I have a Macbook with several user accounts. Most of the accounts have administrator privileges. The computer's hard disk is encrypted using FileVault.
Is there a way to additionally encrypt each users' home directory so as to prevent other admin users from accessing the others' directories?
I'm (always) running the newest OS X (10.10.4 at the time of writing).
Encrypting the entire user folder will break OS X 10.10 since certain user processes get started before you log in.
You might have to re-evaluate your security levels for users if you can't trust administrators to not abuse their power. FileVault only protects you against non-users. Admin users are root and you can only hope they a) don't know that or b) refrain from bad behavior.
Look at a MDM solution like Casper from JAMF. It has a self-service tool that allows app updates and limited administrative tasks without needing end users to be admin. All users can easily be normal users whether you make tools to do the limited things they need or purchase a tool that has those policies already created.