I am interested in making a down top quilt for use in a hammock. I normally sleep in a Hennesy Hammock when backpacking and already have a down underquilt. I want to make my own rectangular top quilt to decrease my packweight though. I would like to be able to use the quilt to about 30*. The quilt’s dimensions will be 80” by 50”. How much 900 fill down should I use to acheive comfort at this tempurature? Also would sewn-through baffles be fine if I only plan to use the quilt down to 30*? Thanks

I’d guess you’ll want at least 2” of loft for a 30 degree bag, so you need to fill 80X50X2=8000 cubic inches.  An ounce of 900FP down fills 900 cubes, so you’ll want a minimum of 8000/900=8.88 ounces of down. 

It sounds like you’re basically trying to make a JRB Hudson River quilt (http://www.jacksrbetter.com/Rectangular Quilts.htm#Hudson River).  That quilt is rated at 25-30 degrees, and has 2” baffles, so sewn-through might make cold spots.  Of course it depends on how cold you sleep, etc.

I actually am not even planning on sewing up or tightening up the bottom.  Because the hammock naturally wraps the Quilt around me and holds it there, all I need is a rectangular quilt like you would find at home.  HOw much heat would be lost if I used sewn through baffles?

It’s really hard to say.  I have a JRB Stealth quilt with sewn-through construction (no baffles) and it works for me, in a bivy to about 30.  It’s rated to 45.  I’m not a hammocker, though.  You might want to ask the people over at hammock forums what they think.  (http://www.hammockforums.net/forum/)