Gemara

(Beraisa - R. Eliezer): "V'Hayah Lecha l'Os" teaches that Tefilin is a sign to you (the wearer), not to others (for it is covered by clothing).

Zevachim 19a - Question (Rava): If the wind lifted the garment up off his skin, what is the law? If it must rest on his skin, the Avodah is Pasul. Or, perhaps this is included in normal wearing of garments.

Question: Is very fine dirt (which one does not notice) a Chatzitzah?

(R. Ami): Tefilin are a Chatzitzah (between the Kohen and Bigdei Kehunah).

Question (Beraisa): If they wore Tefilin, it is not a Chatzitzah.

Answer: That applies to the Shel Rosh (head Tefilin). R. Ami discusses the Shel Yad (Tefilin on the arm).

Question: What is the difference between them?

Regarding the Kesones (tunic), it says "Yilbash Al Besaro." Also regarding the Mitznefes, it says "v'Samta ha'Mitznefes Al Rosho"!

Answer (Beraisa): The Kohen Gadol's hair was seen between the Tzitz and the Mitznefes. There he wore Tefilin. (The Shel Rosh is not a place of Bigdei Kehunah, but the Shel Yad is where the Kesones is worn.)

Erchin 3b: One might have thought that Kohanim are exempt from Tefilin. "L'Os Al Yadecha... l'Totafos Bein Einecha" teaches that the Mitzvah to wear the Shel Rosh applies only to those commanded to wear the Shel Yad. Kohanim do not wear the Shel Yad (at the time of Avodah), for Bigdei Kehunah must be on their skin without any interruption - "Yilbash Al Besaro". One might have thought that they are also exempt from the Shel Rosh!

A Beraisa teaches that they are obligated, for neither Tefilin is Me'akev the other.

Megilah 24b (Mishnah): It is the way of outsiders to cover Tefilin with gold and put it on the sleeve.

Rishonim

Rosh (Hilchos Tefilin Sof Siman 18): The Gemara in Erchin teaches that the Shel Yad must be on the skin. Zevachim 19 says that the Kohen Gadol's hair was seen between the Tzitz and the Mitznefes, and there he wore Tefilin. This shows that one may not wear them on the Mitznefes.

Rosh (Teshuvah 3:4): "L'Os Al Yadecha" teaches that nothing may interrupt between Tefilin and his skin. This is like we expound about Bigdei Kehunah "Yilbash Al Besaro." L'Totafos Bein Einecha" teaches that nothing may interrupt between Shel Rosh and his (forehead) between his eyes.

Ran (Megilah 15b DH Harei): Outsiders put Tefilin on the sleeve to fulfill "it will be a sign", i.e. visible. They do not expound "Lecha l'Os", it is a sign to you, but not to others. The Rashba infers that the Shel Rosh, which should be visible, may be on top of clothing. The Kohen Gadol wore Tefilin between the Mitznefes and the Tzitz (but the Gemara assumed that he could not wear them over the Mitznefes). Perhaps this is because one may not put another Keli of a Mitzvah on top of the Mitznefes.

Poskim

Shulchan Aruch (OC 27:4): Nothing may interrupt between his skin and Tefilin, both of the head and of the arm.

Magen Avraham (4): The Shlah says that one should wash the place of Tefilin.

Kaf ha'Chayim (16): This is because people who rise for Tikun Chatzos put ashes in the place of Tefilin.It was not enough to mention the problem of Chatzitzah, for one could remove the Chatzitzah through a cloth or comb. One must wash the area due to honor of Tefilin. (Note: Kaf ha'Chayim 1:19 DH Zeh says that the ashes are put 'on the forehead, in the place where Tefilin are worn. Perhaps this means at the roots of the hairs just above the forehead.)

Gra (DH Lo): The Shulchan Aruch is like the Rosh. Rashi and Tosfos (Erchin 3b) say that (hand) Tefilin must be under the garment, because 'it is a sign to you, but not to others.' If so, the Shel Rosh, which should be visible, may be on top of clothing.

Kaf ha'Chayim (17): The Gemara did not resolve the law of fine dust that one does not feel. The Rambam says that one should be careful about this. Therefore, when a big wind blows dust into the hair, one should wash the place of Tefilin.

Kaf ha'Chayim (18): Pnei Aryeh (6) says that white powder is not a Chatzitzah, for it is for beauty. However, the Birkei Yosef says that one should avoid interrupt even if they are not Chatzitzos, for the Tefilin should be on his skin itself, like Bigdei Kehunah.

Mishnah Berurah (14): A Chatzitzah of any size is forbidden. L'Chatchilah It is proper to be careful even about a live louse, and surely about a dead louse or dirt. Therefore, some wash the place.

Machatzis ha'Shekel: I vehemently disapprove of those who grow long hair. Aside from haughtiness (see YD 178), it is a Chatzitzah regarding Tefilin. Since they grow it long, we cannot say that it is (a normal) part of the person. The Rashba says that Chatzitzah does not apply to the head, but in such a case perhaps he would agree that it does. He forbids putting Tefilin on a thick cap, for it must be where a baby's head is soft. Long hair is like a thick cap.

Kaf ha'Chayim (15): Also Mor u'Ktzi'ah says that hair that hangs down out of its place is a Chatzitzah. One should avoid the Safek. However, those who grow long hair in front, and when they put on Tefilin they put the hair above on the head and the Bayis on top to stop the hair from going down between their eyes, all agree that this is a Chatzitzah, for it is not normal.

Mishneh l'Melech (Avodas Yom ha'Kippurim 2:2): Whenever the Gemara mentions the Kohen Gadol immersing, it mentions that he dried himself. The Rambam says 'he dries himself.' This connotes that it is obligatory. It seems that this is because even one hair or dirt between his skin and the Bigdei Kehunah is a Chatzitzah, and disqualifies the Avodah. We are concerned lest a hair or dirt from the water remain on his skin. Even if one will say that we are not concerned for this, the Rambam says that he must be careful during the Avodah lest wind enter under his garment and distance it from him. Surely, water is no less than wind! He must dry himself lest water separate between his skin and the Bigdei Kehunah.

Birkei Yosef (1, cited in Sha'arei Teshuvah 6): One who washes the olace of Tefilin, must be careful to dry it well, due to honor of the Tefilin. Also, perhaps water is a Chatzitzah. Tefilin must be on the skin itself, just like Bigdei Kehunah. The Mishneh l'Melech says that the Kohen Gadol dried himself lest water be a Chatzitzah. If so, since the Rosh equates Tefilin to Bigdei Kehunah, one may not have water there.

Shulchan Aruch (5): If one is prone to get sick from the cold if he exposes his head, and cannot put Tefilin directly on his head, he can put it on a thin cap, and cover it lest people see (that his Tefilin is not on his skin).

Beis Yosef (DH v'Lo and veha'Rivash): The Rosh (Teshuvah 3:4) learns from "Bein Einecha" that the Shel Rosh must be directly on the head. The Rashba was unsure about this. Perhaps it merely teaches where we put on Tefilin. It is better to put it on over a thin cap than to be totally Batel from Tefilin.

Rebuttal (Bach DH Makom): A Hekesh equates the Shel Rosh and Shel Yad regarding Chatzitzah. No one is lenient about the Shel Rosh, even if he is prone to get chilled. It was clear to the Gemara (Zevachim 19a) that the Shel Rosh cannot be worn over the Mitznefes, but the Ran showed that we need not say that this is due to Chatzitzah.

Defense (Magen Avraham 6): One may not put the Shel Yad over his garment, due "Lecha l'Os", but not for others.

Kaf ha'Chayim (23): Tashbatz (30) says that the Tefilin must be on his head, but he may cover them with a garment.

Magen Avraham (7, Gra DH ha'Kova): Tefilin over a thin cap can be right over where a baby's head is soft. One cannot position them properly over a big cap. This is why Tefilin could not be worn over David's crown (Avodah Zarah 44a).

Mishnah Berurah (18): If one has a wound on his arm, he may wear Tefilin over a bandage if needed, but not over his garment. Some forbid even if he has a wound and even if another garment covers the Tefilin.

Rema: One who puts it on like this blesses only on the Shel Yad 'Lehani'ach'.

Gra (DH veha'Manicham): This is because all the Poskim disagree with the Rashba, and even the Rashba was unsure.

Mishnah Berurah (20): The Bach connotes that he should try to remove the Chatzitzah at the time of Keri'as Shma and Tefilah.