Many people take showers in the morning before tefillin. Is wet hair a chazizah?
There are some who express concern that wet hair is considered a chatzitzah. In addition, wet hair is liable to damage the tefillin, and one should therefore dry one’s hair before wearing tefillin. If the hair is only a little damp, this would not be a concern.
Sources: See Birkei Yosef 27:1, who writes that one should not wear tefillin with wet hair, because of the concern of chatzitzah, and other concerns.