Staying dry in a bivy sack

[elric14]

Good morning all,
I am brand new to bikepacking.  I have hiked in the past and camped in a tent.  To cut down on weight and bulk, I decided to try a bivy sack as my shelter. 
Last night I camped in my back yard to test the bivy before I venture out on the bike.  Temps were sub freezing, and I was warm for most of the night.  My set up is a EMS Boreal 0° Sleeping Bag inside an Outdoor Research Alpine Bivy. 
My problem:  By morning, condensation had built up so much that my sleeping bag was moist and starting to constrict, and the zippers of the bivy were frozen shut.  Should I switch back to tent for cold weather overnights, or is there a way to control humidity in the bivy?   
Any insights people have would be greatly appreciated.  I have a week to rectify this problem for next weekends adventure.
 

[12wheels]

If you're a member of Backpackinglight you can read an excellent article which addresses the problems you encountered.
http://www.bikepacking.net/forum/index.php?topic=6241.0;topicseen
If you are not a paying member the forum has a follow up discussion with useful information.
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=68395

[evdog]

I have the same OR bivy.  I normally don't have problems with condensation as I try to keep it unzipped as wide as temperatures will allow.  Only when its raining I do zip it up more to keep the rain out, but even then I try to leave at least 6" unzipped.  The wider it is open the less condensation in my experience.  If you had it zipped up tight try it again partially open; you may need to add a warmer head covering but it might solve your problem. 

My other thought is your sleeping bag may be too warm causing you to sweat more which will cause condensation.  If you have the option try a bag that is not rated as warm.  I have a 35* bag and a sleeping bag liner - with the two of them I am pretty comfortable, sometimes I add a layer or two of clothing if it gets real cold.

If the forecast calls for rain I'd much rather be in a tent than a bivy.  Each one will have condensation issues but for me the tent is more comfortable to hang out in if you have to wait out a storm.

[Colorado Cool Breeze]

A small tarp over your head with the Alpine left open above you head should keep your breath out of you bivi.

Here's mine. Never had a condensation issue.

hth