Bikepacking as you know involves spending the night sleeping outside. This could be camping at a campsite where you pay a fee or wild camping out in nature.
It then stands to reason that you will need something to sleep in or under. Here is my opinion on what to consider before buying a tent, shelter or bivy bag.
The Tent:
Choose between a one man or two person tent when you bikepack with a tent.
Compared to a shelter and a bivy bag tents are the largest with space and the ability to store stuff inside the tent while sleeping.
Packing space can be gained by unpacking and then repacking the tent, rolling it more tightly and removing the tent poles that can be tied to your bike's frame will free up valuable space on your bike.
Compact tents weigh around 1.8 kg max, anything heavier is not compact enough.
When traveling with a partner a two person tent can be carried by one rider to free up space on another riders bike for food, clothes...
One man tents are more compact with less space, but still bigger than a bivy bag with space for your stuff inside the tent.
Some tents are constructed from mesh with a rain cover. While great for warm nights, almost giving the feeling of sleeping under the stars when the rain cover is removed, during cold weather these tents do not offer much in terms of heat retention.
Shelter:
A shelter is different than a tent as it has no ground sheet, windows or zipper doors.
Perfect for a light weight bikepacking setup, shelters take up less space than a tent.
Shelters offer protection against unexpected rain while still providing some privacy. Due to the lack of any type of floor cover with water column they can not protect against prolonged rainfall.
Shelters are good for shorter and overnight bikepacking trips with predictable weather, limited rain and wind.
Bivy Bag:
Bivy bags are quick to set up, compact and low profile.
Important when choosing a bivy bag is to be sure that the material is breathable and water proof. Paired with a good quality sleeping bag bivy bags can be very comfortable and keep you warm even when temperatures are low outside.
In some countries setting up a tented camp in the wilderness is considered rude making the bivy bag ideal if you plan to camp wild.
Condensation can sometimes be a concern when using a bivy but there are ways to minimize this. While not unique to bivy bags condensation in tents are not in direct contact with your body.
Conclusion:
Each type of overnight setup has it's own plus and minuses. Ultimately it will be your choice which will best suite your needs. Tents are best if you do mixed bikepacking rides using paid camping and sometimes wild camping options. If you are planning more wild camping where setting up and departing quickly is a consideration then a bivy might be right for you.
Lastly, if you do short overnight trips during months when the weather is pleasant then a sleeping bag and shelter might be all you need.
The most important is that you get out there and bikepack!!