Which adds more warmth to sleep system: bivy or net tent?

Short answer is it depends. Both a bivy and net tent add warmth, but much depends on how much netting there is, how thick the netting is, and how much of the bivy or net tent is solid walled vs netting. After using both a TT Notch full-netting inner and the EE Recon (L) (just for a few cold nights), both definitely add warmth, but the Recon seems warmest of the two, though it’s very subjective. I guy out both ends of the Recon under a shaped tarp and the bathtub walls come up off the ground a good amount, blocking a lot of chilling wind at night. And while the Recon netting could be lighter, it does seem to block noticeably more wind than thinning netting I’ve used – not a bad idea in shoulder season.

Back in the day I used a 1.5 lb Goretex bivy on many a trip in the mountains in all seasons and it definitely added warmth, and a bit of condensation too, but it fit a little too tight. I suspect that having a larger volume inside may also affect warmth – more air volume to keep warm?