Full M90 or M50 bivy

You have stepped into a subject almost as weird and arcane as quantum physics…

OK bit of an exaggeration, but there does seem to be a lot of complexity and controversy about the issue of condensation in relation to bivy fabrics and combinations of fabrics. For example, a number of folks here have claimed to have less condensation with a less breathable fabric like eVent vs say m50 or 90.

I would do a lot of research here before deciding on what fabrics to use.

Multiple fabrics aside, am I missing some gaping flaw with a full M90 bivy other than lack of a built in groundsheet which I trade off for increased breathability?

You can't breath out the bottom anyway, you have a sleeping pad, better to be waterproof, like if you have a 20 inch wide pad that's 1 inch thick, it should be 20 + 1 + 1 = 22 inches width of waterproof

Good point, I was thinking one advantage of this system is that it would be light and tighter fitting enough that it would be very similiar to having a mummy-bag and I could move about in the night the way it is(readjusting my quilt inside) rather than rely on always having the a waterproof portion aligned with the ground

I would definitely recommend the M90 over the M50. The early runs of M50 that we used in our bivys were less breathable than the M90, but still worked well for those in climates that weren't super humid. I can't speak for the newest runs of M50, but the last batch we used was almost unusable for bivys, as they didn't breath hardly at all. I ended up re-purposing most of it in other projects.

That being said, M90 is a terrific fabric, and you will love it for a bivy. Breathes great and has a nice DWR coating on it.

"a very basic full bivy but with either M90 or M50 or a combo of both the whole way around
I use a foam sleeping pad and it doubles as my groundsheet, and when things do get really wet I prefer the space a stand-alone groundsheet gives me over just a bivy"

I concur. I've used a totally uncoated bivy under a tarp for several years now and have been very happy with it.

As you and others have mentioned I use my foam sleeping pad along with other pieces of stand-alone groundsheets.

I most recently used that .7 ounce green uncoated nylon that was around a while back and before that I used uncoated 1.5 ounce nylon.

System works just fine and the whole bivy can be thrown into the washer and dryer for cleaning.

I see no problem with your idea. I can't speak specifically about the breathability of the M90 or M50, however, due to lack of experience with the fabrics.

Edit-I forgot to add that I use the groundsheet inside the bivy. The bivy keeps the groundsheet from blowing away and keeps the edges of the groundsheet bent upwards so as to create sort of a bathtub floor. The uncoated nylon bivy fabric is also the first line of defense against rocks, sticks, etc. that might puncture the otherwise waterproof groundsheet.