What is also appealing is that felt is forgiving. If water gets on the wrong side of a felt-wrapped wall, the felt can absorb the water and over time allow drying to the outside of the structure. Plastic housewraps don't move water this way. Plastic wraps are non absorbent. They rely on vapor diffusion to move water that gets on the wrong side of the wrap.
Diffusion is a slow and weak force.
But this does not mean that it traps water vapor in the wall cavity. As the relative humidity rises, the perm rating of felt rises too- eventually above that of Tyvek. Felt is dynamic. Having said that I also like Tyvek and Rwrap. They cost more than felt, but these products stand out to me as being very strong products.