I have a house built in 1978 that has vinyl siding over a black fiberboard sheathing (not sure of the name of it) with fiberglass batt insulation and a plastic sheet vapor barrier then drywall. I need to pull siding off of our house repair the flashing on a dormer, Depending on the damage to the sheathing I may replace it with polystyrene sheathing or osb. My question is this, should I put tyvek over the sheathing or will this trap moisture between the drywall vapor barrier and the sheathing? thanks!

 

Im pretty sure that actual tyvek isnt a moisture barrier at all. Its just an air infiltration barrier.

 

Ok I went to tyveks website and they say its a gross water infiltration barrier, and a air infiltration barrier. It says its breatheble so I suppose it wouldn't hurt to use it over the black board sheathing or the polystyrene sheathing if I replace the blackboard.