Written by Horav Yosef Yeshaya Braun, member of the Badatz of Crown Heights
There is a product on the market which is an undershirt with tzitzis on it. It’s worn by some people in hot weather, or while exercising or jogging.
In order for such an undershirt to be kosher for tzitzis it must be ascertained first and foremost that most of the length on both sides of the undershirt are open at the seams, not just a small section at the bottom; otherwise it cannot be considered as having four corners and requiring tzitzis.
The holes for the arms cannot be regarded as part of the opening.
The mitzvah min hamuvchar (preferred manner of doing the mitzvah) is to wear a woolen tallis katan, which obviously, the undershirt tzitzis is not.
There are some additional concerns regarding this garment. Among them:
1) There are some poskim who maintain that a garment which was made for absorbing perspiration does not qualify for the mitzvah of tzitzis.
2) Other poskim are of the opinion that although it is kosher, it’s nevertheless not respectful to the mitzvah.
3) When wearing garments made to absorb perspiration one often lacks the kavanah (intent) that he is wearing it for the mitzvah, as the garment is often worn for its “undershirt” aspect. According to some poskim, wearing tzitzis without kavanah is tantamount to wearing a four cornered garment without tzitzis.
4) Many of these garments are made from netted material; according to some poskim netted material does not qualify for the mitzvah of tzitzis. Many are made out of synthetic material, and poskim question whether synthetic material qualifies for this mitzvah. Therefore, one shouldn’t make a brachah on such tzitzis.
5) According to Siddur Harav, an individual who is a yarei Hashem (G-d fearing) and wishes to perform the mitzvah of tzitzis in a manner that conforms to all opinions, should ensure that the garment is completely open on both sides. One shouldn’t even fasten the sides to each other with clips or hooks and certainly one shouldn’t have sleeves or fashion openings for the arms.
6) According to Kabbalah one should not wear tzitzis directly on their body, but should wear an undershirt underneath.
It’s important to note that one may not make a brachah on tzitzis if the garment is exempt from tzitzis, and wearing such a garment on Shabbos might be considered carrying.
The hechsher on the product usually pertains only to the tzitzis and is not an indication of the kashrus of the garment as far as the mitzvah is concerned.
Nevertheless, the consensus among poskim is that if the seams on the two sides are sufficiently open, an undershirt with kosher tzitzis on it, while not recommended, can still be considered kosher if the garment isn’t made from synthetic material – but should be worn only when perspiring heavily or under other extenuating circumstances.
Otherwise, regular tzitzis – and preferably a woolen tallis katan – should be worn over a regular undershirt.