Borah Bivy instead of Tyvek ground cloth?

[mtnrunner260]

I will get condensation in my 15 degree down bag even with the bivy not zipped all the way up. It is only by my feet, I assume it is from wet socks or other things stuffed down in the bottom of the bag. It has never been a significant amount and dries before night even if I stuff bag in a compression bag all day. The bag is likely to warm for what I need. Maybe next year I'll have a quilt.

Has anyone ever done a hot water bottle in the bivy? Wondering if that up condensation significantly.