CI 3.x Folder Permissions?

I've updated from CI 2.x-> CI 3.x. It went quite quickly.

I've updated my production server and as expected am getting a permissions error:

Message: mkdir(): Permission denied
Filename: drivers/Session_files_driver.php
Line Number: 136

Where is the docs page that describes what folders need to be set to what permissions? I was just searching the docs for "permissions", but nothing relevant seemed to come up.

Thanks in advance for the info!