- The material of the walls:[1]
All materials are valid for a Sukkah [if it does not move with a common wind[2] as explained in Halacha B]. Even if the material does not lend shade to the Sukkah[3] [such as glass or transparent plastic], and hence it gives off more sun then shade, it is valid.[4] [Thus, the walls may even initially be made from material that is Halachically unfit for use as Sechach.[5]]
Material with bad odor, or material that dries and shrinks:[6] Nevertheless, one is not to use materials that give off a foul odor or which dry and shrink within the 7 days to below the minimum requirement for the wall.
✍️Using sheets as a wall:[7] It is not proper to use sheets [whether of fabric or plastic] as material for a wall even if it is tied down well.[8] However, it is permitted to use sheets if one places poles within three Tefach of each other and hence uses Lavud to make an additional backup wall.[9] [According to all, Bedieved if one used sheets for a wall, then if they are tied down well that they do not move with the wind, the Sukkah is Kosher even if Lavud was not used.[10]]
Using animals and humans as a wall:[11] Humans and animals are considered valid walls for a Sukkah, if they are 58 cm wide and 80 cm tall, and are not three Tefachim from the ground. However, animals are only valid if they are tied to the other walls of the Sukkah [in order so it does not run away]. On Yom Tov or Shabbos, it is forbidden to make a 1st, second, or third wall using an animal, and it is likewise forbidden to make such a wall using a human, if the human knows that he is being used as a wall. However, if he does not know that he is not being used as a wall, then it is valid to establish him as awall even on Shabbos and Yom Tov. It is permitted to make a 4th wall using a human or animal even on Shabbos. On Chol Hamoed, a human may be used as a wall even if he knows that this is the purpose of his stay. Vetzaruch Iyun if on Chol Hamoed the person himself can be used as a his own wall.
[1] Michaber 630:1
[2] Michaber 630:10
[3] Meaning that although the main aspect of a Sukkah is its ability to provide shade, of which lack there of invalidates the Sukkah, this is only in regards to the Sechach. However, the walls are not required that they provide shade. Hence even if in the morning or evening when the sun shines at an angle towards the walls of the Sukkah there is more sun light than shade, nevertheless the Sukkah is Kosher.
[4] Michaber ibid
[5] Biur Halacha 630:1 “Kol Hadevarim” in name of Elya Raba and Birkeiy Yosef; Sefer Haminhagim p. 65
Other opinions: Some Poskim rule that the wall of the Sukkah follows the same laws as the Sechach, and hence any material that is Biblically invalid for Sechach is likewise invalid to be used as material for the walls of the Sukkah. [Darkei Moshe 630 in name of Hagahos Ashri in name of Or Zarua, brought in Bach 630 and Biur Halacha 630:1] Some Poskim conclude that it is proper to be stringent like this opinion. [Bach ibid, brought in Biur Halacha ibid; Peri Megadim in 630; Biur Halacha ibid according to Gr”a who is Machmir to invalidate an invalid Mamad, and the Chazon Ish extends this to invalidate even a Mamad Demamad]
[6] Rama ibid
[7] Michaber 630:10; Piskeiy Teshuvos 630:9
[8] The reason: Being that it is difficult to ascertain that the sheets will not move with the wind, as even if one ties them well, occasionally the ropes tear unnoticeably, and he ends up having a wall that cannot stand in face of a common wind. [Michaber ibid]
[9] The reason: As in such a case even if the sheets move with the wind the wall remains Kosher due to the poles which are Lavud.
[10] Aruch Hashulchan 630:32
[11] See Michaber 630:11-12; Admur 362:12-17; M”B 630:52-57; Kaf Hachaim 630:74-87