Full list of all 14 Outlast achievements. It takes around 8-10 hours to unlock all of the achievements in the base game on Windows.
The base game contains 8 achievements, and there is 1 DLC pack containing 6 achievements.
Collect all documents and complete all recordings
Finish the game in Insane mode
Collect 15 documents and complete 15 recordings
Finish the game
Collect the key in the Female Ward
Activate the sprinklers in the Male Ward
Drain the Sewers
Restore power to the Administrative Block
Complete all recordings
Collect all documents
Finish the DLC in Insane mode
Finish the Whistleblower DLC
Turn off the electricity
Turn off the gas