Once you encrypt your CyanogenMod 13 device, you will not be able to decrypt the device. You will also notice that when you restart into TWRP, you will not be able to see your sdcard… because it is encrypted! You might think you are stuck on your nightly build now forever… well…

1. Download your nightly build
2. Copy onto your external sd card (not encrypted)
3. Restart into recovery
4. Install the img from the external sd card
5. Ignore warnings about inability to mount /data/
6. Restart into your new nightly!

I thought TWRP had let me decrypt my phone so it could access the /data and /sdcard directories, but that seems to have be removed?