Single wall vent into attic

[dan orourke]

........

[Jim Luttrall]

IRC 2003
G2427.7.6 Installation. Single wall metal pipe shall not originate in any unoccupied attic or concealed space and shall not pass through any attic, inside wall, concealed space or floor....
That is the rule.
The reason is fire safety.

[Michael Thomas]

Also, you are apt to get a lot more condensation, and rusting .