Bivvy bags – synthetic vs. down sleeping bags

Down.

im strongly agree DOWN

I assume your bivy sack will be protected from the rain by a tarp or tent; otherwise life will suck if it rains.
Either insulation will work in a bivy sack, but my concern is that you're using a WPB material. In my experience, WP fabrics don't breathe sufficiently to put over a sleeping bag (UNLESS, you're wearing a VBL sack or clothing inside the sleeping bag to prevent moisture from escaping from your body into the bag's insulation). Even my highly breathable non-WP Pertex Quantum bivy collects moisture on the inside.

WPBs require heat to drive the moisture through them. In a jacket, your body (just mm away from the fabric) does the job. With a sleeping bag, the great distance between your body and the WPB fabric creates a temperature gradient wherein the fabric temperature on the bag side of the bivy is allowed to drop below the dew point (the temperature at which the relative humidity reaches 100%). When this occurs the water vapor condenses into liquid water (or ice) on the inside of the bivy sack rather than passing through the "WPB" fabric.

In this case a synthetic bag would best retain insulative powers as the moisture continues to build inside the insulation during the night and continue doing this into your second night.

Rather than creating this scenario, how about a nice, light down bag with a light, breathable non-WP bivy sack and a tarp or well vented tent for rain protection?

I have found that a down bag inside a WPB bivy will take on a little water over a three-day trip unless you can dry it out.

Not a significant amount – about 2oz/day but enough to show that it might not work for a week.

I did a 3-day mountain trip last year with just a bivy bag and a far-to-warm down bag and it all worked just fine.

If your sleeping bag has some DWR or pertex outer then that will take care of most of the bivy bag issues.

You might also want to consider not using the bivy bag at all unless rain is expected….