A space can't be in channel names according to the IRC RFC, it's a delimiter.
<channel> ::= ('#' | '&') <chstring>
...
<chstring> ::= <any 8bit code except SPACE, BELL, NUL, CR, LF and comma (',')>
irssi might support it for some weird reason, but it doesn't need to be supported because no compliant IRC server should allow it.