diff --git a/.zshenv b/.zshenv
index 0edd2b3..392ac5f 100644
--- a/.zshenv
+++ b/.zshenv
@@ -1,4 +1,15 @@
-: ${XDG_CONFIG_HOME:=~/.config} ${XDG_CACHE_HOME:=~/.cache} ${XDG_DATA_HOME:=~/.local/share} ${XDG_RUNTIME_DIR:=$TMPDIR}
+# Initialise each of the XDG vars to their default values if they're unset.
+: ${XDG_CACHE_HOME:=~/.cache}
+: ${XDG_CONFIG_HOME:=~/.config}
+: ${XDG_DATA_HOME:=~/.local/share}
+
+# For XDG_RUNTIME_DIR, pick a temporary directory - officially it should be
+# destroyed when the user logs out but close enough. The reason for suffixing
+# /xdg-$USER is that TMPDIR does not necessarily belong to the current user
+# exclusively (although it does under MacOS and PAM).
+[[ -z $XDG_RUNTIME_DIR ]] && XDG_RUNTIME_DIR=${TMPDIR%/}/xdg-$USER
 export XDG_CONFIG_HOME XDG_CACHE_HOME XDG_DATA_HOME XDG_RUNTIME_DIR
+
+# The real zsh config lives in XDG_CONFIG_HOME! ;)
 : ${ZDOTDIR:=$XDG_CONFIG_HOME/zsh}
 source $ZDOTDIR/zshenv