House Ventilation
A little humidity is important to comfort in a home, particularly in the winter.
But too much vapor, combined with fumes from synthetic materials, pesticides, cleansers and household chemicals can make a house’s air not only uncomfortable but downright toxic. The answer? Ventilation.
Houses should breathe. They should draw in fresh air and exhaust stale air. In fact, some experts recommend that one half of a home’s air volume should be exchanged every hour.
You can do this, of course, by opening the doors and windows. But the trick is to provide needed ventilation without sending expensive home-heating and cooling energy dollars out the window.
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A house accomplishes this feat by having the proper type and combination of vents and fans in unoccupied portions of the house-such as attic and crawl space-and by venting specific areas of the interior.
Bathroom Exhaust Fan Venting Options
There are two common ways to route ductwork to vent bathroom air outside: through the ceiling and out through an exterior wall(wallvent), or through the ceiling and out through the roof (roofvent). (You can also buy a fan that vents directly through an exterior wall, but this typically requires running a new electrical line to the wall.)
Of the two ceiling options, a vent through a wall is less likely to leak.
Wallvent
Roofvent
Source:http://www.hometips.com/hyhw/comfort/58vent.html