I personally grow out my hair a bit in the front of my head. A chup I believe it’s called.
I’ve heard from different people that this is Halachically unacceptable as it presents a problem when wearing Tefillin as it is a Chatziza (interposition).
Any sources to back that claim up?
R Daniel Mann from Eretz Hemda answers your question at length here. He concludes it is not an issue in most cases although he does bring opinions (Machatzit Hashekel, Mishna Brura 27:15) that raise the issue of much more hair than normal and/or the tefilin sitting on so much hair that it doesn't sit on the head anymore. See there for details.
The summary of his answer is
Some poskim say that long hair is a chatzitza for tefillin. However, regarding most cases of long hair, these opinions are difficult, and the length is not the main issue, as we will see.
One puts the tefillin shel rosh where the hair grows (Shulchan Aruch, Orach Chayim 27:9). As one is not required to shave his head frequently, hair could not possibly be a chatzitza regarding tefillin. [...]
In summary, in all but the most extreme cases presented above, there is ample reason to say that long hair does not prevent the fulfillment of the mitzva of tefillin. We would note that many of the poskim who raised the issue combat the phenomenon of long hair for males (primarily?) based on other halachic, social, and philosophical elements (see Bemareh Habazak V, 25).
I heard once in a shiur that as long as the hair is growing in its natural position that it is not an issue. I don't recall who gave the shiur, but they commented that they personally push the hair on the front of the head forward before putting on their Tefillin. But if the hair is folded back, as is often done with a chup, and the Tefillin is resting on the part that's folded back it would be an issue since it's not the natural way for the hair to grow.
