Downproofness depends on yarn density (thread count) AND on the grade of down you use. Real down - the 800+ cu.in. stuff - will work with fabric that is less dense because it has fewer quills to poke through. Lower fill-volume down needs tighter fabric.
In general, real Supplex (DuPont trademark) will hold 800+ down just fine. BUT, but why would you use fabric that heavy? Even if you put it on the inside of the bag (for comfort?) and used a lighter fabric for the outside, you will be adding 4 to 8 ounces to the weight of the bag (depending on what weight of Supplex you have).
Supplex has many good uses where abrasion resistance or cotton-like feel is desired. Hiking shorts or, if you insist, sleeping bags for car camping come to mind.
Remember, you will spend nearly $100 for enough good down. After that investment, why not use a quality, lightweight shell fabric such as Momentum from Thru-Hiker.com?
Edited by Spock (02/02/09 12:25 PM)