Location of .emacs file under `su` and `su -`?

[Adam Funk]

When I use `su` and run emacs, I get errors because it loads
~adam/.emacs as the init file, which contains 

  (add-to-list 'load-path "~/.local/site-lisp/")

but then doesn't find the *.el and *.elc files there, because (as far
as I can tell) it's looking in /root/.local/site-lisp/, which doesn't
exist.  Does that seem to be a correct interpretation?

When I use `su -` and run emacs, it loads ~root/.emacs as the init
file.

In both cases, `env` tells me
 
  EDITOR=/usr/bin/emacs
  HOME=/root
  USERNAME=root
  USER=root

so I think the difference is here:

  su      -->  LOGNAME=adam
  su -    -->  LOGNAME=root

When emacs starts up, does it actually look up "~${LOGNAME}/.emacs"
instead of (what I would expect) "${HOME}/.emacs"?