You will usually only get condensation to excessive levels if a) you cook or make a brew inside your bivvy, b) sleep with your door completely closed, and c) only use a single skin bivvy.
In reverse order c) it only takes a few minutes to put on the Winter skin or overwrap (whatever), and it makes a huge difference to the warmth and comfort levels. PLUS, if it starts to rain heavily during your session, its the easiest part of a bivvy to dry off when you get home. Dont EVER pack away a bivvy wet and then leave it till you go again. Mold and rot sets in easily in the damp areas like folds in the cloth. It discolours the material, stinks badly, and is very bad for your health. If you ever come across a striped bivvy that reeks, you wont have to ask what went wrong, you now know.
b) Easy to sort out, leave your front door open, (this means you are not groping for stuck zips in the dark when you get a run*, always a good thing imop). It also means you need to set up your bivvy so the door is not facing directly into the wind and rain, yet gives you a view of your rods and indicators.
a) I know its tempting in the colder months to lay in your sleeping bag and just slip on a cuppa, but its not worth it. There is a photo going the rounds on the web, of the results of a bivvy fire. Doesnt take a lot to magine being zipped up in your bag with the bivvy doorway on fire and what the results will be. Its not the only occasion its happened, so please dont let it happen to you.
Its far better to set up your cooker just outside and to one side or the other of your bivvy doorway and while you are cooking / boiling your brew, you are also getting a view of the lake.
Even in the pitch dark (which it rarely is), you can learn a lot from hearing fish crashing or topping in your swim.
* One day remind me to tell the story of the 6ft green one armed caterpillar.
Hope you are ok with this lot.
BOF