In a steam shower, when you're done "steaming" all you do is turn the main shower on and the spray of cool water will almost instantly cause the air temp in the shower to drop enough to cause the steam to condense, it clears the air almost immediately. It can be advantageous to have a vent inside most any shower. But you just don't want the duct work accessible to steam when using the steam function, thus the desire for a positive closing damper. You could go with a single fantech fan with a couple of vent grills, one with a positive damper in the steam shower itself, the other outside the shower in the main area of the bath. Only turn on the fan when using the shower as a regular shower, or only when done using the steam shower. In a typical small bath you can run a fan with about 1.1 CFM per sqft of floor space. In larger baths you can size it off of fixtures; 50cfm per "typical" fixture and 100cfm for things that generate more moisture like jetted tubs, steam showers, etc. When you start getting towards the 200cfm fans, it's best to install more than one vent grill in the ceiling. Pulling that much volume through a single grille can result in turbulent air noise. If you have a long/twisting duct then more CFMs might be needed due to static losses in the duct. Your builder would be able to help you with that though. Spring-loaded and gasketed dampers are about $30-$50 depending on size.

mike -- Don't know that we will need a timer. It's just DH and I -- retired -- can remember to turn it off, I think. BTW, how long should it run after a bath or shower? energy rater -- The specs say the backdraft damper is within the 8" duct connection portion of the fan, not whether it faces in or out. I need to ask that they face it away from the fan itself, right? Or are you saying the damper should be not at the fan, but at the soffit? The bath is on the NW corner of the house. Air and moisture will go UP through the bathroom ceiling into the fan, ACROSS about ten feet of attic floor, then out a West soffit. Is it at the soffit that you want metal-flex flaps? Does moisture acclumlate there? Drip outside the exterior wall? (Yeah, where DOES the water go?) I see round aluminum-looking flex-ducting in the attic. Is that what I should expect to see on this install -- not hard metal ducting like a forced air furnace? Is this capacity appropriate? I want to eliminate excess moisture, but don't want to be taking baths or showers in a wind tunnel! (It is meant to run *while* one bathes and a short while afterwards, right?) Is a sone rating of 1.7 good? Is Broan a good brand? This installer is also going to correct three other, smaller bathroom exhaust fan ducts that now empty INTO the attic instead of outdoors. Price is $900 all in for Broan unit and all work. He says it will take about four hours. We are north of Chicago in a high-*address tax*-suburb, as my DH terms it. Thanks so much for your time and trouble. Much appreciated. I'm probably overly concerned, but we keep finding faults (like these fan exhausts) in a house that passed city inspection when it was built only 10 years ago. I'm sure it was not code to vent into an attic!

this was NOT elegant... and was 20 years ago... in the north east in a house with a basement laundry, with a finished "rec" room that you walked through to get the "laundry/furnace/hot water heater/sump pump area which was about 1/3 of the basement.... My father vented my dryer with rigid metal piping into a 5 gallon bucket that he had cut 4 holes about 3" in diameter in the sides of near the top, and he had then cut a hole large enough for the vent pipe to fit into... I had one elbow off the dryer, a straight run and then another elbow into the bucket... I made sure there was water almost to the holes in the sides... when the dryer ran, lint would collect in the water, and yes water did evaporate into the air... since the basement was "damp" to begin with (I believe from the sump pump hole) I had a dehumidifier down there in the "finished" part... with a hose attached that ran to the sump pump hole.... Not sure how your "lowest level" with the cement wall is finished or viewable... but.. this was a cheap alternative and it did work... btw... yes the bucket got ugly... I would take the lid off and use a metal kitchen soup type strainer to "fish" out the lint and dump it in the trash when I was done with my laundry... and before I started I would make sure the water level was alright... like I said at the start... this was certainly NOT elegant... but very functional...

"energy_rater, if you vent the humid air from a bathroom to the soffit, what's going to keep that moist air from going right into the attic?" back draft damper attached to bath fan housing. venting attached to back draft damper. venting exits soffit to exterior. fan being on will push air out of venting, venting to exterior of attic will keep moisture out of attic. also, if you ever take off the cover of the bath fan from inside the house..there is an oversized cut in the ceiling that is covered by the cover of the bath fan. sealing this cut is recommended to keep attic air/temps/ insulation particles out of houses. I use Hardcast brand mastic tape #1402. I also use this tape to attach back draft damper to housing of bath fan for air tight seal, and to attach venting to back draft damper. best of luck.