Lightweight synthetic vs down quilt for summer w/bivy

"Plus, this might be my system for my planned thru-hike of the AT."

I would not use The bivy you listed for a thru-hike of the AT. It will not be water proof enough for that by its self.

Down quilt is going to be lighter and more compressible. I would stick with down if trying to get your base weight down to "4-6lbs". Also condensation is not humidity. Condensation is what you need to worry about. There are several threads on this site that explain condensation indepth. but to be short (and I could be wrong) condensation requires a significant temperature difernece to condense the moisture in the air. I would think that you would have more condensation in cooler climates giving that your body and gear are supplying the humidity not the ambient air when it comes to bivys.

Plus in a warmer climate you would have more opertunity to dry out down making it more suitable to a warm climate than to colder. Atleast until you get to the super cold, where there is no liquid water just frozen stuff.

I think I made the mistake of including too much info in my initial post.

My planned thru-hike of the AT is obviously different from my shorter summer trips in the redwood forests of Central and Northern California.

I'm not replacing a waterproof system with my Borah bivy. The bivy is an experiment for me on my home trails to cowboy camp and decide if I'm capable of managing the condensation issues. I'll still carry my Gatewood Cape even on local trails I know well.

I got away from my main point, which is to find a light and cheap summer bag, whether it's synthetic, down, a sheet, whatever.

I'm wondering if it makes more sense to use a synthetic quilt for the summer? Or stick with down?

Maybe bringing the bivy into the conversation was a mistake as well. However, I thought I should include it so people understood my sleep system besides the quilt/bag.

I skip the groundsheet a lot when using a bivy up here. So much duff on the forest floor that things stay dry and clean enough.

I use down year round with no issues. A cheap, light summer synthetic quilt is on my list though.

By your weight standards (around 16oz), your only options would be the MLD Spirit 48 which weighs 12oz for Regular or the EE Prodigy 50 which weighs 15oz. Add an ounce to each if large.

EE is $20 cheaper.

Ive struggled with paying $200 for a 40 or 50 degree quilt, although I would use it dual purpose as an overbag in winter and a summer bag.

Ive taken a Walmart fleece sleeping bag down to 50. $17. (but heavier of course).

i would say synthetic, but there is no right answer.

Ive spent $1000s experimenting and figuring out what is right for me. still dont have it pegged down.

A cheap option would be to take a Walmart 40* synthetic sleeping bag and using it as a quilt to see how you like it. I did the same thing with a USGI Patrol bag for $30 off of Amazon and removed the zipper. It's a bit heavier at 29oz but I learned on the cheap that I'm not a quilt guy when there are comfy 1.5lb 30* down sleeping bags available on the market.

I'm glad it only cost me $30 instead of $200 to learn this but YMMV. As mentioned above, I will keep it as an overbag for when the temperatures really drop.

Personally I have never foud synthetic insulation to be as vapor permeable as down, so I would never reccoend it unless a trip specifically required it.
Given your setup, why not bring a down hooded jacket and use that as needed, and nothing for your lower half (but perhaps a pair of pajama pants.) Since you have the bivy and the Gatewood cape, you are already pretty well protected.

Personally, I'd definitely try that for a summer weekend trip anywhere near Big Basin or Big Sur.

Sure it might get "chilly-ish", but if you are insulated from below, protected from wind above, and your upper half is warm, I'd bet you'll be just fine in that particular climate.

Matt
(A former resident of Pacifica)

The AT gets cold at the beginning in March and again at the end up in NH/ME. Most use shelters most of the way so a bivy plus tarp when needed would probably be fine. could always supplement the quilt with a cheap fleece blanket for a few weeks then drop it in a hiker box.

my MYOG synthetic quilt is probably 45-50 and only weighs 12oz for what would be 5'6 on EE's sizing. for under 100 bucks i used M90 and Apex 2.5

Have you checked out this quilt?

http://www.jacksrbetter.com/shop/shenandoah-standard-rectangular-quilts/

Rated 40-45*,15oz,$170.