Using a bivy with overbag as a sleep system? Will it work?

Bivys come with several disadvantages, condensation and poor transfer of body evaporation being one. Usually a bivy is a bandaid for an inadequate sleep/shelter system. Years ago many of us used them to supplement a smallish poncho/tarp. New lighter shelter materials eliminated the need for bivys for many of us. As far as adding warmth, a 4 ounce bivy, if you could find one, is not going to be warmer than an additional 4 ounces of down. I bet that Gore-Tex bivy weighs at least 16 ounces. IMO, using a bag or quilt matched to the expected conditions is the lightest most efficient method. Supplementing a bag or quilt with down clothing you are already carrying is acceptable.

Given your suggested kit, I would probably be very cold at 0c.

1700g = 60 ounces = 3.75 pounds which heavy as a sleep system, it is even heavy if the bivy is your shelter. Many people here do trips where ALL their gear weighs 5 or less pounds.

good that Nick is posting : )

maybe an overly heavy shelter is a bandaid for an inadequate bivy

about half the time, when it's not raining or there's excessive condensation, I'll sleep without a shelter. Then, a bivy is good. Waterproof bottom. The top prevents wind from blowing my heat away.

Especially with down, even if there's no wind, there's a lot of convective heat loss. The properties that make down such a light weight insulation also result in warm air flowing out of the down. You need a cover that's dense enough to prevent air flow. WPB laminates are more than necessary. And when it's windy, it's even worse.

A bivy will weigh a little less than the down required to provide the same warmth.

Then, as long as I have a bivy, when it's rainy, I can use a shelter that has no floor and has a lot of ventilation like a pyramid.

Just what works for me and what I've measured.

Thanks for the feedback guys.

It was 15c last night so I tested the combo in the backyard over night. The thermometer showed an extra 7c inside the closed bivy over my torso and head. Where the bivy touched directly against the sleeping bag on my legs was cooler.

I certainly was warm enough all night at 15c in a 15c bag and goretex bivy.
I'll need to try on a colder night how much colder I can go with adding a layer of fleece and or my down puffy jacket.

The bigger problem was lack of comfort on the inflatable pad. I tossed and turned all night but couldn't fall asleep on the 1" self inflating pad. I'd need to add a zlite pad under or to buy a thicker more comfortable pad. I hear the neoair xtherm is the best.