Here is a quick and easy pattern to build your own a 3-Layer Insultex Underquilt. I wish I had dejoha's illustration skills, but I don't. You'll have to settle for this. :(
Insultex (IX) requires that the layers not be "squished", but rather hang loosely. The small gap between layers (and it can be very very small and work effectively) minimizes the heat transfer, allowing the Insultex to perform at it's best. The differential cuts on this quilt design force those gaps, so no matter how tight you pull the quilt up under you, the layers will not compress.
I have put this together as an average size - modify to your needs, longer or shorter, increase or decrease the width. The picture is a general guideline, but I have found this particular design to fit very well under a hammock and work effectively.
3yds Shell material (unnecessary, but I prefer 1.1oz ripstop to add durability)
20ft 1.5" Gross Grain (channels)
24ft 1/8" shock cord (suspension)
3 cordlocks (suspension adjustment)
1. Cut out the (3) raw sized panels of Insultex, as seen in the drawing.
2. Mark your V-cut-outs (aka-pleats). A sharpie works well.
3. For each V-pleat, fold in half the long way, and sew your lines together (you can cut the flap out or leave it, doesn't matter much either way). When you're done, all three layer pieces will have equal lengths and widths - 42" wide and 60" long - by these specs offered.
4. Shell Material Sub-Directions: Shell material is useful for adding durability to the Insultex, but it also adds weight. One good option is to add shell material only to the outside, as the inside will be against your hammock and be protected.
--4.a. Cut and sew to the same pattern as the corresponding IX layer.
--4.b. Fold V-pleat over after sewing and sew flat to fabric (similar look to a flat felled seam).
--4.c. When sandwiching layers together, be sure the proper side of your shell piece will be facing out.
5. Sandwich all layers together in order. Use lots of pins here, as the differentially sized layers will fight each other.
6. Sew the perimeter of your quilt, approx. 1/2-5/8" in from the edge. If you sew too close, you may find you don't get through all layers as some shifting may occur. If you've sewn too far from the edge so that the gross grain will not cover your thread, feel free to trim to approx. 1/2" from your sew line.
7. Cut (4)1.5" Gross Grain lengths to the height (2 pieces) and width (2 pieces), plus 2 inches. For example, the width is 42", so cut (2) 44" long pieces of gross grain. For a finished edge, fold the ends in, or heat seal with a candle or lighter.
8. Fold Gross Grain in half (long ways, so the 1.5" gg is now 3/4" wide), pin pieces to corresponding perimeter edges of your quilt, and sew in place, being sure you are sewing through all layers. Sew two parallel lines for added durability.
9. Use 36" of shock cord running through each of the short edges of your quilt channels. Sew one end of shock cord to the end of your channel, and use a cord lock at the other end for adjustment (Be sure to use a good knot on the end of the shock cord so it doesn't slip out of the cord lock). Alternately, you could put a cord lock on either end if you don't want to sew the shock cord in place. This will require two extra cord locks.
10. Run the remainder of your shock cord through one long channel, and back through the other long channel. Slip the two loose ends through your last cordlock and tie a knot. (Heat seal your shock cord with a candle or lighter to keep the sheath from fraying.)
11. Place the shock cord over the end knots of your hammock and use the cord lock to adjust. The under quilt will be able to slide back and forth along the channels for easy positioning.
Notes: Insultex does not stuff well, but it does roll well. Fold and roll your quilt, rather than attempt to stuff it into a sack. This will save a lot of space.
Currently, IX can be purchased here:
Backwoods Daydreamer Gear
TreeToTree Trail Gear