In unconditioned attics or crawlspaces, insulate the duct. Bathroom vent ducts must always terminate on the outside of the house. Vent to the outside of the house using a backdraft-protected hood.

If the dryer is against an exterior wall that is not underground ... If the dryer is an interior room of the house, vent through the attic to the roof. Note: Vent a dryer through the roof only ...

Absolutely. Opening up your windows is an effective form of flush ventilation (flushing the warm air out of your house) - so long as the air outside isn't warmer than the air inside your house.