Cracked tube guy here. Here’s my experience from 3 separate setups at 2 different makerspace locations (same org, just found a better building across town):
Setup 1: Venting directly outside with an exterior louvered vent under an overhang
Setup 2: Shared venting (with a bathroom) with a blocking diverter and a blast gate
Setup 3: Dedicated venting with a blast gate and a louvered vent without an overhang
Setup 1 (old space) used flexible 4" dryer tube between the laser and blower, then solid ducting to an existing exhaust port on the wall (intended as kitchen exhaust), and worked OK…until the winter of 2013 brought a super-hard freeze (by our standards: -18F) to our area. Even though the exterior louvers were closed and the space was heated (50s-60s when no one was there), enough cold air came in to freeze the coolant water in the tube.
After this, we added the blast gate (similar to http://www.rockler.com/4-stablegate-blast-gate), which sat between the blower fan and the laser.
Setup 2 (initial setup in the new space) had the same dryer line running between laser and blower, then switched to solid ducting to run to a ceiling vent that went to a common exhaust worked well for the laser, but not so well for the bathroom. Even though we made sure the diverter was doing its best at keeping the exhaust airflow from coming into the bathroom, it still smelled like fresh hot laser in there. I can’t recommend having a laser share an exhaust with anything, even if you have a good diverter.
Setup 3, the current setup, returned the laser to a dedicated exhaust line, put a better louvered vent on the exterior wall, switched ducting from using any floppy dryer tube to solid aluminum flex duct, and has the blast gate after the blower on the exhaust route. Some water got blown into the exterior vent during a particularly heavy storm, which the gate prevented from flowing further into the blower. We’re still trying to figure out whether we need to change this, or are fine, since that type of storm almost never happens (we’ll end up fixing it, but other projects have taken precedence). This setup works pretty well so far.