dryer vent between joists?

I'm installing a stacked washer/gas dryer on the second floor. The units will sit in a closet that is about 4 feet from an exterior wall. I would like to run 4" rigid pipe down through the floor and turn to run parallel between the floor joists and exit through a rim board between the 1st and 2nd floor.
My questions are about clearances. 1st of all, is advisable to bury the pipe between a floor and the ceiling? Do I have to keep the pipe a certain distance from drywall or joists or subfloor? Also, can I put insulation around the pipe?
I don't imagine that the pipe itself gets hot enough to combust unless it is clogged. I would have access to clean the pipe from the closet or from the termination hood on the side of the house.

My dryer vents in that manner - it was installed at the time of construction. Mine goes up, follows the joist bay along the ceiling and exits to the outside with a cap. It needs to be smooth metal duct, and you'll want to seal the joints in case it was venting CO (you don't need that or moisture pushed into the house!). As long as you keep the number of turns to the minimum and within the manufacturer's maximum length and turn limits, I think you'll be okay. See if someone can verify the code requirements, but I've seen it done that way and have passed, at least previously (codes do change).

Perusing my 2006 IRC, I noticed that the termination of the duct shall be not less than 3 feet from any opening into a building (window, door, etc). I can't quote chapter and verse but I definitely remember that.

SP

By- laws and codes vary from State to State, and from Province to Province.
It is always best to give the local authorities a call and find out for sure.