air gap private key?

[refreshing at tormail.org]

I could air gap my private key. Put it on a machine with no network
access. Then replying to mails becomes awful?

This requires transferring incoming mail onto a usb device as text file
and put it into the other machine. Write an answer, sign and put it back
on usb an
finally put it back on the machine with internet.

More paranoids could say that the offline machine could get infected by
the usb.

To be more paranoid I could not put anything form the online machine to
the offline machine. Answer without quoting and only store on usb. Never
import to offline machine should be quite secure?