Hello,
I’m finally honing in on making a hybrid sleeping bag/quilt pretty soon here.  I had a few questions before I go ahead and started buying supplies, though.  I’ll be using Primaloft Sport, and quilting it to the liner side of the quilt only, using Momentum fabric for shell and liner.  The shell will be theoretically “unpunctured” except at the edges of the footbox where different panels come together.  I’ve heard that people like DWR bivies to use in conjunction with quilts because they add some degree of water repellency from rain when used under a tarp, reduce drafts, and overall add some wind resistance to sleeping systems.  Ron Bell raves about the Momentum fabric he uses in his bivies, and cites it as being quite water resistant.  But…

My main question is, if I’m using Momentum as the shell material for my synthetic quilt, and my design includes a draft under-flap that seals the quilt into essentially draft-free system (and if I seam seal all the stitching), then, considering all the uses that DWR bivies have, would my quilt do the same thing a DWR bivy would do?  This is assuming that there’s a waterproof groundcloth under the quilt at all times, and that there’s a separate system for bug netting. 

How good is Momentum at repelling morning dew?  I like to wake up pretty early on backpacking trips and don’t like to wait around camp for my sleeping bag shell to dry out in the sun.  Does water just bead up on Momentum and fall off with a quick shake, or is there some absorption into the fabric that requires extra drying time?

Do you think the single layer of Momentum in a quilt shell would start to wet out over the course of sleeping under a tarp during a moderate to heavy overnight rain if the tarp was minimally sized?  For instance, the MLD bivies use eVent in the footboxes of their Momentum bivies, I guess that’s done as a safeguard in case you are too close to the foot-end of the tarp’s coverage and get a lot of rain-spash down where you can’t see. 

What about if I used Epic fabric as shell material over my legs and on the footbox?  Any serious drawbacks or benefits to be had from this half Momentum, half Epic top configuration?

Thanks to all who reply

Sure- I agree: no point in adding a splash bivy if you have a momentum shelled bag.  For the quilt, like you say as long as you can seal it up properly no need for an overbag.  Avoiding an extra layer of fabric, no matter how breathable, decreases the chance of condensation (and of course weight).

Momentum’s DWR works very well, but don’t expect it to be a magic bullet.  As it gets soiled and greasy, the water repellence is reduced.  Keep it clean and it works well.  Most of the time my gear skews towards the nasty end of the disgusting spectrum, identified by the gag reflex.  On an overnight it’d probably be at its best, but if you’re like me I wouldn’t expect a miracle as the interval between launderings increases.  Thinking about it in this light, though, I suppose another reasonable argument for an overbag could be made if it entailed some kind of extra-ordinary cleanliness and the resulting better performance of the DWR. 

I have bags made from epic and momentum (not together).  At an extra .7oz/sq yd, you’d get more bang for the weight by increasing the size of your tarp to prevent the problem you’re considering addressing with epic. And tarps repel water no matter how dirty they get.

I do not think you will need the bivy protection.  My experience, however, is limited to the east where rain has not ben much of a problem in recent years.  I use down, sleep under a tarp, and opjnly use a bivy for wamrht in late fall or winter.  I am not sure how much water protection you get from epic over Momentum that would be worth the extra work.  Momentum dried quickly after the foot end got wet from condensation one time.  In fact, if you put Momentum under a faucet of running water the water runs off quickly, rainfall beads up and rolls off, too.