Exhaust fans and attic ventilation questions

Presently, my two bathrooms have exhaust fans venting straight into the roof attic space. I do not believe that my roof has any ventilation system at all. I am concerned that this may promote unhealthy air which filters down into the living areas below. Are these concerns valid?
I am considering installing ducted versions of the exhausts fans which exhaust air outside through the roof. Or I could install a draught stopper version of the exhaust fan to prevent air from coming down from the roof. I would also like to install some sort of roofing ventilation like those twirling whirlybird ventilators. Would these address the issues above?
Chris
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I vented my bath fans through the roof. (Used Galvanized dryer vent. Galvanize pipe from fan thru to vent). It shows moisture on the roof just below where it vents. So you definitely are getting moisture in your attic. Not good.

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Most residential bath fans are vented into the attic. Every attic I've been in has been dryer than a popcorn fart so the minimum amount of bathroom steam you will push up there won't hurt anything and that is just one less future roof leak which WILL hurt something. Bath yes, kitchen stove no, vent it out.

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This is a good place to start. http://www.askthebuilder.com/546_Bathroom_Fan_Ventilation.shtml
Terminating a mechanical vent fan into an attic is against all building codes; and has been for quite some time. The code requirements for attic ventilation are there specifically to rid the attic space of natural moisture due to infiltration from the conditioned spaces below. Dumping mechanical ventilation into the attic is far more than the code minimums were ever designed to take and have the potential to cause all kinds of problems. (Attics and eaves, are not considered part of the "exterior").
Mold and mildew aside, it will cause a dramatic decrease in insulation value. The amount of moisture going into the attic may not be visible, but it is there nonetheless. As the warm moist air hits the cold air of the attic, it condenses into a water film and clings to individual insulation fibers (insulation works by trapping a tiny amount of air film around each fiber). A small amount of moisture will dramatically lower the ability of the fibers to resist heat transfer.
If you have loose-file insulation in your attic, you start out with one strike against you already. In tests conducted by the Oak Ridge National Laboratory, loose fill insulation can loose as much as 1/2 its R value do to conduction in very cold weather (when you need it most). This is especially true of insulations with a high air permeability like fiberglass and to a lesser degree cellulose. Adding excess moisture from a bath or kitchen vent fan will cause insulation R value to drop even further. (Much further).
http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/92/920510.html
Condensation occurs when a surface is cooler than the dew point temperature of the air next to the surface. Any time condensation occurs, mold and decay are possible. Mold can grow when the air is damper than 80% rh.
Surprising little moisture is needed to condensate. Consider that it only takes from 4 to 6 pints of water to raise the rh inside a 1000 sq ft home from 15% to 60%. Now turn on your bath fan and dump that moisture laden air at the rate of 50 cfm (usually more like 80-100) into your cold attic and the warm, moisture laden are from a bath or shower will begin condensate and fall back onto the insulation.
It's very inexpensive to install vents (with dampers) on the exterior roof. Some metal flex connected to the fan outlet is all it takes. (You may have to pull a few shingles, but it shouldn't take more than a couple of hours. Your home will be warmer next winter and you'll use less fuel.
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Code typically demands vented attic spaces. This can be handled many ways. Look at the soffits to see if you have screens or a perforated strip at the eave. Google <continuous ridge vent> <gable end vent> . The wind turbines were all the rage a few years ago, and they make powered vents. Each has its pros and cons.
I doubt you will suffer any ill effects if your bathroom fan isn't vented to the outside if there is any kind of attic ventilation, but venting to the outside eliminates any possibility of increased humidity, etc.
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While its possible that your attic has no visible ventilation system, its extremely doubtful. Would consider verifying your assumptions before seeking a solution.
An uncomfortable way is to go into the attic on a bright sunny day. Look for daylight in the case of naturally aspirated ventilation. Soffit vents are obvious outside, even if ducted, or idiotically covered with insulation in the attic. Ridge vents and gable vents are obvious externally as well. Same with turbine vents. Dave
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