchased from Thru-Hiker
35.5 oz finished weight. 25-26 oz down.
2.25" tall baffles, for a projected loft of 2.75", which I figure should "rate" at about 25 degrees. 5" chamber width. [edited 10/10/2012: lowest temp on recent trip was 26 degrees, no wind, very humid, double walled-tent REI Quarter Dome tent with mesh doors closed but both fly doors wide open, base layer of clothing and hats -- we were warm enough. Much colder though and we would probably have put on a second layer of clothing or closed the fly doors. On nights when temp was >40 we were very warm and sticking arms and/or legs out from under quilt.]
Main body is 68 inches wide by 70 inches long, not tapered. At the foot end, I added a "vertical" box, 7" tall by 33" wide, which is sewn to the main body without sewn-through seams and effectively forms a foot box. The interior vertical wall of the footbox is 7" tall and the exterior wall is 10" tall -- that prevents the feet from pushing the inner wall to hit the outer wall. And at the head end I added 10" (2 chambers) that have pleats -- more on that later, as it solves a problem unique to double quilts and is, as far as I know, a solution not previously documented on BPL. We are about 68 and 69" tall.
I didn't taper because we sleep better, and we like each other more in the morning, when we have enough room to bend our knees, kick around, and spread out.
I used all black, instead of a contrasting color, so that the 5 yards of fabric that comes in a Thru-Hiker kit was continuous, giving me far more efficiency in making use of the yardage. Not an issue for a single quilt, but important for a double quilt - more on that later.
I used the shop-vac method of moving down. What a great tip that was, easy and mess-free.
I used four different colors of thread (none black), so that the top and bottom of each seam is in two different colors, and seams that overlap or intersect are of different colors. Purpose: I could monitor my tension easily, and I could rip out seams more easily in case I made mistakes. Effect: Looks like a pre-school group with a box of crayons had a party, not very professional, but it got the job done.
I used more down than most people use. Assuming an overall height of 2.75", I filled with 2.0 oz per yard-inch of "900-fill" down (i.e. one square yard at 1" tall). To state it in a different way: To nominally fill a square yard of quilt that has 2.25" baffles using 900 fill down, you would need 3.24 oz of down (2.25*36*36/900). First, I assumed that my volume would be BaffleHeight+0.5", so I wanted to fill to 2.75". That would have required 3.96 oz of 900 fill down to nominally fill each square yard. But I used 5.5 oz per squre yard instead, as per Nisley's statements that down that is more compressed (to a factor of ~2.5) will perform as well as fully lofted down, and my own experience that humid or "old" down will start to shift.
By my calculations (please correct me if I mis-calculated), Nunatak Backcountry Blanket is filled with 1.2 oz per yard-inch ("800 and higher") ; Jacks-R-Better with ~1.6 oz (800+ fill power) or ~1.8 oz if you order overstuff; Enlightened Equipment to ~1.5 oz per yard-inch ("850fill"); ZPacks to ~1.5 oz per yard-inch ("900fill"). My thinking was heavily influenced by Richard Nisely's discussions of down and my own frustration at having down shift when it is humid or old, and wanting to prevent that shifting. Also, my chambers are 70" long with lots of opportunity for shifting, unlike in a single bag. I wasn't so confident in breaking from all trends that I ventured to have shorter baffles for an effective density of 3 oz per yard-inch, but I seriously considered it. I couldn't find a straight-forward thread about this from Richard, but the info is nested into comments such as this one:
Double-quilt design issue #1: The fabric is 58" wide, and the bag is 68" wide. So you can't just cut a top piece and a bottom piece and be done. I didn't want a seam running head to foot (visually gawky, and the inevitable puckering/tension that a seam adds changes the length of the fabric in the direction of the seam (unless tension is so loose as to be too loose)). So the main body of the bag (finished size = 68x70 inches) is constructed out of two pieces at 72" x 58", and one piece at 72" x something-less-than-58-but-I-don't-remember-what.
Double-quilt design issue #2: The gap between the two heads is prone to open up and let air flow in. Nunatak solves this with a little flap that hangs between the two people (not too effective in my experience). I made the two baffle chambers closest to the head of an entirely separate piece of fabric 16" wider and added four two-inch pleats when I attached it to the main body (see red arrows in the images below). This way, when the bag is nestled around my neck, it doesn't pull it tight between Jim and me. It's pretty nifty, very effective, and I'm pretty proud of how it turned out.
The little white blot and yellow line in the first photo is a loop of gros-grain. This lets me attach a thin shock cord which can be hooked to a spare shirt (or to a sock with a weight inside it) in order to anchor the center, so that as we thrash in the night we don't displace the quilt, which would let in a draft of cold air. The gros-grain runs through the bag and emerges on both sides, so when you pull on one loop it pulls both top and bottom fabrics in unison instead of pulling them apart. Since there are loops on both interior and exterior I have flexibility with how I attach the anchor. The system worked out very well.
Details of the neck anchor. It's a 14 inch loop of thin shock cord, with two mini cord locks. I used two cord locks so I can pinch my weight between the two and slide the whole unit closer or farther from the bag.
The neck anchor is a wadded up red shirt held in place by the two cord locks, and it's extended away from the quilt. I could easily slide the pair of cord locks closer to the bag to better secure the area against drafts.
Here's a link to an entirely different design to address this same problem. I'm not sure how his design would be implemented for baffled down, but it's more food for thought.
The image below shows the underside of bag. The piece outlined is just a single layer of M50. It forms the floor of the footbox. The end of this toward the knees has a thin shock cord sewn into a sleeve, and a mini cordlock that allows me to adjust the width. When it's cold I can draw it closed, but when it's warm I can let it out giving us a very wide comfortable foot box. The fabric does not reach our knees, and it's easy to pull a leg out if it gets too warm. I used shock cord instead of regular cord so there's never excessive tension on the fabric.
I posted photos and a description of how the quilt attaches to the pads in a separate thread: Pad and Quilt Coupler
Impressions, and open questions:
We didn't have any really cold nights on our trip. One night with light frost, and a second night with heavy frost and ice skim in the platy bottles that were under the fly of the tent. We slept in a traditional double-wall tent with the netting closed but the doors of the rainfly wide open, wearing thin base layer, hats, and socks. On the colder of those nights, by morning, I was feeling a tiny bit of a chill - not enough to put on a second layer since it was about time to get up, but if it had been earlier in the night I would have taken the time to add a windshirt. Jim was plenty warm enough even on our coldest night.
I like the idea of the water-repellency of the M50 and believe it will keep the down drier. On the other hand I don't think it will dry out as fast when left in the sun. When I make my next bag (a warm weather version using the same design) I may use nobull or pertex for part of the liner, just so it has more air passing through. I'm undecided about this. Also, with a fully M50 bag, it does loft a bit slower than a bag where air moves through the fabric more freely, but it's fast enough that it wasn't an issue for us. I can imagine it being a "sales" issue if a user in a store was assessing an M50 bag and was not impressed by how it looked 5 minutes after removing it from a stuff sack.
We really liked the dimensions. Our last quilt was ~64" wide instead of 68", and the extra few inches made for a better wrap-under at the edges. We were able to still have fabric effectively wrapping under our sides even when we were both on our sides and facing opposite directions - the most "stressful" position in terms of width. The roomy foot-box was like heaven - we could bend our knees and spread our legs and feet with luxury. I think 68" wide is our sweet spot!
I thoroughly enjoyed making the bag, but spent DOZENS of hours thinking about the design/construction details. I won't have to redo all those hours for the next quilt, because I'm happy with my decisions. I didn't count the hours it took to construct. [edited Oct 22: I have completed my second quilt with identical design and it took FORTY-SEVEN hours to build!] Anybody who thinks they are getting a bargain by making their own baffled down bag instead of buying from Katabatic or Nunatak or Enlightened or Jacks or ZPacks -- that's only true if you are retired/unemployed and don't assign much $ value to your time! A sewn-through bag would be a lot faster and might be a bargain, but a baffled bag with a real footbox is quite time consuming. Fortunately for me, I'm retired.
I took my sewing machine to the shop for a much needed overhaul before I started. That was a critical step, boy did things work more smoothly after a cleaning and tuning.