Some poskim say that long hair is a chatzitza for tefillin. However, regarding most cases of long hair, these opinions are difficult to justify, and as we will see, length is not the main factor.
The place to put the tefillin shel rosh is where the hair grows.[1] Since one is not required to shave his head every morning, hair could not possibly be a chatzitza regarding tefillin. The question is: why not?
The gemara[2] discusses whether hair that hangs down from the head and reaches between a kohen’s clothes and his body is a chatzitza or whether it is considered to be a part of the body itself. The gemara does not resolve the question. The Machatzit HaShekel[3] says that the doubt relates only to the hair at the back of the head. A blorit (probably a clump of hair in the front of the head) is a chatzitza for tefillin. He says that in the case of a blorit, one cannot apply the rule that anything that exists normally (revitaihu) is not a chatzitza because it is “abnormally big.” Presumably, if long hair whose roots are in the part of the head where the tefillin rest were combed down neatly (even if that hair extends to one’s waist), it would not pose a problem because the part of the hair that the tefillin is on is the same as it would be if one cut the hair closer to the head. Rather, the problem must be only if there is noticeably more hair than usual at the point where the tefillin sit. Thus, slightly long hair that is parted on one side of the head could be more problematic than longer hair arranged with a middle part,[4] and we have not heard of people being careful about the former.
Igrot Moshe[5] goes further, concluding that even transplanted hair is not a chatzitza, since it cannot be removed from the head without cutting (unlike a toupee) and, furthermore, the person wants the hair to remain. Rav Ovadia Yosef[6] points out that mourners must let their hair grow for at least thirty days, and some do so for twelve months, yet the poskim do not warn about this being an issue regarding tefillin. He also points out that there is a s’fek s’feika (double doubt) strengthening the contention that hair is not a problem of chatzitza, since in addition to the possibility that hair is considered part of the body, the Rashba[7] posits that chatzitza is not a problem for tefillin shel rosh.
There are two scenarios in which the concerns raised by poskim are particularly pertinent for those people who, by our standards, have long hair. One is that there could be several layers of hair on the sides of the head, where the straps of the shel rosh hold down the tefillin. Much of this hair would not be there at all if one’s hair was of normal length. On the one hand, this seems not to be a problem according to the Rama,[8] who says that chatzitza applies only to the boxes of the tefillin, not the straps. On the other hand, the Mishna Berura[9] says in the name of “the Acharonim” that one should not be lenient in regard to the part of the straps that are used to fasten the tefillin to the body. Another issue raised by the Mishna Berura[10] is that in addition to posing possible problems of chatzitza, a lot of hair can prevent the tefillin from being secured properly in place. In other words, although the tefillin can be considered to be resting on the head even if there is hair in between it and the scalp, it is not supposed to be resting on a clump of hair that happens to be rooted in the scalp.[11] Sometimes, especially when long hair is also thick and/or curly, the tefillin does not seem to be resting on the head to a significant degree.
In summary, in all but the most extreme cases presented above, there is ample reason to say that long hair does not interfere with the fulfillment of the mitzva of tefillin. We would note that many of the poskim who raise the issue do so in the context of combating the phenomenon of long hair for men based on other halachic, social, and philosophical considerations, rather than the concern about chatzitza for tefillin.[12]